Compounds, and how they differ from mixtures

Joining elements

Hydrogen is a gas that burns. Oxygen is a gas that helps things burn. When they combine, they make water, a liquid that puts out fires. The properties of water are not like those of hydrogen or of oxygen.

A compound is a pure substance made of two or more elements, joined chemically in a fixed ratio.

• Water: hydrogen and oxygen.
• Carbon dioxide: carbon and oxygen.
• Common salt (sodium chloride): sodium and chlorine.
• Sugar: carbon, hydrogen and oxygen.

The fixed ratio

In water, the mass of hydrogen to the mass of oxygen is always $1 : 8$. This is true for rain water, for sea water that has been purified, for water from a tap, and for the ice in a freezer. Any sample of pure water has $11.1\ \%$ hydrogen and $88.9\ \%$ oxygen, by mass.

Another compound of the same two elements, hydrogen peroxide, has a different ratio, $1 : 16$, and it is a different substance with different properties.

A compound is a new substance

• Its properties are different from those of its elements. Sodium is a soft metal that reacts with water, and chlorine is a poisonous gas. Together they make salt, which we eat.
• A compound can be split into its elements only by a chemical change. For example, passing an electric current through water splits it into hydrogen and oxygen.
• It has a fixed melting point and boiling point.

Compounds and mixtures

Compare a mixture of iron filings and sulphur with the compound iron sulphide.

You should find that $21\ \text{g}$ of iron uses exactly $12\ \text{g}$ of sulphur, and any extra of either is left over unchanged.

Practise with new numbers

Try it in Python

This program finds the percentages of hydrogen and oxygen in water, and the masses in different samples.

Remember these

• A compound has two or more elements that are chemically joined in a fixed ratio by mass.
• Its properties are different from those of its elements, and only a chemical change splits it.
• A mixture has any ratio, and its parts keep their properties. They can be separated by physical means.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. How much hydrogen and how much oxygen are there in $45\ \text{g}$ of water? Use the ratio $1 : 8$.

2. Is sea water a compound or a mixture? Give a reason.

3. Give two ways in which iron sulphide differs from a mixture of iron and sulphur.

4. Hydrogen burns and oxygen helps burning. Why does water put out fires?