In everyday life, there are three states of matter – solids, liquids and gases. The differences between the three states are due to the arrangement and spacing of the particles and their motion.
The particles in a solid:
The particles in a liquid:
The particles in a gas:
Adding or removing energy from a material can change the state. Heating a solid material will cause it to melt from a solid to a liquid. Continued heating will cause the liquid to boil or evaporate to form a gas. In some instances, when heating the solid material, it can go straight to being a gas without being a liquid - this process is called sublimation.
Boiling is an active process. It occurs at a certain temperature, called the boiling point. People actively apply energy to a liquid to turn it into a gas using a heater such as a kettle.
Evaporation, on the other, hand is a passive process. The liquid will slowly absorb energy from the surrounding area so that some of its particles will gain enough energy to escape from the surface of the liquid and it can occur at any temperature.
Throughout all of these changes, the number of particles has not changed, just their spacing and arrangement. As a result, the total mass has not changed. It does not matter if a substance melts, freezes, boils, evaporates, condenses or sublimates. The mass does not change.
These changes in state are called physical changes because the process can be reversed (eg cooling instead of heating). This is different to the changes seen in a chemical reaction when the changes cannot be reversed so easily.