All changes that you see can be broadly divided into physical and chemical changes.
Some changes affect only the physical properties of the substance that is undergoing change. Physical properties include shape, size, colour and state of matter.
- Some properties of physical changes are:
A physical change is temporary.
No new substance is formed when a physical change takes place.
Forms of energy, such as heat, light and electricity, are neither emitted nor absorbed in a physical change.
A physical change is generally reversible.
Some changes are in the form of chemical reactions, which lead to the formation of new substances. These are called chemical changes. These changes may be accompanied by the emission of heat, light or sound, a change in colour, or the formation of a gas.
- Some properties of chemical changes are:
A chemical change is permanent.
A new substance is formed.
Forms of energy, such as heat, light or electricity, may be emitted or absorbed during a chemical reaction.
A chemical change is generally irreversible.
Crystallisation is a physical change, since the process involves a change in the state of matter. No new substance is created in the process of crystallisation.
Water and oxygen in the air react with iron to create a new substance called rust. The properties of rust are different than that of iron. This is a chemical change.
The rusting of iron can be prevented by coating iron with something else, such as paint, or with zinc through a process called galvanisation. The process of galvanisation is also used in many other places, such as to prevent the rusting of iron pipes that carry water to our homes.
Crushing of a chalk piece
Boiling of water
Melting of ice-cream
Evaporation of water
Digestion of food
Formation of calcium carbonate
Rusting of iron
Burning of magnesium ribbon