This is one of the most exciting topics that you will study in Year 7 and you can do some of the experiments for yourself in your very own chemistry laboratory (this is your kitchen NOT your bedroom), but please don’t do any experiments without permission from an adult. Well some of them are quite safe, even so, get permission first!
All you need to do with this page is scroll down and read it. If there are any sections that you do not understand you should ASK THE CAT for help: use the form at the bottom of this page.
Please be specific with your question.
The page is divided into 9 sections:
- How can we tell whether a liquid is a mixture?
- How much salt can we get from rock salt?
- What happens to the solute when a solution is made?
- How can we separate solvents from solutes?
- How can chromatography separate and identify substances in mixtures?
- Checking progress
- Is there a limit to the amount of solid that will dissolve in a liquid?
- What else affects solubility?
- Reviewing work
You should know that:
- some solids dissolve in liquids and others do not,
- many common materials are mixtures,
- mixtures can be separated.
Well you will already know this from everyday life. Salt and sugar will dissolve in water, but sand will not dissolve. So we say that salt and sugar are soluble and sand is insoluble.
You know that a cup of tea is a mixture because when you make it you put boiling water over the tea leaves or teabag and can see a change of colour. You might then add some milk and lots of sugar. You made the mixture. You should also know that tap water is not pure. It usually contains some calcium carbonate. This gets left behind in your kettle after water has boiled. Eventually the inside of your kettle gets furred up with the impurities from tap water. If you let the kettle boil and collect the steam, you can make some pure water. We usually call this distilled water. In a cup of tea, the water is the solvent and the sugar and other flavours dissolved in it are the solutes. The tea leaves at the bottom of the cup are insoluble. Together, the mixture of solvent and solutes are a solution.
Air is also a mixture, though it is a little bit more difficult to separate air into its components. These are oxygen, nitrogen, carbon dioxide, argon and other rare gases and some water vapour. Air is not a solution because it does not consist of chemical substances dissolved in a liquid.
Rock salt is another mixture.
Rock salt is the slightly pink mixture of salt and gravel that is spread on our roads in freezing weather. The salt in it helps to melt ice and the gravel makes the surface of the road rough to help prevent cars and motorbike from skidding. You should know how to:
- get pure salt from rock salt,
- evaluate methods used in terms of the mass of salt obtained,
and know that:
- salt comes from a variety of sources and
- has many uses.
You can do your own experiment with rock salt, but you will have to wait for some freezing weather when the Council will spread grit on the roads.
Put some rock salt in an old cup or glass and fill it up with water. Now stir it until the salt dissolves. Carefully pour the liquid through a filter. You can use some kitchen towel for this. The salty water will go through, but the sand and gravel will be caught on the paper towel. Pour more water over the sand if you want to get “pure” sand. If you want pure salt, you can leave the cup of salty water to evaporate. After about a week in a warm place, the water will have evaporated leaving the salt behind.
Table salt can be obtained in two different ways. Some of it comes from the sea. Seawater is allowed to flood special ponds. Very gradually, the seawater in these ponds becomes stronger and stronger as the sun evaporates the water. Eventually the water is so salty that it starts to crystallise. This concentrated salt solution is called “brine”. Particles of salt stick together because there is not enough water to keep them apart. After the salt has been scraped out of the dried up pond, more seawater is allowed to flood back into it.
We can also get salt from salt mines. These are just places where seawater dried up in the past forming layers of rock salt. This can be purified by dissolving the salt in water, then filtering it to remove the sand, and finally crystallising the brine.
Solvents and solutes
You should know that when a solute dissolves:
- mass is conserved
- the solute and solvent particles intermingle.
By looking at a glass of water you cannot tell whether the water is pure or impure. As you dissolve salt or sugar in water, the solute seems to disappear, but it is actually still there. You know this because you put the substance in the water. You can prove that it is still there by tasting it. Don’t do that in a chemistry laboratory because it is not safe.
What has happened is that the particles of the solid have separated and become mixed in between the liquid particles. With an accurate chemical balance, you could show that the salty or sugary water is heavier than the original tap water. You may have see photographs of people floating in the Dead Sea where the water is very much denser than ordinary sea water.
Separating solvents from solutes
You should know that distillation:
- can be used to separate a liquid from the solids which are dissolved in it
- is a process in which evaporation of a liquid is followed by condensation.
You should know that:
- a mixture of two or more solutes which are soluble in a particular solvent can be separated by chromatography
- how to separate and identify materials using chromatography
- how chromatography can be used to compare mixtures of solutes
- how scientists use evidence from chromatography
Have a look at my page on chromatography to extend your knowledge
You should know how:
- particle theory can be used to model changes that take place when solutions are formed or components of solutions are separated.
Is there a limit to the amount of solid that will dissolve in a liquid?
You should know that:
- when a solid is added to a liquid, eventually no more will dissolve
- different masses of different solids dissolve in the same volume of a particular solvent
- solids can dissolve in liquids other than water
So how much sugar do you put in a cup of tea? My pet human put FOUR large spoonfuls in every cup. He would put more, but it would just leave a sticky mess at the bottom of the cup.
This is a very simple experiment for you to try out at home. See how many spoonfuls you can dissolve in a cup of water. You can use sugar or salt and you will get different results. So which is more soluble, salt or sugar? Can you dissolve more in hot water or in cold water? Does stirring make any difference to the total amount that can be dissolved?
Try to make your experiment a fair one by having the same amount of salt or sugar in each teaspoonful. You would be able to do the experiment more accurately in the school laboratory where you are able to use an accurate balance. Weighing the substance on a kitchen scales won’t help because they are not accurate enough.
What else affects solubility?
- that many solutes are more soluble at higher temperatures
- to use tables of data to calculate quantities of material to use
- to make comparisons, identify patterns and make predictions from graphs
You should be able to:
- identify key points about changes involving making and separating solutions,
- explain changes and techniques.