Pure Substances and Mixtures
- Chemistry
- Ages 11–14 · Grades 6–8
- Hands-on experiment
What you’ll find out
- Elements contain one kind of atom; compounds contain elements joined in a fixed ratio.
- In a mixture the parts are not chemically joined and keep their own properties.
- Mixtures can be separated using the different properties of their parts.
No sign-up and no real names. In the lesson you check your understanding as you read, run the experiment yourself, and practise until it sticks.
Elements, compounds and mixtures
Everything you can hold is a pure substance or a mixture. An element is a pure substance made of one kind of atom, like iron, copper or oxygen; there are 118 known elements. A compound is two or more elements chemically joined in a fixed ratio: water is always two hydrogen atoms to one oxygen, and table salt is always sodium and chlorine one to one. A compound usually behaves nothing like its elements. Sodium is a soft metal that reacts violently with water and chlorine is a poisonous gas, yet together they make the salt on your chips. A mixture is two or more substances mingled but not joined, in any amounts, each keeping its own properties. Sand and salt, air and seawater are mixtures, which is why they can be separated.
Separating by what each part does
Suppose you are handed a jar of sand, salt and iron filings, all stirred together. No single trick will separate it, but each part has a property the others lack. First, run a magnet over it: the iron filings are magnetic and jump to it, while the sand and salt stay behind. Next, add water and stir. The salt dissolves but the sand does not, so pouring the mixture through filter paper catches the sand while the salty water passes through. Finally, warm the salty water until the water evaporates, leaving salt crystals. Magnetism, dissolving and evaporating each did one job. Industry works the same way: magnets pull iron from recycling, and salt is harvested by evaporating seawater.
Ink is a mixture too
Some mixtures look like a single substance. Black felt-tip ink seems to be one colour, but it is usually a blend of several dyes. Chromatography separates them. Put a dot of ink near the bottom of a strip of filter paper and stand the strip in a little water, keeping the dot above the waterline. As the water creeps up the paper, it carries the dyes with it, but not at the same speed: some travel far and others lag behind. After a few minutes the black dot has spread into bands of blue, purple and yellow. The pattern is like a fingerprint for that ink. Scientists use similar methods to compare inks on documents or check the colourings in sweets. A pure dye stays as one spot.
Curriculum links: NGSS MS-PS1-1 · NGSS MS-PS1-2