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What is a mixture?

A mixture contains two or more substances that are mixed together, but have not chemically bonded with each other.

A pure substance contains just one substance on its own. For example, table salt (sodium chloride) is a pure substance.

For example, seawater is a mixture of water, salt and other substances.

Can you name some different mixtures, and pure substances, that you might find in your house?


What are the properties of a mixture?

The substances in a mixture are not chemically bonded together. This means that it is usually quite easy to separate the substances (e.g. to get salt from seawater).

The proportions in a mixture are not fixed (for example there might be lots of salt in seawater, or very little).

The properties of a mixture are often an ‘average’ of the properties of the substances it is made from (e.g. a mixture of a red liquid and a blue liquid is a purple liquid).


Odd-one-out


A mixture or a pure substance?



What is a solution?

A solution is a special type of mixture that is made when one substance dissolves and mixes fully with another.

For example, a cup of instant coffee is a solution.

The solid that dissolves (e.g. coffee granules) is called the solute.

The liquid that does the dissolving (e.g. hot water) is called the solvent.

How many other solutions can you think of?


Solvents and solutes

tea

flavours and sugar

salt

flavours/sugar/carbon dioxide

flavours and alcohol

water

water

water

water

water

black tea

orange squash

seawater

wine

fizzy drinks

substance solvent solute

What is the solvent and what is the solute?

How many other examples of solutions can you think of?


Special solutions

Solutions do not have to be made from a solid and a liquid.

Solutions can also be made by dissolving a gas into a solvent.

Liquids can also be dissolved in other liquids, like alcohol in wine, and solids can even dissolve in other solids. For example, some alloys can be classified as solid solutions.

For example, it is the dissolved oxygen in water that allows fish to breathe.


Mixture or solution?


Does everything dissolve?


Calcium carbonate is not soluble in water because the calcium carbonate and water particles are not able to mix.

Copper sulfate is soluble in water because the copper sulfate and water particles are able to interact and mix together.

calcium carbonate copper sulfate

water

water

What happens when something dissolves?


Dissolving in action?


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50 g 10 g+

Conservation of mass

If 10 g of salt is added to 50 g of pure water, what is the mass of the solution?

When the salt has dissolved, you can’t see it any more. How could you check that the salt is still there?

?60 g


How much salt?


Conservation of mass – extension

If 10 g of salt is added to 50 g of seawater, what is the mass of the solution?

How much salt will be recovered if the mixture is separated by evaporation?

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50 g 10 g+ ?60 g


Does a solid keep dissolving?


How does temperature affect solubility?

Does sugar dissolve in cold tea?

It does, but not as much as in a cup of hot tea.

The sugar is more soluble at higher temperatures.

The amount of a solute that can dissolve at a given temperature is called its solubility.

How does temperature affect the solubility of a substance?

The solubility of a substance usually increases as the temperature increases.


Supersaturated solutions

Supersaturated solutions are very saturated indeed.

The solute will stay in solution until a ‘seed’ crystal is added, and then it will crystallise out of the solution very quickly.

When it does this, it gives out heat energy. Supersaturated solutions are therefore used in heat packs.


Supersaturated solutions in action


Solubility of gases

Gases are unusual because their solubility decreases when the solvent gets hotter.

Fish and other organisms that live in water survive by taking in oxygen that has dissolved in the water.

Some scientists think that global warming is causing sea temperatures to rise.

What effect do you think rising sea levels will have on the creatures that live in the sea?



How could you separate these mixtures?

Suggest some methods by which you could separate out these mixtures:

chocolate buttons and raisins

different coloured sweets

pebbles and sand

salt and sand

mud and water

oil and water

gold and iron.


Separating an insoluble solid

How could you separate an insoluble solid like sand from a mixture of sand and water?

It is easy to separate an insoluble solid by filtering the mixture.

The insoluble solid cannot pass through the filter paper but the water can.

The sand that is trapped by the filter paper is called the residue. The water that passes through the filter paper iscalled the filtrate.


Filtering apparatus


Separating a soluble solid

How could you separate a soluble solid, like salt, from a seawater solution?

To separate a soluble solid from a solution, evaporation can be used.

The solution is heated so that the water evaporates and leaves the dissolved solid behind.


Evaporation apparatus


Salty water

Where does the salt around the Dead Sea come from?


Separating salt from seawater

The Dead Sea is a salty lake, located between Jordan and Israel.

The Dead Sea is almost six times more salty than the ocean, so nothing is able to live in it and that’s why it is called ‘dead’.

The heat of the Sun evaporates the water from the Dead Sea, and the salt that is left behind is collected in salt beds.

The Dead Sea contains some of the saltiest water in the world!


Separating salt from rock salt

We also get salt from rocks called ‘rock salt’. Rock salt is a mixture of salt with sand and bits of rock.

Today, rock salt is mined using earth-moving equipment before being purified.

How could you use dissolving, filtering and evaporation to separate the salt from rock salt?

Rock salt was traditionally extracted by hand from underground mines; a very dangerous process.


Separating immiscible liquids

Liquids that do not mix together are described as immiscible.

On a small scale, immiscible liquids can be separated by simply removing the top layer using a pipette.

Can you think of any examples of immiscible liquids?

oil

water

In laboratories, chemists use a separating funnel to separateimmiscible layers.

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Separating miscible liquids

Liquids that do mix together are described as miscible.

How could you separate a mixture of miscible liquids?

Can you think of any more examples of miscible liquids?

An example of this is water and alcohol – these two liquids mix together easily.

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Distillation

The technique used to separate a liquid from a mixture is called distillation.

Distillation has three steps:

1. evaporation

2. condensation

3. collection

The solution is heated so that the liquid evaporates and is turned into a gas. Everything else is left behind.

The gas cools in the condenser and turns back into a liquid, which can then be collected.

Could distillation be used to make seawater safe to drink?


Distillation


Chromatography

Chromatography means colour-writing.

Chromatography is used to separate mixtures of coloured or non-coloured substances that are soluble in the same solvent.

A spot of the mixture is placed on some filter paper.

In method B, the solvent is slowly dripped onto the paper.

In method A, the solvent is soaked up the paper.


Chromatography experiment


Identifying dyes in a mixture

Which dyes does the mixture contain?

Dots of single dyes are placed alongside a dot of unknown mixture.

The solvent washes up the paper, and then the pattern of the dyes in the mixture can be compared with the single dyes.


Which dyes?


Uses of chromatography

How many uses of chromatography can you spot?


Which separation technique?


Separating mixtures



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