The pure substances composing matter are capable of existing in three states or conditions:
and . Under normal conditions of temperature and pressure water is a liquid (exists in the liquid state). If water is suffi-ciently cooled, it becomes ice, the solid state. If water is suffi-ciently heated, it boils and becomes water vapor, the gaseous state. In each of these states, however, water is fundamentally the same substance having a definite chemical composition.
According to the kineticΓÇômolecular theory of matter, a pure substance is composed of small par-ticles, called molecules. A is the smallest quantity of a substance which has the properties of the mass (a recognizable quantity of the material). The molecules of the substance are in a continuous, never-ending motion. The three states of a sub-stance and their recognizable characteristics may be explained on the basis of this theory.