Pure and impure substances Flashcards
Evaporation occurs when a ___ slowly turns into a ___ below its boiling point.
liquid
gas
Crystallisation is a ___ technique used to obtain ___ of a ___ solute
separation
crystals
solid
When a ____ is heated, the ___ evaporates and ___ of the ___ are left behind
solution
solvent
crystals
solute
What would be left behind if a bowl of salty water was left out in the sun?
Salt crystals. The water would evaporate so the dissolved salt would end up left behind in the bowl.
Why do salt crystals appear faster in the salt solution when it is heated using the Bunsen burner?
Because the hotter a liquid is, the more energy the particles have, so the faster the liquid will evaporate.
What is evaporation?
Evaporation describes the process of a liquid turning into a gas.
What is faster? Evaporation or Boiling?
Evaporation is a slower process than boiling because it occurs at temperatures below the boiling point of the liquid.
Why do puddles dry up?
Puddles dry up because of evaporation. Puddles do not boil because the temperature on earth is always below 100°C - the boiling point of water.
How is evaporation involved in the water cycle?
Water evaporates from the seas, oceans, lakes and rivers. It condenses in clouds to form rain.
What is a technique to separate a solid solute from a solvent in a solution?
Evaporation
The solvent is the liquid, and when the solution is heated, the solvent evaporates. The solute is left behind as crystals.
Describe and explain how evaporation can be used to obtain copper sulfate crystals from copper sulfate solution.
Copper sulfate solution is placed into an evaporating basin and heated using a Bunsen burner. The water (the solvent) heats up and evaporates. The copper sulfate (the solute) does not evaporate. The copper sulfate turns into dry crystals when all the water has evaporated.
What is a solute?
The solid (or occasionally a gas) which dissolves into a solvent (liquid) in order to make a solution. For example, the main solute in sea water is sodium chloride.
What is a solvent?
The liquid in a solution which dissolves the solute. For example, the solvent in sea water is water.
What is a solution?
A mixture made when a solute (usually a solid) dissolves into a solvent (a liquid). Sea water is a solution of salt dissolved into water.
What is are two techniques to separate a solid solute from a solvent in a solution?
Evaporation
Crystallisation
Which technique to separate a solute from a solvent is faster? Evaporation or Crystallisation?
Evaporation is faster.
Crystallisation is similar to the evaporation technique used to separate a solid solute from a solution but is done over a longer period of time.
What produces larger crystals? Evaporation or Crystallisation?
Crystallisation produces the largest possible crystals of the solute, because the slower the evaporation process, the larger the crystals.
Desribe the 4 steps of the Crystallisation process
- Rather than heating the evaporating basin directly with a Bunsen burner, it is often better to warm it gently and slowly over a beaker of boiling water.
- Heat the solution until half the solvent (water) has evaporated, which makes the solution more saturated.
- Leave the solution to cool and large crystals will form slowly.
- Put the evaporating basin in a warm place so the rest of the water evaporates.
What is the main difference between what is produced in crystallisation and evaporation?
Crystal size.
This is because crystallisation produces larger crystals than evaporation.
What term is used to describe a mixture which contains a solid dissolved in a liquid?
Solution
What term describes the liquid in a solution?
Solvent
What is the name of the process when a liquid turns into a gas below the boiling point of the liquid?
Evaporation
What term describes a solution which contains the maximum amount of solute?
Saturated
What is the best way to get the largest possible crystals when evaporating a solution?
Warm the solution gently for a long time
What piece of apparatus is usually used to heat up a solution during evaporation?
Bunsen burner
Which word is used for the solid which has dissolved to make a solution?
Solute
Distillation is a separation technique used to separate a ___ from a ____.
solvent
mixture
Water can be separated from salt solution by ____.
distillation
What two processes occur in the water cycle to turn sea water into rain water?
Evaporation and condensation.
Water evaporates from the surface of the sea. The salt gets left behind in the sea. Then, the water vapour in the clouds cools and condenses to form rain.
What is the name of the piece of apparatus that cools down the hot gas after the mixture has been heated?
Condenser.
Distillation involves___ the solution and then ___ the___ back into a liquid by ___ it down.
boiling
condensing
vapour
cooling
For example,___ water can be distilled to produce ___ water, which is safe to drink.
dirty
pure
Name 6 Separation Techniques
Evaporation
Crystallisation
Distillation
Fractional Distillation
Filtration
Chromatography
Which two changes of state occur during distillation?
Boiling (liquid to gas), and then condensing (gas to liquid).
Desribe the steps for distillling a salt solution
The salt solution is placed into a flask and heated until it boils.
The water turns into a gas but the salt stays behind in the flask.
The steam passes into the condenser. The condenser is a tube which is surrounded by a layer of cold water. This cools the steam, which turns it back into a liquid.
The distillate is pure water.
When a solution made from salt dissolved in water is distilled, the temperature reading on the thermometer will be 100°C.
Why is this?
100°C is the boiling point of the water.
Ethanol boils at 78°C, whereas water boils at 100°C. A mixture of ethanol, water and salt can be separated using ___.
Ethanol boils at 78°C, whereas water boils at 100°C. A mixture of ethanol, water and salt can be separated using fractional distillation.
Describe the 3 steps to separate Water and ethanol
- Heat the mixture in a flask until the gas above the mixture is 78°C. The ethanol boils and can be re-captured using a condenser in the same way as the water in the salt water example.
- Keep the temperature at 78°C until the liquid ethanol coming from the condenser stops or slows down to almost nothing.
- The water will remain in the original flask.
___ is a technique also used in the chemical industry to obtain petrol and diesel from crude oil (a fossil fuel).
Fractional distillation
Crude oil is a mixture of lots of different liquids (fractions) and we can use their different ___ to separate them into more useful substances.
boiling points
Which technique would be used to separate a mixture of the solid solute potassium bromide dissolved into water?
Distillation.
There is no need to use fractional distillation, as the mixture contains only one liquid.
Describe 5 hazards in the distillation process
- Knocking over glassware – risk of broken glass and cuts
- Hot liquids – risk of spillage and scalds
- Bunsen burner flame – risk of burns or fire
- Heater – risk of burns
- Flammable liquids (e.g. ethanol) – risk of fire
Diffusion is the movement of a substance from an area of ___ concentration to an area of ___ concentration.
high
lower
Diffusion occurs in __ and __ when their particles collide randomly and spread out.
liquids
gases
Diffusion is an important process for living things - it is how substances move in and out of __ .
cells
If someone sprays deodorant from the other side of the room, how does the smell reach your nose?
Through a process called diffusion. The deodorant particles bump into air particles and are spread throughout the room. We smell the deodorant particles that reach our noses.
Why does diffusion happen faster in gases than in liquids?
The particles in gases have more energy and are more widely spaced out than the particles in a liquid. This means that gas particles will bump into each other harder and more often. Which makes gas particles spread out more quickly than liquid particles can.
What is diffusion?
Diffusion is the process by which particles of one substance spread out through the particles of another substance.
___ is how smells spread out through the air and how concentrated liquids spread out when placed in water.
Diffusion
___ is not the same as diffusion. ___ is the term used to describe when a substance breaks up and mixes thoroughly with a solvent to produce a solution.
Dissolving
Dissolving
Why doesn’t diffusion happen in solids?
Diffusion doesn’t happen in solids because the particles in a solid cannot move around, instead they vibrate about a fixed position.
Which two factors affects diffusion?
- The type of substance - substances diffuse more quickly through gases than through liquids
- Temperature - the hotter the gas or liquid, the faster diffusion happens.
Most materials that we use are ___ , and just a few are pure elements or pure compounds.
mixtures
In chemistry, a ___ substance is a single substance made of only one type of particle.
pure
___ change the temperature at which a substance melts and boils.
Impurities
Cartons of fruit juice often say they contain ‘pure’ juice. Does the word ‘pure’ have the same meaning for a scientist as it does in everyday life?
Natural fruit juice is made up of sugar, water and many other naturally occurring chemicals. In chemistry this is not considered ‘pure’ and is a mixture.
Is the air that we breathe a pure or impure substance?
The air that we breathe is impure. It is a mixture of oxygen, nitrogen, carbon dioxide, carbon monoxide and many impurities such as methane.
__ substances are made from only one chemical element or one compound.
Pure
Salt is a __ substance made only of sodium chloride.
pure
Can you name two examples of compounds?
Examples include pure water, which is H₂O, and carbon dioxide, CO₂.
A label for a bottle of water will often include a list of small amounts of other substances. These are called ___ .
impurities
Which of the following, if any, is a pure substance?
Sea water
Water in a mountain stream
Distilled water in the school science laboratory
None of the above
None of these examples is a chemically pure substance.
Sea water is not pure because it is a mixture of water, salt, and other dissolved substances.
The water in a mountain stream contains much less salt than sea water. It still contains some impurities, such as dissolved oxygen, carbon dioxide, minerals and microorganisms.
Distilled water contains very small amounts of impurities.
A pure substance has a fixed __ point and __ point.
melting
boiling
The melting point of __ water is 0 °C and its boiling point is 100 °C.
pure
When a substance contains ___ , its melting and boiling points change.
impurities
When __ is added to water, the mixture freezes below 0 °C, and boils at a temperature over 100°C.
salt
It is possible to find out if a substance is pure or not by measuring its __ or point.
melting
boiling
4 steps for using a thermometer
- Make sure that the bulb of the thermometer is covered by the liquid but not touching the bottom of the container.
- Give the thermometer enough time to reach the same temperature as the liquid you’ve dipped it into. Stir the liquid with the thermometer to make sure all the liquid has the same temperature.
- Watch the coloured liquid in the thermometer move. When it stops moving, it’s ready to read.
- Hold the thermometer in the liquid and get down to read the temperature at eye level.
How does filtration work? (3 steps)
- The filter paper has lots of tiny holes in it, which are far too small to see.
- The particles of the liquid including any dissolved solutes will pass through the holes in the filter paper.
- If the liquid contains an insoluble solid made of large particles, these larger particles cannot pass through the holes in the filter paper.
What is filtration?
Filtration is the process of separating solids from liquids using filter paper.
When is filtration used?
The process can be used to separate an insoluble solid, for example stone or sand grains from a liquid. The liquid could be a pure liquid, for example water, or it could be a solution, for example, salty water.
When a mixture of sand and water is filtered:
the sand stays behind in the filter paper, it becomes the ___ .
the water passes through the filter paper, it becomes the filtrate.
residue
filtrate
What is a residue?
The solid which has not passed through the filter paper during filtration.
What is a filtrate?
The liquid which has passed through the filter paper during filtration.
Filtration is used to separate an __ solid from a pure liquid or a solution.
insoluble
Filtration usually involves a circle of __ folded to make a cone and placed into a filter funnel.
filter paper
The __ is the liquid which passes through the filter paper and the __ is the solid left on the filter paper.
filtrate
residue
A sieve can be used to separate small stones from fine sand. Why does sand pass through a sieve but stones do not?
Sand passes through a sieve because the sand particles are small enough to go through the holes in the sieve. The stones are too large to fit through.
If a solid does not dissolve in a specific solvent, it is ___ in that solvent.
insoluble
A solution is made when a __ dissolves into a .
solute
solvent
If a substance can __ into a solvent, it is soluble. If it cannot __ , it is described as insoluble.
dissolve
dissolve
__ , __ and using __ are all ways to speed up dissolving.
Heating
stirring
fine powders
When sugar is added to hot water and stirred, it seems to disappear. What has really happened to it?
It has dissolved. The sugar crystals break into tiny particles which are too small to see and spread out through the hot water.
What happenes to the solubility of sugar when the temperature of the tea is hottest?
The solubility increased with temperature. The hotter the water, the more sugar can dissolve.
A __ is made when a substance dissolves into a liquid.
solution
A solution can also be called a __.
mixture
When a solid dissolves, it breaks down into smaller particles that spread out through the ___ .
solvent
When a white solid dissolves, it makes a ___ solution, for example, salt or sugar in water.
When a coloured solid dissolves, it makes a __ solution, for example, copper sulphate dissolves to makes a blue solution.
colourless
coloured
There is a limit to the mass of solute that will dissolve in a particular volume of the solvent. When no more solute dissolves, the solution is __ .
saturated
When copper sulfate is added to water to make copper sulfate, which substance is the solvent, and which is the solute?
Copper sulfate is the solute. Water is the solvent.
What happens to particles of a solid solute when it dissolves? (3 Steps)
- The solute is added to the solvent.
- The individual solute particles break apart and spread out.
- The solute is now fully dissolved. The solute particles are widely spaced throughout the solvent.
A red solute is dissolved into a colourless solvent making a pink solution. Why do you think it is pink?
Because the red solute has broken apart and the particles are now spread throughout the solvent.
Insoluble or soluble
Wax is ___ in water.
insoluble
Insoluble or soluble
Chalk is ___ in water.
insoluble
When an insoluble white powder is stirred into a colourless liquid it does not dissolve. At first the liquid will appear to turn __ , but after a few minutes the liquid will return to __ as the powder settles on the bottom of the container.
white
colourless
___ measures how much solute can dissolve in a volume of solvent at a specific temperature.
Solubility
For example, sugar has a higher solubility in water than salt. Therefore, more sugar than salt can dissolve in a volume of water at a specific temperature.
You can speed up dissolving a solid by:
- Heating up the solvent. The solubility of a solid solute usually increases when the temperature increases.
- Stirring.
- Using fine powder rather than large pieces of solute.
