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:
Transferring energy to or from a substance can change its state. Heating a substance in the solid state will cause it to melt, which changes it to the liquid state. Continued heating will cause the substance to evaporate or boil, which changes it to the gas state.
Some substances in the solid state can change straight to the gas state. This process is called sublimation.
During evaporation, particles with enough energy can escape the liquid as a gas. Unless energy is transferred from the surroundings by heating, the liquid gradually becomes colder. Boiling happens when enough energy is transferred that the substance evaporates as fast as it can.
The number of particles does not change during a change of state, only 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. For example, ice is water in the solid state:
This is different from the changes seen in a chemical reaction, when the changes cannot be reversed so easily.