What happens to the particles in a substance when heat is added?
The atoms in the substance/element start vibrating faster. Kinetic energy increases.
Solids are tightly compacted and have more energy holding them together. For example, a rock has many more atoms/molecules holding it together then styrofoam. They have a definite shape.
Liquids are less compacted than solids. They conform to their containers. For example, water does not have a definite shape and fits to its container. They don't have a definite shape.
Gases are all over the place. They have much more space in between each other and have no definite shape whatsoever. Gases are usually found in the air.
Ice can go through all three of these phases. First from a solid, then to a liquid (in the form of water) and then to a gas (by evaporation).
When particles are heated up, space is being created. The atoms started to get "overly excited" and started to move faster than they usually do. When this happens, energy is released in the form of heat, light or etc. Because of this, kinetic energy increases and atoms colliding with each other happens more often.