Final answer:
Physical changes involve changes in size, shape, form, or state of matter without altering chemical composition. The identity of the substance remains the same, as do its mass, the number of atoms, and the number of molecules.
Step-by-step explanation:
A physical change involves a change in size, shape, form, or state of matter without a change in chemical composition. When we discuss physical changes, we are talking about changes that can be seen or felt, but that do not involve the break up of the particles in the reaction. During a physical change, the form of matter may change, but its identity remains the same.
The arrangement of particles may change in a physical change, but crucially, the mass, the number of atoms, and the number of molecules will stay the same. Examples of physical changes include the melting of ice to liquid water, the evaporation of water to water vapor, and the dissolution of salt in water.
In each of these cases, the substance retains its chemical properties even though its physical form has changed.