What are materials made of?
            3. Separating material mixtures
last page  page 15 Next page
Salt and water
Salt is a useful compound. For thousands of years, it has been used for preserving foods and for flavouring them. Luckily, we have a ready supply of salt - the sea.
Distillation animation
Picture 3.1 Distillation is used to recover a solute from a solution.

Getting salt from sea water
Sea water is salty because it has dissolved salty solids. These have come from active volcanoes on the floors of the oceans.

We can recover the salt using evaporation. In some hot countries, they build a low wall round a shallow section of the sea. This traps the seawater in an enclosed lagoon. The water evaporates and leaves the salt behind.

Evaporation is useful for recovering the solute from a solution. However, the solvent is lost. Let's see how we can catch it and save it.

Getting pure water from sea water
In some hot countries there is lots of sea water but little fresh water for people to drink. The sea water is purified in a de-salination process.

The sea water is heated until it boils. The water changes to steam but the salt remains behind. The steam is collected and condensed back into water. The pure water is then directed away in pipes to a collecting container.

We call this process distillation. In distillation, the solvent is boiled, captured, condensed and collected. It allows us to separate a solvent from a solute without losing either of them.

Getting salt from rock salt
In the U.K., we get salt from rock salt rather than from the sea. Rock salt is a mixture of salt and earthy impurities. It was left behind millions of years ago when enormous shallow seas evaporated. To use the salt, we need to separate it from the earthy impurities. Here are the steps to do this:
Rock salt sequence
Picture 3.2 Separating salt and sand in rock salt.

Step 1. Rock salt is crushed into little pieces and added to water. The salt dissolves in the water but the earthy impurities are insoluble.

Step 2. The solid sandy bits are separated from the salt solution using filtration. The mixture is passed through some filter paper held in a funnel. The salt solution can pass through the small holes in the filter paper because all the particles are tiny. The earthy impurities can’t pass through the holes, because the particles are too big.

Step 3. The salt solution is changed to salt by evaporating the water. The solution is heated so that the water evaporates, leaving the salt behind.

Filtration is often used to separate the solid and liquid parts of a suspension. The liquid part is called the filtrate. The solid that remains on the filter paper is called the residue.

top of page top of page last page Next page
Question 14
Look at the descriptions of processes below. In each case, type the word that best describes it into the box. You can choose from the list at the bottom. Make sure you spell the word correctly.
Pure steam is turned back into pure water.

Solids are removed from a suspension of sand and rock in salt solution.

Fresh water is separated from salty water by boiling it, collecting the steam and letting it turn back into water.

Salt is recovered from salt solution by heating the water so it disappears.

Solid salt is removed from rock salt forming a solution in water.
Choose from these words: