Substances join with others to form new substances in what is called a chemical reaction. Reactions can take place naturally. For example, the food we eat is changed into energy through a series of chemical reactions inside our bodies. In industry, we use chemical reactions to create, for example, medicines, along with many other useful substances.

Chemical change

Baking a cake is an example of using chemical reactions to make things. The molecules that make up a cake’s ingredients—flour, eggs, butter and others—are re-arranged using energy (the heat of the oven) during the baking. The cake is chemically different from its ingredients.

Substances can be mixed together to make new materials without a chemical reaction taking place. For example, sand, cement, rubble and water can be mixed together to make concrete. The molecules in these substances do not change.

Examples of reactions

Certain gases in the air react with rainwater to form acids. Over time, acids may dissolve rocks such as limestone. This causes buildings and statues to crumble.

If an iron nail is left out in the rain or damp air, its molecules will react with the water to produce a new kind of molecule: iron oxide, or rust.

Photosynthesis

The way a plant makes its food is an important kind of natural chemical reaction. It takes in carbon dioxide from the air and water from the soil and turns them into carbohydrates. These are food for the plant.

As part of the process, photosynthesis also produces oxygen, which is released into the air. A certain chemical contained in a plant’s leaves, called chlorophyll, uses the energy from sunlight to help the chemical reaction take place.