matter in our sorroundings

Question 1

what is matter?

Answer 1

anything which occupies space and has mass is called matter. air, water, hydrogen, oxygen, sugar, sand, silver, steel, ice, iron are all different kinds of matter, because all of them occupy space ( that is, they have volume), and mass

Question 2

how do we classify matter?

Answer 2

matter can be classified in a number lf ways. ancient Indian philosophers said that all matter ( padarth ), living or non living, was made up of five basic elements ( panchn tarva ) : air, water, fire, sky, earth. moder day scientists classify matter in two ways, on the basis of its physical properties and on the basis of its chemical properties.

Question 3

how is matter classified on the basis of its physical & chemical properties ?

Answer 3

on the basis of physical properties, matter can be classified as solids, liquids or gasses. on the basis of chemical properties, it can be classified as elements, compounds and mixture’s

Question 4

what is matter made up of?

Answer 4

everything around us is made of tiny pieces or particles. the number of particles in everything is however, very large. for e.g, a small rain drop contains around 10²¹ particles of water in it. these particles are so small that they can’t be seen even with a high power microscope. however, even without seeing them, we have certain evidence that everything is made up of matter. evidence also shows that the particles in matter are also constantly moving. the particles which make up matter are known as atoms or molecules.

Question 5

give examples to prove that matter is made up of particles.

Answer 5

most of the evidence for the existence of particles in matter and their motion comes from the experiments on diffusion and brownian motion. diffusion is the mixing of different substances on their own. without the concept of particles, things become difficult to explain
1. dissolving a solid in a liquid
potassium permanganate is a purple coloured solid substance and water is a colourless liquid. when a crystal of potassium permanganate is placed in a beaker filled with water, water slowly turns purple without any external stirring. both the crystal and the water are made up of tiny particles. the particles of potassium permanganate, when putinto water separate from one another and they get into the spaces between the particles of the water. can conclude that particles are in motion from this experiment. moment of different particles among one other on their own, so that they become mixed uniformly, is called diffusion.
2. mixing of two gases
air is a colourless gas or mixture of gases. when a gas jar is empty, it is actually filled with air. however, we can’t see air because it is colourless. bromine is a red-brown liquid. it forms vapour easily. bromine vapour is red-brown in colour and is heavier than air.
a gas jar containing air is placed upside down on a gas jar containing bromine vapour. we will see that the red-brown vapours of bromine from the lower gas jar spread up into the air from the upper gas jar and after some time, containing air also becomes completely red-brown in colour. the spreading up of bromine vapour into the air is because, both air and bromine gas are made up of tiny moving particles which collide with each other and bounce about in all directions, due to which they get mixed uniformly. is another example of diffusion. even though, bromine gas is heavier than air, it still goes up, against gravity and mixes with the air in the upper jar because its particles are moving with high speed due to which they have sufficient kinetic energy to overcome the force of gravity and move the gas jar containing air. the process of diffusion gives evidencethat matter is made up of tiny particles and these particles are constantly moving
3. movements of pollen grains in water
the best evidence for the existence and movement of particles in liquids was given by Robert brown in 1827. Robert brown suspended extremely small pollen grains in water. on looking through the microscope, it was found that the pollen grains where moving rapidly throughout water in a very irregular or zigzag way. it was also observed that warmer the water, faster the pollen grains move on the surface of water. movement of pollen grains on the surface of water is because water is made up of tiny particles which are moving very fast and the pollen grains move on the surface of water because they are constantly being hit by the first moving particles of water. although the water particles are two small to be seen, their affect on the pollen grains can be seen clearly. the random motion of visible particles caused by the much smaller invisible particles of water is an example of the brownian motion which is named after the scientist Robert brown who first observed this phenomenon. brownian motion can also be observed in gases for example when sunlight enters a room, we see tiny dust particles suspended in air which are moving rapidly in a very haphazard way. the dust particles move here and there because they are constantly hit by the fast moving particles of air. brownian motion is the zigzag movement of the small particles suspended in a liquid or gas. brownian motion also gives evidence that matter is made up of tiny particles and that these particles are moving constantly

Question 6

what are the characteristics of particles of matter

Answer 6

important characteristics of particles of matter are:
1. particles of matter are very, very small
the extremely small size of the particles of matter can be shown by performing the following experiment. two or three small crystals of potassium permanganate and dissolve it in 100 ml of water in a beaker. we will get a deep purple colour solution of potassium permanganate. take 10 ml of the purple solution of potassium permanganate from the first beaker and mix it with 19 ml of water present in the second beaker to dilute it. due to the dilution, the colour of potassium permanganate solution in the second beaker becomes a bit lighter. now take 10 ml of potassium permanganate solution from the second beaker and mix it with 90 ml of water present in the third dilute it further and we will see the colour of the solution becomes more lighter. we keep the utens the potassium permanganate solution like this a number of time so that we get a very dilute solution of potassium permanganate in water but the water is still coloured. it should have a light pink colour by now. the experiment shows that just two or three tiny crystals of potassium permanganate can impart colour to a very large volume of water. from this observation we conclude that each crystal itself must be made up of millions of small particles which people on spreading an important colour to more and more of water ( on dilution ). each tiny crystal of potassium permanganate is supposed to be made up of millions of particles, therefore, these particles must be extremely small.
2. the particles of matter have spaces between them
the spaces between the particles of matter can be shown by performing the following experiment. we take about 100 ml of water in a beaker and mark the level of water in the beaker with the marking pen and also takes 50 g of sugar. dissolve the sugar in the water by stirring it with the glass rod and once the sugar has dissolved we get a sugar solution. if you look at a level of sugar solution in the beaker we find that the level of sugar solution in the beaker is at the same mark where water level was intially. that even after dissolving 50 grams of sugar in 100 ml of water, the volume has not increased. this can be explained by assuming that the particles of water has some space in between. when sugar is dissolved in water, it’s crystals separate into tiny particles. these particles of sugar go into the space between the various particles of water due to which there is no change in the volume of water dissolving sugar in it. the fact that there is no change in the volume of water on dissolving sugar in it tells us that there are spaces between the particles of matter. these pieces accommodate the sugar particles and this leads us to another conclusion being that in water the molecules are not tight tighly packed and they are somewhat loose, having between them.
3. particles of matter are constantly moving
when we light an incense stick in one corner of a room, its fragnance spreads in the whole room quickly. this is because the burning of incense stick produces gases which move rapidly in all directions and mix with the moving particles of air in the room and reach every part of the room quickly along with air. if, however, the particles of gases produced by the burning of incense stick and the particles of air were not moving, then the fragnance of incense stick could not spread in the whole room quickly. so, the observation that the fragnance of a burning incense stick spreads in the entire room very quickly tells us that the particles of matter are constantly moving.
another example would be the diffusion of copper sulphate into water. copper sulphate crystals are blue in colour and when a few crystals of copper sulphate are placed at the bottom of a beaker containing water then water in the whole beaker turns blue slowly. the crystals of copper sulphate dissolve in a little of water around them to form fine particles of copper sulphate in solution. due to the motion of the particles, the blu dopper sulphate particles in solution move upwards in the beaker and the colourless water particles move downwards and mix with each other this is called diffusion. if we carry out the experiment by using hot water in the beaker, we will find that the water turns blue at a faster rate. this is because, on heating, the particles of water and that of copper sulphate gain kinetic energy and move faster. due to faster movements, they mix into each other more quickly
3. particles of matter attract each other
there are some forces of attraction between the particles of matter which bind them together. the force of attraction between the particles of the same substance is known as cohesion. the force of attraction/cohesion is different in the particles of different kinds of matters.
if you take a piece of chalk, a cube of ice and an iron nail, and beat them with a hammer, we will find that it is very easy to break the piece of chalk into smaller particles, it requires more force to breake a cube of ice, whereas the iron nail does not break at all even with a large force. this shows that the force of attraction between the particles of chalk is quite weak and the force of attraction between the particles of an ice cubeis a bit stronger whereas the force of attraction between the particles of iron nail is very very strong.
let’s take another example. we can move our hand through air very very easily because the force of attraction between the particles of air is very small. it is negligible. we can also over hand through water in a bucket fairly easily because the force of attraction between the particles of water is also small but we cannot move our hand through a plank of wood because the force of a attraction between the particles of wood is very strong and if we try breaking a plank of wood with the blow of our hand, our hand will get hurt. the force of attraction is maximum in the particles of solid matter and minimum in the particles of gaseous matter.

Question 7

explain the meaning of rigid and fluid.

Answer 7

rigid means unbending or inflexible. a stone is rigid. fluid means a material which can flow easily and requires a vessels to keep it. water is a fluid because if you pour water on the top of a table, it flues over the surface of the table. requires vessel to keep it.
a solid is rigid form of matter. due to their rigidity, solids have a tendency to maintain their shape when subjected to outside force. the main characteristic of a solid is its rigidity. solids do not require a container to be kept in for example a brick and a log of wood. a liquid is a fluid form of matter which occupies a lower part of the container in which it is kept. a liquid does not fill the whole container. liquid require a container to keep them. liquid has a well defined surface. so, a liquid can be kept in an open container and it will not escape from the container by itself for example water and milk.
a gas is a fluid form of matter which fills the whole container in which it is being kept. gas is also required a container to keep them and gases do not have a free surface and they have to be kept in a closed container because a gas will escape escape in an open container for example LPG this is why gases are usually kept in a tight gas cylinders. liquids and gases are known as fluids. the characteristic of liquids and gases of flowing easily is called fluidity.

Question 8

matter exist in three physical states. name those three states.

Answer 8

solid liquid and gases are called the three states of matter. in other words, matter exists in 3 physical states, namely, solid, liquid and gas. they have different properties

Question 9

what are the properties of solids?

Answer 9

ice, wood, stone, iron, brick etc are some of the common solids around us.
the solids have the following characteristic properties:
1. solids have a fixed shape and a fixed volume.
2. solids cannot be compressed.
3. solids have high densities and are heavy.
4. solids do not fill their containers completely.
5. solids do not flow.

Question 10

what are the properties of liquids?

Answer 10

water, milk, juice, ink, oil, petrol, kerosene etc. are some of the common liquids.
the liquids have the following characteristic properties:
1.liquids have a fixed volume but they have no fixed shape. they take the shape of the vessel in which they are placed.
2. liquids cannot be compressed much.
3. liquids have moderate to high densities. they are usually less dense than solids.
4. liquids do not fill their containers completely.
5. liquids generally flow easily.

Question 11

what are the properties of gases?

Answer 11

air, oxygen, hydrogen, carbon dioxide, nitrogen, steam etc. are some of the common gases.
the have the following characteristic properties:
1. gases have neither a fix shape nor a fixed volume and they acquired the shape and volume of the vessel in which they are kept.
2. gases can be compressed easily into a small volume.
3. gases have very low densities. they are very light
4. gases fill their containers completely.
5. gases flow easily

Question 12

why do solid, liquids and gases have different properties?

Answer 12

matter in all the three states, solid, liquid, and gas, is made up of very small particles known as atoms and molecules. according to the kinetic theory of matter, particles of matter are in continuous motion and possess kinetic energy. some forces of attraction also exist between the particles of matters and are called interparticle forces. the force of attraction tend to hold the particles together and control their movements. the interparticle force of attraction are the maximum when the particles are close together. as the distance between the particles of matter increase, the forces of attraction between them decrease. the movement of particles or the kinetic energy of particles can be changed by heating or cooling the matter. higher the temperature, greater the moment of particles. the properties of particles decide a substance is a solid liquid or a gas.
1. the space between the particles.
2. the force of attraction between the particles.
3. the amount of moment of particles or the kinetic energy of particles.

Question 13

what are solids?

Answer 13

in solids the particles are closely packed and there is a strong force of attraction between the particles of a solid which holds them together in fixed positions. thus, the position of particles in a solid are fixed. the particles of a solid only vibrate on their fixed positions. they cannot move from one position to another. if a solid is heated, its particles start vibrate faster. the spaces between the particles of a solid are very small. particles of a solid have the minimum kinetic energy and solids have the most orderly arrangement of particles.
a solid has a fixed shape because the particles of a solid are closely packed and their positions are fixed due to the forces of attraction between them. solid has a fixed volume because the spaces between its particles are fixed. solid cannot be compressed much because its particles are already very closely packed and there are hardly any spaces between them. a solid has a high density because its particles are very close together. a solid does not feel a container completely because it’s particles are hed tightly by strong interparticle forces and hence they cannot leave their positions to fill the whole container. a solid does not flow because its particles are held very strongly and they are unable to leave their fixed positions.
we have just now that a solid has a fixed shape. now, a rubber band changes its shape on stretching, then why do we call it a solid? a rubber band is considered a solid because it changes shape under the action of force and when the force is removed, the rubber band regains its original shapes. like all other solids, if a rubber band is stretched with the large force, it breaks.
salt and sugar are considered to be solids because the shapes of individual crystals of salt and sugar remain fixed. sponge can be compressed easily but it is still considered a solid because it’s compressibility is due to the presence of minute pours in it which are filled with air. when we press the sponge, air is expelled from its pores making it highly compressible

Question 14

what are liquids?

Answer 14

in liquids, the particles are close together, they are not as close as in solids. the particles are somewhat loosely packed in liquids and the spaces between the particles of a liquid are slightly more than that in a solid but they are still very small. there is a quite strong force of attraction between the particles of a liquid which holds them together but the force is not strong enough to hold the particles in fixed positions. due to comparatively less strong interparticle forces, the positions of particles in a liquid or not fixed. the particles of a liquid have more kinetic energy then the particles of a solid. the liquids have a more disorderly arrangement of particles then solids. the particles and liquid are also vibrating and when a liquid is heated, then it’s particles begin to move faster.
a liquid does not have a fixed shape because the positions of its particles are not fixed due to comparatively less strong forces of attraction. liquid takes the shape of its container because the particles of a liquid can slide over one another easily. liquid has a fixed volume because, at a given temperature, the spaces between its particles are fixed. a liquid cannot be compressed much because its particles are still quite close together and have small spaces between them. a liquid has comparatively high density because its particles are quite close together. liquid does not fill its container completely because the particles are held fairly strongly by the interparticle forces and hence cannot leave the body of the liquid to fill the whole container and a liquid generally flows easily because particles are able to slide over one another due to slightly weaker interparticle forces of attraction.

Question 15

show how liquids do not have a fixed shape but they have a fix volume with an experiment.

Answer 15

we take some water in a beaker and mark the level of water in it and we will find that water takes the shape of the beaker. now, we put this water from the beaker into other containers of different shapes and we will notice that water takes the shape of all these tumblers for example when we pour the water into a glass tumbler, the water takes the shape of the glass tumbler and when we pour it into a conical flask, it will take the shape of the conical flask. experiment we conclude that a liquid has no fix shape and takes the shape of its container. now, let us pour the water from the conical flask back into the original beaker. we find that the water fills the beaker up to the same mark. thus we come to the conclusion that a liquid has a fixed volume even if it is poured into different containers

Question 16

what are gases?

Answer 16

in gases, particles are much farther apart from one another as compared to solids and liquids. the spaces between the particles of gases are very large and the force of attraction between the particles of a gas is negligible. the particles of a gas are free to move in any direction. the position of the particles of gases as well as the spaces between the particles of a gas are not fixed. the particles of a gas have the maximum movement or the maximum kinetic energy. due to this, the gases have the most disorderly arrangement of particles. because of high kinetic energy and negligible forces of attraction, the particles of a gas move with high speeds in all directions. when those fast moving gas particles hit the walls of a container, exert a pressure known as gas pressure. the pressure exerted by a gas is due to the collision of the fast moving gas particles against the walls of the container. when gas is put in an empty container, it quickly spreads throughout the container and fills it completely and when the gas is heated, the particles start moving faster. a gas does not have a fixed shape because the position of its particles are not fixed and a gas does not have a fixed volume because the space between its particles are not fixed. a gas takes the shape and volume of its container and it can be compressed easily because its particles are far apart and there are large spaces between them which can be reduced by compression. gas has a very low density because its particles are far apart. a gas fills its container completely because due to high kinetic energy and negligible forces of attraction, the particles move with high speeds in all directions. a gas also flows easily because its particles are free to move anywhere.

Question 17

show that gases do not have a fixed shape or a fixed volume with an experiment

Answer 17

we take two balloons of different shapes and sizes and fill these balloons with equal amounts of air. we will find that the air takes the shape of the balloon in which it is filled for example, if the balloon has a round shape, then the air takes the round shape of the balloon. and if the balloon has a long shape, then air takes the long shape of the balloon. this shows that the air has no fixed shape of its own and takes the shape of its container. if we look at the round balloon and the long balloon, we will find that they have different volumes and this proves that a gas has no fixed volume and takes the volume of its container.

Question 18

what is diffusion?

Answer 18

diffusion is the spreading out and mixing of a substance with another substance due to the motion of its particles. the diffusion of one substance with another substance goes on until a uniform mixture is formed. diffusion is a property of matter which is based on the motion of its particles. diffusion occurs in solids liquids and gases and is the fastest in gases and slowest in solids. diffusion is fastest in gases because the particles in gases move very rapidly. the diffusion is lowest in solids because the particles in solids do not know much. the diffusion in liquids is much faster than that in solids. the rate of diffusion increases on increasing the temperature of the diffusing substance. this is because when the temperature of a substance increases by heating, it’s particles gain kinetic energy and move more rapidly. and this increase in the speed of the particles of a substance increases the rate of diffusion. the phenomenon of diffusion tells us that the particles of matter are constantly moving.

Question 19

explain the diffusion in gases

Answer 19

diffusion in gases is very fast and this is because the particles in gases move very quickly in all directions. in the laboratory, diffusion of gases is usually demonstrated by setting up a gas Jar of bromine vapours and a gas Jar of air. an example of diffusion in our everyday life is the spreading of the smell of food when it is being cooked. when food is cooked, substances in food release gases having the smell of food in them and these particles of gases move very quickly and mix up with air by diffusion. another example is the lighting of the incense stick. of the burning stick spreads all around the room due to diffusion and the smell of perfume spread due to the diffusion of perfume vapours into air. when a perfume bottle is opened, then the liquid perfume quickly changes into vapour and moves very rapidly in all directions.

Question 20

explain the diffusion in liquids.

Answer 20

diffusion in liquids is slower than that in gases. this is because the particles in liquids move slower. here are some examples of diffusion in liquids.
1. if a crystal of potassium permanganate is placed at the bottom of water in a beaker, then the purple colour of the crystal spread into the whole water, slowly.
2. if a crystal of copper sulphate is placed in bottom of the water in a beaker, then the blue colour of the copper sulphate spreads into the whole water slowly.
3. if a drop of ink is put into a beaker of water, then the colour of ink spreads into the whole water of the beaker.

gases like carbon dioxide an oxygen are essential for the survival of aquatic plants and animals and those gases present in the air diffuse into water and dissolve in it and aquatic plants use this dissolved carbon dioxide for preparing food and aquatic animal use the dissolved oxygen for breathing and this is an example of diffusion of gases into liquids.
solids liquids and gases can diffuse into liquids and the rate of diffusion in liquids is much faster than that in solids as particles in a liquid move much more freely have more species between them as compared to particles in solids.

Question 21

explain the diffusion in solids

Answer 21

diffusion can also take place in solids but diffusion in solids is a extremely slow process. we will now give two examples of diffusion of solids in solids.

1. if we write something on a black board and leave it and clean for a considerable period of time, we will find that it becomes quite difficult to clean the blackboard afterwards. this is due to the fact that some of the particles of chalk have diffused into the surface of blackboard.
2. if two metal blocks are bound together tightly and kept understood for a few years, then the particles of one metal are found to have diffuse into the other metal.

diffusion of a solid substance into another solid substance is so slow that many people think the decision does not take place in solids at all. diffusion in solids is slow because particles in solid do not move from their fix position and the particles only vibrate about their fixed positions. due to lack of appreciable moments in the particles of solids, the diffusion of solids in solids is rare.

Question 22

what is the common unit of temperature and the SI unit of temperature?

Answer 22

we use celsius scale of temperature for measuring temperatures in our everyday life. common unit of measuring temperatures is degrees celsius ( °C ). the laboratory thermometer are all calibrated in degrees celsius and even the clinical thermometer is calibrated on the Celsius scale. the melting point of ice is zero degree Celsius and boiling point of water 100 degree Celsius.
there is another scale of temperature called Kelvin scale of temperature which is used by mainly scientists for research. the SI unit of measuring temperature is kelvin ( K ).
the word degree and the ° sign is NOT used with kelvin scale temperatures. an advantage of the kelvin stage is that all the temperature on this scale are positive. the melting point of ice is 273 K and the boiling point of water is 373 K.
0°C = 273
temp. on kelvin scale = temp. on celsius scale + 273
to convert a temperature on Celsius scale to the Kelvin scale, we have to add 273 to the Celsius temperature
to convert a temperature on Kelvin scale to Celsius scale, we have to subtract 273 from the Kelvin temperature

Question 23

explain the change of state of matter.

Answer 23

matter can exist in all three states : solid state, liquid state and gaseous/vapour state.
we can change the physical state of matter in two ways:
1. by changing the temperature.
2. by changing the pressure.
when we see that we can change the state of matter by changing the temperature, we mean that the state of matter can be changed by heating or cooling it. and when we see that we can change the state of matter by changing the pressure, we mean that the state of matter can be changed by increasing or decreasing the pressure on it. the spaces between the particles, force of attraction between the particles and the amount of moment or kinetic energy of particles can be changed by changing the pressure and/or temperature of a substance. the same substance can exist in all the three physical states. the two factors which decide whether a given substance would be in a solid, liquid or gaseous state are: temperature and pressure

Question 24

explain what is meant by melting.

Answer 24

by increasing the temperature, a solid can be converted into its liquid state. this is known as melting e.g if we heat ice, it changes into water. the process in which a solid substance changes into a liquid on heating, is called melting/fusion. the melting of a solid substance takes place at a fixed temperature. the temperature at which a solid substance melts and changes into a liquid at atmospheric temperature, is called the melting point of the substance. different solids have different melting points. the melting point of wax is 63°C.
the melting point of a solid is a measure of the force of attraction between its particles. higher the melting point of a solid substance, greater will be the force of a fraction between its particles. the melting point of iron metal is very high ( 1535°C ) which tells us that the force of attraction between the particles of iron is very strong.
when a solid is heated sufficiently, it changes its physical state and becomes a liquid. when a solid substance is heated, the heat energy makes its particles vibrate more vigorously. at the melting point, the particles of a solid have sufficient kinetic energy overcome the strong forces of attraction holding them in fixed positions and break to form small groups of particles.

Question 25

explain what is meant by boiling/vaporisation

Answer 25

liquid can be converted into its gaseous or vapour state. water normally exists in the liquid state. if we go on heating water, it ultimately starts boiling and changes rapidly into gas called steam. the process in which a liquid substance changes into a gas rapidly on heating is called boiling. a change of state takes place during boiling. the boiling of liquid takes place at a fixed temperature. the temperature at which a liquid boils and changes rally into gas at atmospheric pressure, is called the boiling point of the liquid.
different liquids have different boiling points. the boiling point of alcohol is 78°C. the boiling point of water is 100°C, whereas the boiling point of mercury is 357°C.
the boiling point of a liquid is a measure of the force of attraction between its particles. higher the boiling point, greater will be the force of attraction. when a liquid is heated it changes its physical state and becomes a gas. this is because. when a liquidis heated, the heat energy makes its particles move even faster. at the boiling point, the particles of a liquid have sufficient kinetic energy to overcome the force of attraction holding them together and separate into individual particles and liquid boils to form gas.

Question 26

explain what is meant by condensation and freezing.

Answer 26

condensation ——
if we cool steam by lowering its temperature, it is converted into liquid water. in this case a gas changes into a liquid, so a change of state takes place. the process of changing a gas or vapour into liquid by cooling it is called condensation. when a gas is cooled enough, it’s particles lose so much kinetic energy that they slow down, closer to each other until they start being attracted to each other, and form a liquid. condensation is the reverse of boiling.

freezing ——
when water is cooled by lowering its temperature in a refrigerator, it changes into solid ice. the process of changing a liquid into a solid by cooling is known as freezing. freezing means solidification. this is a change from liquid to solid state. freezing is the reverse of melting. the freezing point of a liquid is 0°C. when a liquid is cooled by lowering its temperature, its particles lose energy due to which they move slowly. if the liquid school enough after its freezing point, it’s each particle stops moving and vibrates about a fixed position, as in itfreezes and becomes a solid
we can continues that the state of matter can be changed by changing the temperature.

solid can be changed into liquid by heating it ( melting ) and the liquid can be changed back into a solid by cooling it ( freezing ). a liquid can be changed into a gas by heating it ( boiling ) and the gas can be changed back into a liquid by cooling it ( condensation ).

Question 27

what is latent heat?

Answer 27

normally, when heat is given to a substance, then its temperature rises. this, however, is not so when a change of state takes place. because when heat is given to change the physical state of a substance, there is no rise in temperature of the substance. the heat energy which has to be supplied to change the state of a substance is called latent heat. latent heat does not raise the temperature. latent heat is always supplied to change the state of a substance. the word ‘latent’ means ‘hidden’
it is called latent heat as it is hidden in the substance undergoing the change, and it does not show its presence by raising the temperature. so, latent heat cannot be detected by a thermometer.

Question 28

why does latent heat not cause a rise in temperature?

Answer 28

every substance has some forces of attraction between its particles which holds them together. now, if a substance has to change the state, then it is necessary to overcome/break these forces of attraction between its particles. the latent heat which we supply is used up in overcoming the forces of attraction between the particles of a substance during the change of state. the latent heat does not increase the kinetic energy of particles and hence, th temperature of the substance does not rise.

Question 29

what are the types of latent heat?

Answer 29

latent heat is of two types:
1. latent heat of fushion.
2. latent heat of vaporization.
heat required to convert a solid into a liquid state is called latent heat of fusion or the latent heat of melting and the heat required to convert a liquid into the vapour state or gaseous state is called a latent heat of vaporisation.

Question 30

explain the latent heat of fusion.

Answer 30

the latent heat of fusion can be studied by performing an experiment. we take some crushed eyes in a beaker and suspend a thermometer in it. note the temperature of the ice. if is found to be 0°C. heat urgently by using a small flame of the burner and on heating, the eyes starts melting to form water and we keep on recording the temperature of melting ice on the thermometer every minute and as more heat is given, more ice melts to form water but the temperature remains 0°C. as long as there remains even a little bit of ice, there is no rise in the temperature during the melting of the ice. only when all the ice is melted then the temperature of water starts rising on further on heating. going into the ice but not increasing his temperature, is the energy required to change the state of ice from solid to liquid. the latent heat of fusion of a solid is the quantity of heat in joules required to convert 1 kg of the solid to liquid, without any change to the temperature.
3.34 × 10⁵ joules of heat has to be supplied to change one kilogram of ice into water. the latent heat of fusion of ice is 3.34 × 10⁵ joules per kilogram. the heat energy is used up in changing the state of the solid substance by overcoming the force of attraction between. ice is a solid substance and the particles of ice attract one another with strong forces and these forces of attraction hold particles slowly packed in solid ice. the heat we supply is all used up to overcome the force of attraction and this he does not increase the kinetic energy and hance no rise in temperature occurs. it has then found that ice at 0°C is more effective in cooling a substance and water at 0° c. this is due to the fact that for melting, each kilogram of ice takes its latent heat of rules from the substance and hence cools of substance more effectively. water at 0°C cannot take any such latent heat from the substance
when a solid melts, it absorbs heat to form liquid. the reverse of this is also true. when a liquid freezes to form a solid, an equal amount of heat is given out. when ice at zero degree celsius melts, it absorbs latent heat of fusion to form water at the same temperature. when water at 0° c freezes to form ice at 0° c, it gives out an equal amount of heat

Question 31

what is the latent heat of vaporisation.

Answer 31

take some water in a beaker and suspend a thermometer in it. we heat this water by using a burner and note its temperature after every minute. as heat is given, the temperature of water rises gradually until 100°C is reached. at the temperature of 100°C, water boils and starts changing into steam. as more heat is given to the water, more steam is formed but the thermometer reading remains at 100°C showing that there is no rise in temperature during the boiling of water. once the water has begun to boil, the temperature remains constant at 100°C until all the water has changed into steam. the heat which is going into boiling water but not increasing its temperature is the heat required to change the state of water from liquid to gas. this is known as the latent heat of vaporisation of water. the latent heat of vaporization of a liquid is it quantity of heat in joules required convert 1 kilogram of the liquid at its boiling point to vapour or gas without any change in its temperature and it is in found by experiment that 22.5 × 10⁵ joules per kilogram is the latent heat of water ( 22.5 × 10⁵ J/KG )
we see that when a liquid boils on heating, it’s temperature remains the same. the heat energy supplied to a boiling liquid is used up in changing the state of the liquid substance by overcoming the force of attraction between particles. the temperature of a boiling liquid does not rise even though heat is being given continuously. this is because the particles of water attract one another with certain forces and these forces of attraction hold the particles together in the liquid state in the heat which we supply to water during boiling is used to overcome or break the force of attraction between water particle so that they become totally free and change into a gas this. the latent heat does not increase the kinetic energy of water particle and hence there is no rise in temperature
if he put a little of spirit at the back of our hand and wave it around, desperate evaporates rapidly and our hands feel very cold. this is due to the fact that spirit needs latent heat of vaporization change from liquid to vapour state and the spirit take the latent heat of vaporization from our hand and so our hand lose heat and feel cold.
when water changes into steam, it absorbs latent heat, but when steam condenses to form water, an equal amount of latent heat is given out. since steam gives out more heat then boiling water, it causes more severe burns. this also explains why steam is better than boiling water for heating purposes

Question 32

what is sublimation? explain.

Answer 32

most of the solid substances, when heated, first change into a liquid and then into vapours/gas. this vapour, on cooling, first forms a liquid and then a solid. but there are a few solid substances which change directly into vapours on heating and vapours give back solid substance on cooling.
the changing of a solid directly into vapours on heating, and of vapours into solid on cooling, is known as sublimation.
the solid substance which undergoes sublimation is said to be sublime. the solid obtained by cooling the vapour of the solid is called a sublimate.
ammonium chloride undergoes sublimation. when solid ammonium chloride is heated, it directly changes into ammonium chloride vapour. and when hot ammonium chloride vapour is cooled, it directly changes into solid ammonium chloride. sublimation occurs in only a few substances. so it is a rare process. the common substances which undergo sublimation are: ammonium chloride, iodine, camphor, naphthalene and anthracene
when these solids are heated, there particles move so quickly that they separate completely to form vapour. another example of sublimation is provided by solid carbon dioxide which is commonly known as dry ice. solid carbon dioxide sublimes to form carbon dioxide gas.

Question 33

perform an experiment to demonstrate sublimation

Answer 33

take some ammonium chloride in a China dish and place the China dish on a tripod stand. the China dish is covered with an inverted glass funnel. a loose cotton plug is put in the upper, open end of the funnel to prevent the ammonium chloride vapours from escaping into the atmosphere. the China dish is heated using a burner. on heating, ammonium chloride changes into white vapours. the vapour rise up and get converted into solid ammonium chloride on coming in contact with the cold, inner walls of the funnel. in this way, ammonium chloride collects on the inner side of the funnel in the form of a sublimate and can be removed.
in many households, small naphthalene balls are kept in stored woollen and silk clothes to protect from the attack of moths and other insects. it is noticed that these naphthalene balls disappear with time without leaving behind any residue. this is because naphthalene balls undergo sublimation. the naphthalene balls keep on forming naphthalene vapour slowly which disappears into the air.

Question 34

what is the effect of change of pressure? explain.

Answer 34

the physical state of matter can also be changed by changing the pressure. the physical state of matter can also be changed by increasing or decreasing the pressure. gases can be changed into liquids by increasing the pressure and some solids like solid carbon dioxide can be changed into gases on decreasing the pressure

Question 35

explain how gases can be liquefied by applying pressure and lowering temperature.

Answer 35

when a high pressure is applied to a gas, it gets compressed into a small volume and when we lower its temperature, it gets liquefied. so, we can say that gases can be liquefied by compression and cooling.
there is a lot of space between the particles of a gas. we can reduce the space between the particles of a gas by enclosing it in a cylinder and compressing it by pushing in the piston. if enough force/pressure is applied to the piston, the gas is highly compressed and the particles of the gas get so close together that they start attracting each other sufficiently to form a liquid.
when a gas is compressed too much,then heat is produced due to compression. so while applying pressure to liquefy gases, it is necessary to cool them to take away the heat produced due to compression. cooling lowers the temperature of compressed gas and helps in liquefying it. we conclude that gases can be liquefied by applying pressure and lowering temperature. a gas has to be cooled below a certain temperature before liquefication by pressure can occur. cooling is usually done by pouring water over the coils carrying the compressed gas. ammonia gas can be liquefied by applying high pressure and lowering the temperature. that is, ammonia gas can be liquefied by compression and lowering the temperature.
decreasing the pressure and increasing the temperature can also change the state of matter.
solid carbon dioxide/dry ice is stored under high pressure. on decreasing the pressur on dry ice, it gets converted directly into carbon dioxide gas. when a slab of solid carbon is kept exposed to air, then the pressure on it is reduced to normal atmospheric pressure, its temperature rises, and it starts changing into carbon dioxide gas. the conversion of solid carbon dioxide into carbon dioxide gas is a change of state ( from solid to gas ) which is caused by the decrease in pressure nd rise in temperature.

Question 36

what is solid carbon dioxide?

Answer 36

solid carbon dioxide is a white solid called dry ice. solid carbon dioxide is an extremely cold substance. it is used to deep freeze food and to keep ice cream cold. since solid carbon dioxide directly changes into carbon dioxide gas, and does not melt to produce a liquid, it is called dry ice. dry ice can produce much lower temperatures that that produced by ordinary ice. thus, it is much more effective for cooling purposes than ordinary ice.

Question 37

what is evaporation? explain.

Answer 37

when a liquid is left exposed to air, its volume decreases gradually. this is due to evaporation. in this process some of the liquid at the surface turns into vapour and mixes with the gases in the air. when a liquid turns into a liquid, it is said to have evaporated. the process of a liquid changing into vapour/gas even below its boiling point is called evaporation. evaporation can occur even at room temperatures, but its faster at higher temperatures.
whatever be the temperature at which evaporation takes place, the latent heat of vaporisation must be supplied whenever a liquid changes into vapour. the wet clothes dry due to evaporation of water present in them. rain water puddles also dry up because of evaporation of water.

the process of evaporation:
some particles in a liquid always have more kinetic energy than the others. so, even when a liquid is well below its boiling point, some of its particles have enough energy to break the forces of attraction between the particles and escape from the surface of a liquid in the form of vapour. thus, the fast moving particles or molecules of a liquid are constantly escaping from the liquid to form vapour.

Question 38

what are the factors affecting evaporation?

Answer 38

the factors affecting evaporation are:
1. temperature
2. surface area
3. humidity
4. wind speed

Question 39

explain how temperature and surface area affect evaporation.

Answer 39

temperature ——
the rate of evaporation increases on increasing the temperature of the liquid. in other words, the rate of evaporation of a liquid increases on heating. when the temperature of a liquid in increased by heating it, more particles of the liquid get enough kinetic energy to go into the vapour/gaseous state. this increases the rate of evaporation. the rate of evaporation becomes the maximum at its boiling point.

surface area of the liquid ——
the rate of evaporation increases on increasing the surface area of the liquid. if the surface area of a liquid exposed to the air is increased, the rate of evaporation of the liquid increases. if some liquid is kept in a test tube and in a China dish, then the liquid kept in the China dish will evaporate more rapidly because more of its surface area is exposed to air. we spread out the washed wet clothes while trying to increase the surface area of rapid evaporation. after rain the wet roads dry quickly because the rain water is spread over large area of the road and this gives the particles of water a greater chance of escaping from the liquid and from this discussion we conclude the rate of evaporation of a liquid can be increased by increasing the surface area of the liquid.

Question 40

how does humidity of air and wind speed affect the rate of evaporation? explain.

Answer 40

humidity of air ——
the amount of water vapours present in air is represented by a term called humidity. when the amount of water vapours present in the air is small, the air appears to be dry. on the other hand, when the amount of water vapours in the air is large, the air appears to be damp and we say that the humidity is high.
humidity of air tells us the degree of dampness in the air. when the humidity is low, the rate of evaporation is high. under these conditions, sweat from our body evaporates readily and we feel cool and comfortable. when the humidity of air is high, then the rate of evaporation is low. in the later part of summer, the humidity is high. people sweat a lot in such weather but this sweat from our body does not evaporation and such weather becomes muggy/damp and we feel hot and uncomfortable. this type of weather is experienced during cloudy days in the rainy season and in areas close to the sea ( coastal areas )

wind speed——
the rate of evaporation of a liquid increases with increasing wind speed. when speed of wind increases, the particles of water vapour move away with the wind, decreasing the amount of water vapour in the surrounding. this increases the rate of evaporation of water.

Question 41

explain what is meant by the cooling caused by evaporation.

Answer 41

the cooling caused by evaporation is based on the fact that when a liquid evaporates, it draws or takes the latent heat of vaporization from anything it touches. by losing heat, the thing gets cooled. if we put a little of spirit/ether/petrol at the back of your hand and wave it around, the spirit evaporates rapidly and our hands feel very cold. this is due to the fact that to change from the liquid to the vapour state, spirit requires latent heat of vaporization. the spirit takes the heat from our hands. the hand loses heat and gets cold.
during hot summer days, water is usually kept in an earthen pot to keep it cool. the earthen pot has a large number of extremely small pores in its walls and some of the water continuously keeps seeping through these pres to the outside of the pot. this water evaporates continuously and uses the latent heat required for vaporization from the earthen pot and the remaining water. in this way, the remaining water loses heat and gets cold.
at many places, especially villages, people often sprinkle water on the ground in front of their homes during the hot summer evening. this water evaporates by taking the large latent heat of vaporization from the ground and the surrounding air. by losing heat, the place becomes cool and comfortable. water vaporising from the leaves of trees also causes surrounding air.
perspiration or sweating is our body is method of maintaining a constant temperature. when our body temperature tends to rise too much, are sweat glands give out moisture or sweat on our skin and this sweat evaporates, it takes the latent heat of vaporization from our body.
we should wear cotton clothes in hot summer days to keep cool and comfortable because we get a lot of sweat on our body in hot summer days and cotton is a good absorber of water so it absorbs the sweat from our body and exposes it to the air for evaporation. the evaporation of the sweat cools our body. synthetic fibres do not absorb much of our sweat and fail to keep a body cool in summers. a fan increases the rate of evaporation of sweat from our skin and keeps us cool and comfortable.
the cooling in a desert room cooler is caused by the evaporation of water and a desert cooler cools better on a hot and dry day because the higher temperature on a hot day increases the rate of evaporation of water, and the dryness of air also increases the rate of evaporation.
we are able to sip hot tea faster from saucer than from a cup. saucer has a larger surface area and the larger surface area of the saucer the evaporation of hot tea from the saucer is faster and its faster evaporation cools the hot tea more quickly making it convenient for us to drink.

Question 42

show the presence of water vapour in air with an example.

Answer 42

there is always some water vapour in the air around us. the amount of water vapour in air keeps on changing. water vapour coffee into the air from the evaporation of water present in ponds, lakes, rivers and oceans. water vapour is also given out by plants by the process of transpiration. animals give out water vapour when they breathe out. land also gives water vapour on being warmed by the sun.
take a steel tumblr and put some well crushed ice into it. wipe the tumblr from outside with a piece of clean, dry cloth, so as to make its outer surface completely dry. allow the Steel tumbler to stand undisturbed for about 5 minutes. large number of tiny drops of water appear on the outer surface the Steel tumbler. this happens because the air around us contains water vapor in it and when these water vapour come in contact with the cold,voutside surface of the Steel tumbler they condense to form tiny drops of liquid water.

Question 43

are there only three states of matter?

Answer 43

scientists now say that they are actually 5 states of matter: solid, liquid, gas, plasma and Bose-Einstein condensate

plasma :
plasma is a mixture of free electrons and ions and it is considered the fourth state of matter. plasma occurs naturally in stars. inside the stars, the temperature is so high that the atoms break up and some of the electrons break away from the atoms converting the rest of atoms into electrically charged particles called ions and this mixture of free electrons and eyes in a star is called plasma. the sun nd the stars glow because of the presence of plasma in them.
plasma can also be made on the earth by passing electricity through gases at very low pressure taken in a glass tube called a discharge tube. the fluorescent tubes and neon sign bulbs from plasma when they are switched on. a florescent tube may contain helium gas or some other gas, and a neon sign bulb contains neon gas. when electricity is passed through a fluorescent tube or a neon sign bulb, the gases present in them get ionized to form plasma. this plasma makes a fluorescent tube or neon sign bulb glow.

Bose-Einstein condensate :
in 1920, an Indian scientist Satyendra nath Bose did some calculations for the fifth state of matter. on the basis of these calculations, Albert Einstein predicted the existence for a new state of matter called Bose-Einstein condensate or BEC. the fifth state of matter was finally achieved by 3 scientists, Cornell, ketterle, wiemen of USA by cooling a gas of extremely low density ( about 100 thousandth the density of normal air ) to super low temperatures.

Question 44

show that solids and liquids cannot be compressed but gases can be compressed with an experiment?

Answer 44

let us take 100 ml glass ranges having frictionless Pistons. the nozzles of all the syringes are closed by inserting them in separate rubber corks as to prevent leakage. remove the Pistons from all the syringes.
in the first syringe, we will fill common salt, which is a solid. then insert the piston back into the syringe. (after applying some vaseline on it for smooth moment )
in the second syringe, we fill water and then insert the piston back in to the syringe.
in the third syringe we do not fill anything. it is already filled with air. we put the piston back in it
the piston of the first syringe does not move down on putting weight. this shows that on applying pressure, the volume of common salt in the syringe does not get reduced and thus common salt, which is a solid cannot be compressed. the piston of the second. syringe containing water also does not move down on putting weight. this also shows that water cannot be compressed. the piston of the third syringe containing air move down considerably on putting weight which shows that the volume of the air is reduced and hence, gases can be compressed. gases have high compressibility and they can be compressed into very small volumes by applying large pressures and due to this high compressibility, fairly large mass of a gas can be put in a small metal cylinder by compression and such gas cylinders can be transported conveniently from one place to another. from this activity we can also conclude that solids and liquids cannot be compressed