Commonly, a given kind of matter exists in different physical forms under different conditions. Water, for example, exists as ice (solid water), as liquid water, and as steam.

The main identifying characteristic of solids is their rigidity: they tend to maintain their shapes when subjected to outside forces. Liquids and gases, however, are fluids; that is, they flow easily and change their shapes in response to slight outside forces.

These two characteristics, rigidity (or fluidity) and compressibility (or expansibility), can be used to frame definitions of the three common states of matter:

Solid is the form of matter characterized by rigidity; a solid is relatively incompressible and has fixed shape and volume.

Liquid is the form of matter that is a relatively incompressible fluid; a liquid has a fixed volume but no fixed shape.

Gas is the form of matter that is an easily compressible fluid; a given quantity of gas will fit into a container of almost any size and shape. The term vapor is often used to refer to the gaseous state of any kind of matter that normally exists as a liquid or a solid. These three forms of matter—solid, liquid, gas—comprise the common states of matter. illustrates the three physical states of matter.

Changing state of matter

You would have observed changing states of matter when ice cubes melt from solid into liquid water or when water boils into vapor, but have you wondered why substances change form? Changing states of matter occur when matter loses or absorbs energy.

Changes Between Liquids and Solids

How would you make ice cubes in a tray? First, you would fill the tray with water from a tap. Then you would place the tray in the freezer compartment of a refrigerator. The freezer is very cold. What happens next?

Freezing

Heat transfer occurs between the warmer tray and the colder air in the freezer. The warm water loses heat to the cold air in the freezer. This heat transfer occurs until no energy is available for the particles to slide past each other. This forces them to remain in fixed positions, locked in place by the force of attraction between them.

Melting

If you took out the ice cubes from the freezer and placed them in a warm room, the ice would absorb energy from the warmer air around them. This absorbed energy would facilitate them to overcome the force of attraction holding them together, enabling them to slip out of the fixed position that they held as ice.

Changes Between Liquids and Gases

If you fill a pot with cold tap water and heat it on a hot stovetop, the water heats up. Heat energy travels from the stovetop to the pot, and the water absorbs the energy from the pot. What happens to the water next?

Vaporization

If the water is hot enough, it starts to boil. Bubbles of water vapor are formed in the boiling water.

Condensation

When you take a hot shower in a closed bathroom, the mirror is likely to fog up. You may wonder why does this happen? Some hot water from the shower evaporates and when it comes in contact with cooler surfaces such as the mirror, it cools and loses energy.

Changes Between Solids and Gases

Solids that change to gas passes through the liquid state first. However, sometimes solids change directly to gases and skip the liquid state.

Sublimation

The process in which solids directly change to gases is known as sublimation. This occurs when solids absorb enough energy to completely overcome the forces of attraction between them.