Elements and Compounds: Quick Revision

A brief note on Elements and Compounds. Read it before a test. Try each question at the end before you open its answer.

The big ideas

• A physical change makes no new substance. A chemical change makes new substances; gas, colour, smell, heat, light or a new solid are its clues.
• An element has one kind of atom and cannot be broken down by a chemical change. Most elements are metals; the rest are non-metals and metalloids.
• A compound is two or more elements joined chemically in a fixed ratio by mass, with new properties.
• A mixture can have any ratio, and its parts keep their properties.

Compound or mixture at a glance

A worked example

How much hydrogen and oxygen are in $81\ \text{g}$ of water? Then $63\ \text{g}$ of iron is heated with $40\ \text{g}$ of sulphur; they join in the ratio $7 : 4$. How much iron sulphide forms?

1. Water. One part is $\dfrac{81\ \text{g}}{9} = 9\ \text{g}$: hydrogen $9\ \text{g}$, oxygen $8 \times 9\ \text{g} = 72\ \text{g}$.
2. Sulphur needed. $63\ \text{g} \times \dfrac{4}{7} = 36\ \text{g}$, and there is $40\ \text{g}$, so the iron runs out first.
3. Iron sulphide. $63\ \text{g} + 36\ \text{g} = 99\ \text{g}$, with $4\ \text{g}$ of sulphur left over.

Practice with new numbers

A program to try

The program works out the worked example.

Mistakes to avoid

• Calling every change of appearance chemical. Melting and dissolving are physical.
• Writing $\text{CO}$ for cobalt. Cobalt is $\text{Co}$; $\text{CO}$ is carbon monoxide.
• Dividing by $8$ for water. The ratio $1 : 8$ has $9$ parts.
• Calling an alloy a compound. Brass and steel are mixtures.

Check yourself

1. Is curdling of milk a physical or a chemical change?

2. Name a metal that is a liquid and a non-metal that conducts electricity.

3. How much oxygen is there in $36\ \text{g}$ of water?

4. Why can a magnet separate iron from a mixture of iron and sulphur, but not from iron sulphide?