Copper carbonate always has 5.3 parts copper to 4 parts oxygen to 1 part carbon. In other words, the ratio of copper to oxygen to carbon is 5.3:4:1. The picture shows that in a 103g sample of copper carbonate, there are 53g of copper, 40g of oxygen, and 10g of carbon. A sample of copper carbonate was found to contain 15.9g of copper. How many grams of copper carbonate were in the sample? Use the ratio of copper to oxygen to carbon.

Atoms are rearranged during chemical reactions, and are not lost or gained. Chemical reactions can be represented using equations. Catalysts speed up reactions without being used up.

Part ofChemistryChemical reactions and tests

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Thermal decomposition

Some compounds break down when heated, forming two or more products from one reactant. This type of reaction is called thermal decomposition.

Many metal carbonates can take part in thermal decomposition reactions. For example, copper carbonate breaks down easily when it is heated:

copper carbonate → copper oxide + carbon dioxide

CuCO3 → CuO + CO2

Copper carbonate is green and copper oxide is black. You can see a colour change from green to black during the reaction. The carbon dioxide produced can be detected using limewater, which turns milky.

Green copper carbonate is heated in a test tube. It changes to black copper oxide and gives off carbon dioxide gas.

Copper carbonate is easily decomposed by heating

Other metal carbonates decompose in the same way. Here are the equations that represent the thermal decomposition of calcium carbonate:

calcium carbonate → calcium oxide + carbon dioxide

CaCO3 → CaO + CO2

Thermal decomposition is an example of an endothermic reaction, a reaction that gains energy from the surroundings. This is why thermal energy must be supplied constantly for the reaction to keep going. Note that some other reactions are exothermic reactions – they give out energy to their surroundings.

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