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2. An endothermic change transfers energy from the surroundings.
6. A fuel made from the remains of animals and plants that died millions of years ago. Fossil fuels include coal, oil, and natural gas.
10. An exothermic change transfers energy to the surroundings.
11. In a balanced formula equation, chemical formulae represent the reactants and products. The equation shows how atoms are rearranged, and gives the relative amounts of reactants and products.
12. A starting substance in a chemical reaction.
15. A ratio is a way of comparing values.
16. A variable that can only have whole-number values.
17. In a chemical reaction, the total mass of reactants is equal to the total mass of products. This is conservation of mass. Mass is conserved in chemical reactions and in physical changes.

1. A change that is reversible, in which new substances are not made. Examples of physical changes include changes of state, and dissolving.
3. Some fuels are non-renewable. They form over millions of years, and will one day run out.
4. A change in which atoms are rearranged to create new substances.
5. A catalyst speeds up reactions without being used up.
7. A chemical reaction in which substances react with oxygen to form oxides.
8. A chemical reaction in which a compound breaks down to form simpler compounds and/or elements.
9. A substance that is made in a chemical reaction.
13. A chemical reaction in which a substance reacts quickly with oxygen and gives out light and heat.
14. A material that burns to transfer useful energy.