When water changes its state, its molecules are still made of hydrogen and oxygen — the chemical itself has not changed. When elements combine to make a compound, new molecules are formed. This is a ‘chemical change’.
We see examples of compounds forming every day. Most metals combine with chemicals in the air. Copper roofs slowly turn green as the copper combines with water and oxygen to form a new compound. An iron nail left outside soon starts to turn brown. The iron has reacted with water and oxygen in the air to form ‘rust’. Cars are made of several metals including iron. They have to be coated in paint to try to prevent them from rusting.
Baking a cake
When you bake a sponge cake, one of the ingredients used is baking powder. As it is heated in the oven, the baking powder breaks down into different chemicals. One of these is a gas — carbon dioxide. The bubbles of carbon dioxide throughout the sponge make it light and fluffy. The baked sponge is now a new mixture of many different compounds.
Copper roofs slowly turn green as the copper combines with oxygen to form a new compound.
Iron bridges need to be coated with chemicals to prevent them from rusting.
Picture Credit : Google