Lessons 3 to 5 asked what kind of change it is. This one asks a different question: what caused it to happen at all?
A chemical change can involve elements, compounds and mixtures, and it results in the formation of one or more new substances. Your textbook groups the causes into five, and exam questions often ask you to name which one is at work.
When two or more reactants are mixed together, they can combine irreversibly to form one or more products. All three reactions of acids from the last lesson — with alkalis, metals and carbonates — happen this way.
Heat, or an increase in temperature, is needed for many chemical reactions to occur. Both combustion (burning coal) and thermal decomposition (heating calcium carbonate) belong here.
Photosynthesis is a chemical reaction that occurs in green plants. Chlorophyll in their leaves absorbs light energy, and carbon dioxide and water are turned into glucose and oxygen.
carbon dioxide + water --light--> glucose + oxygen
The same idea explains something much more ordinary: colours in clothes fade over time when they are exposed to sunlight, because ultraviolet rays drive a chemical reaction in the dye. It is also why old photographs and book covers look faded.
Rusting and cellular respiration are examples of oxidation, which you met in Lesson 3. Objects made of copper interact with water and oxygen too — that is why old copper coins turn green, and why the red and brown colours of tropical soils are the result of chemical reactions with oxygen.
An electric current can break a compound down into simpler substances, or drive metal out of a solution and onto an object. That is electrolysis and electroplating — the whole of the next lesson.
A well-known example: all the medals for the Tokyo 2020 Olympic Games were made from metals extracted from recycled electronic products. The gold medal is actually a silver core that has been electroplated with gold.