Theory - All substances are made of which are in constant motion. (Kinetic means "movement".)

For most simple pure there are three ; solid, and gas, and they can be converted from one to another by changing energy. (Substances may also be called "stuff" colloquially or "" in some books.)

have a fixed shape and volume. The particles are held together by in a regular ; if this pattern is repeated millions of time, a of the substance can form. Solids are incompressible because the particles lie together. The particles do not have free movement, but can [?] about fixed positions - a bit like line dancing! As increases, the particles gain more kinetic energy, the vibration increases, so the solid slightly - the dancers move a bit faster so they take up a bit more room! If they vibrate (dance fast) enough, the heat energy is sufficiently strong to break the pattern of particles of the , but it is not strong enough to overcome the force of attraction completely; the particles become to move randomly and this is called . At the melting point, energy goes into breaking forces between the particles, and so there is temperature increase until the solid has melted. The at which the solid melts is called the melting . The reverse of this change of state (from to ) is called .

have no fixed shape, but do have a fixed . Its particles are often slightly further apart (though is an important exception) but they are still incompressible because the between particles are very short. Particles in a liquid can exchange places so liquids flow - it's disco dancing - the move randomly. a liquid makes the particles move even faster. Some particles move faster than others and have more energy, so they overcome the forces of attraction and these may . If the temperature is increased further, the kinetic energy of the particles increases to the point when the forces between the particles are completely overcome by the heat energy. At the boiling point goes into breaking the forces between the particles, and so there is no temperature increase until all the liquid has changed into .

Gases have no shape or volume. The particles are very far apart, so are easy to compress - the dancers are so frantic that they cannot stand the smell of each others' sweat! Particles move in a fashion. In all state changes, the size of the particles remains the ; when volume changes, only the average between the particles alters. The average of the particles increases as the temperature of the gas .

A pure substance will have a higher point than an sample of the same substance; the pure substance has a point range of less than 1 C while an impure substance melts over a range of several degrees. Impures substances have a boiling point than pure substances.

is the process by which a gas fills all the space available to it. Gases will diffuse (mix with each other) because the particles are moving randomly and quickly in all directions. Lighter particles diffuse than heavier particles at the same temperature.