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L.Gardiner/Windows to the Universe
Any substance, called matter, can exist as a solid material, liquid, or gas. These three different forms are called states. Matter can change its state when heated.
As a solid, matter has a fixed volume and shape and is usually unable to flow, except in the case of glaciers. For instance, an ice cube or snowflake is the solid state of water.
When the solid state of matter is heated, it turns into a liquid. As a liquid, a substance has a fixed volume, but its shape changes to fill the shape of its container. For instance, a glass of water is the liquid state of water.
When the liquid state of matter is heated, it turns into a gas. As a gas, a substance does not have a fixed volume or shape. Gas expands to fill the shape and volume of its container. For instance, the steam that comes out of a hot teakettle, making the whistle blow, is water as a gas.
Heat causes matter to change its state because, when heated, the molecules within the substance to move around faster. The faster the molecules bounce about, the weaker they are held together.
source : windows to the universe