Phases of Matter
There are three phases of matter that you experience every day. They are solid, liquid and gas. Each phase has specific properties.
Solids have fixed volume and shape. This means that if you drop an ice cube on the floor it does not get any larger and it keeps the cube shape.
Liquids have a fixed volume, but not a fixed shape. If you fill a glass with water, it takes the shape of the glass. If you spill the water on the floor, the water does not take up any additional space, but it does try to spread out to match the shape of the floor.
Gasses don't have a fixed shape or volume. If you boil a pot of water on the stove, the water vapor expands to fill the kitchen and then the house, as the gas molecules want to be as far apart as they can. While the vapor is expanding, it takes on the shape of your house.
Phase Changes
When matter moves from one phase to another, that is called a phase change. The easiest way to change matter from one phase to another is to add or remove energy. The difference, on an molecular scale, between ice and water, is the amount of energy stored in each water molecule. When you take energy out of water, the molecules slow down and become locked in place with their neighbors. If you take the energy out of water, you get ice. If you take the energy out of water vapor, a gas, you get snow.
Melting
Melting is the name for the phase change from solid to liquid. Melting is done by adding energy, usually in the form of heat, to a solid. The energy can come from the sun to melt snow or the ice in your drink, or the energy can come from a furnace to melt metals.
Sublimation
Sublimation is when matter goes from a solid to a gaseous state without being liquid in between the two phases. The most familiar example is dry ice. Dry ice is solid carbon dioxide, CO2. CO2 cannot exist as a liquid in the atmosphere of the earth, so dry ice can't melt. Under the right conditions snow and ice can sublimate directly into water vapor without being a liquid.