Temperature is a major determining factor for the thermodynamic state or phase of a material.
Normally we encounter three states of matter: solid, liquid, and gas. However nature has many other phases of matter.
To understand what causes different thermodynamic states, we need a basic fact of matter. A quote by the famous physicist Richard Feynman says it all:
Question to Feynman: If, in some cataclysm, all of scientific knowledge were to be destroyed, and only one sentence passed on to the next generations of creatures, what statement would contain the most information in the fewest words?
Feynman's answer: The atom is the
fundamental structure of matter: All things are made of atoms - little
particles that move around in perpetual motion, attracting each other when
they are a little distance apart, but repelling upon being squeezed into one
Armed with this deep insight, we can discuss three major states of matter:
Gas: Each molecule moves freely through space except for occasional collisions with another molecule. A gas is therefore an easily compressible fluid.
Solid: The molecules are so close together that repulsion dominates---it is hard to squeeze a solid. The thermal energy is sufficiently low that each atom/molecule only jiggles about its equilibrium point near the bottom of a potential well. Thus a solid maintains a definite shape.
Liquid: A liquid is held together by weak molecular forces that are insufficient for a liquid to maintain its shape. Thus a liquid can flow. The density is high so that a liquid is an incompressible fluid.
As the temperature is varied, a material can undergo a phase change or phase
transition between the different states. A
phase diagram encapsulates the basic properties of a material as the
temperature is changed (see transparency).