Solids, Liquids and Gases (incl density and pressure)

Question 1

By what process can a substance change from a solid to a liquid?

Answer 1

Melting

Question 2

By what process can a substance change from a liquid to a gas?

Answer 2

Evaporation

Question 3

What happens during the melting process?

Answer 3

Heat energy is absorbed without the temperature of the substance increasing. Thus the heat appears to be ‘hidden’ by the substance, and is referred to as the latent heat of melting. It is actually used to loosen the bonds between the particles in a solid

Question 4

What happens during the evaporation process?

Answer 4

Heat energy is absorbed without the temperature of the substance increasing. Thus the heat appears to be ‘hidden’ by the substance, and is referred to as the latent heat of melting. It is actually used to break the bonds between the particles in the liquid.
Evaporation can cause cooling, because it is the fastest moving molecules that leave the liquid, so the average speed - and thus the temperature - of the molecules in the liquid drops

Question 5

What are the particles like in a solid?

Answer 5

They vibrate about fixed-positions within a close-packed regular structure

Question 6

What are the particles like in a liquid?

Answer 6

They have a random motion within a close-packed irregular structure

Question 7

What is one important thing to remember about the particles when melting?

Answer 7

The particles in a solid do NOT get a lot further apart when the solid melts - the structure is still ‘close-packed’, but the particles can now ‘change places with their neighbours.’
e.g. when ice melts the water molecules actually get a bit closer together initially (the density of ice is less than that of water)

Question 8

What is the pressure law?

Answer 8

The kelvin temperature of a gas is proportional to the pressure of a fixed mass of the gas at a constant volume

Question 9

What is the relationship between the pressure and Kelvin temperature of a fixed mass of gas at a constant volume?

Answer 9

P1 / T1 = P2 / T2

Question 10

What is the relationship between density, mass and volume?

Answer 10

Density = mass/volume

Question 11

How would you determine density of an object with a regular shape using direct measurements of mass and volume?

Answer 11

By measuring its mass on a top-pan balance and then by calculating the volume after taking some measurements e.g. of length, width and breadth.
Once the mass and volume are known, the density can be calculated using the formula: density = mass/volume

Question 12

How would you determine density of an object with an irregular shape using direct measurements of mass and volume?

Answer 12

Measure the mass on a top-pan balance
The volume can be found by measuring the volume displaced when the object is totally immersed in water
The best way to do this is to use a displacement can, which is filled to the brim with water to start with - when the object is lowered into the can, the water displaced is caught in a measuring cylinder so that its volume can be measured (this is the volume of the object)

Question 13

What is the relationship between pressure, force and area?

Answer 13

pressure = force / area

Question 14

Why do gases exert pressure?

Answer 14

Because of all the collisions that randomly moving particles make with the walls of any container that they are in

Question 15

What is the direction of the pressure in a gas or liquid (at the same depth)?

Answer 15

The pressure has no specific direction - any surface that is introduced into a gas at any angle experiences it
It acts equally in all directions

Question 16

What causes pressure in a liquid?

Answer 16

Pressure is caused by the weight of liquid pressing down from above. Again, the randomly moving particles are making collisions with the walls of the container, acting equally in all directions

Question 17

Why does hot air cause an increase in pressure in a hot air balloon?

Answer 17

The air molecules have more energy, therefore more random motion, and collide with the wall of the air balloon more. A force is produced by the colliding molecules, and pressure is a force on an area.

Question 18

How does a decrease in air pressure outside the hot air balloon affect the hot air inside?

Answer 18

Hot air cools down or air inside the balloon expands

Question 19

What is the relationship between pressure difference, height, density and g?

Answer 19

Pressure difference = height x density x g

Question 20

What did Robert Brown observe to come up with Brownian motion?

Answer 20

He was observing pollen grains in water through a microscope and he noticed that the grains appeared to move around in random movements.
The movement is cause by millions of random collisions with the much, much smaller water molecules
It was the first visible evidence for the existence of molecules in gases and liquids in constant motion

Question 21

How does the particle theory of a gas explain gas pressure?

Answer 21

Gas molecules are in constant random motion
When a molecule collides with a surface it exerts a force on the surface as it changes its direction
The pressure exerted by the gas is equal to the total force exerted by the molecules on an area of the surface, divided by the area

Question 22

Why does reducing the volume of a gas increase the pressure?

Answer 22

Because the particles collide more frequently with the walls of the container, exerting more force and therefore pressure

Question 23

What is Boyle’s Law?

Answer 23

The pressure of a gas is inversely proportional to its volume. This is for a fixed amount of gas at a constant temperature
P1 x V1 = P2 x V2

Question 24

What happens at Absolute Zero?

Answer 24

As temperature decreases, the average speed at which molecules move decreases
Absolute Zero is the temperature at which all molecules will cease moving

Question 25

What temperature is Absolute Zero?

Answer 25

0K or -273°C

Question 26

Describe the Kelvin scale

Answer 26

Starts from Absolute Zero

A change of one kelvin is the same as a change of one °C

Question 27

How do you convert celsius temperature to kelvin?

Answer 27

Kelvin temperature = celsius temperature + 273

Question 28

What happens to gas pressure when you increase the temperature?

Answer 28

When temperature increases, the kinetic energy of the molecules increases and they move more quickly in their random motion, colliding with the walls of the container more often and with a greater force. Therefore, this produces an increase in gas pressure as pressure is force over an area

Question 29

What is the unit of Kelvin?

Answer 29

K (no degree sign)

Question 30

What is the unit for density?

Answer 30

kg/m3

Question 31

What is the unit for pressure?

Answer 31

N/m2 (=1Pa)