GoalTell mixtures from pure substances, and homogeneous from heterogeneous mixtures, and choose a way to separate a mixture.
New words
mixture · two or more substances mixed but not chemically joinedsolution · a homogeneous mixture, such as salt dissolved in waterpure substance · matter made of one substance only: an element or a compound
Explanation
A mixture has two or more substances that are not chemically joined, so we can separate them by physical means: filtering, using a magnet, or evaporating the water and collecting what is left. If a mixture looks the same all through, like salt dissolved in water, it is homogeneous, and we call it a solution. If its parts can be seen, like sand in water, it is heterogeneous. Pure substances are of two kinds: elements and compounds. In science, “pure” means made of only one substance, not “clean”. Tap water and air are mixtures, even when they look clean. A mixture keeps the properties of the substances in it, and the amounts can change from one sample to another.
Mixture
Kind
How to separate
salt in water
homogeneous
evaporation
sand in water
heterogeneous
filtering
iron nails in sand
heterogeneous
a magnet
Worked examples
Two jars: salt in water and sand in water. The salt disappears into the water and the liquid looks the same everywhere, so it is a solution. The sand stays visible, so that mixture is heterogeneous.
To get the salt back from salt water, an adult can leave a saucer of it on a sunny windowsill. The water slowly evaporates and the salt stays in the saucer. No heating is needed.
Is clean tap water a pure substance? No: it has other substances dissolved in it. Orange juice, soil and air are mixtures too. Distilled water and aluminium foil are close to pure.
Class activity
Jar test, with an adult. Fill two jars with tap water. Stir a spoon of salt into one and a spoon of sand into the other. Look at both. Pour the sandy water through a paper towel in a funnel or sieve: what stays behind? Put some salt water in a saucer on a sunny windowsill and look again after a few days. Then use a magnet to pull a few iron nails out of dry sand.
Practice
1
What is a mixture?
Two or more substances mixed but not chemically joined.
2
Is salt dissolved in water homogeneous or heterogeneous?
Homogeneous: it looks the same everywhere, and it is a solution.
3
Which method separates iron nails from sand?
A magnet.
4
Is air a pure substance? Explain.
No. Air is a mixture of gases such as nitrogen and oxygen.
