According to particle theory, all matter is composed of tiny particles called protons, neutrons and electrons. These tiny particles make up the atom. Atoms, ions and molecules are also particles. In a gas these particles have a large amount of space between each other. In liquids the particles are closer together. In solids they are even closer together.
The natural movement within gases and liquids that causes odors and colors to spread is called diffusion. The odor of bread cooking spreads throughout a room. The color in food coloring spreads throughout a glass of water. This occurs without any apparent movement, and this spreading does not require heat. It occurs because the particles in gases and liquids are always moving. The motion of these particles is random, which means without a pattern. This motion of particles in gases and liquids causes diffusion. Diffusion occurs faster in gases than in liquids. This is true because the particles in a gas are further apart. Because there is more space in between the particles of a gas other substances can diffuse into a gas very easily. Increasing the temperature increases the speed of diffusion. Increasing temperature increases the speed of the particles and causes them to expand and become further apart. Both of these factors increase the speed of diffusion. This is why things dissolve better in hot liquids than in cold liquids. Something that is moving fast has more kinetic energy than something that is moving slow. Heat and temperature are related to kinetic energy, because heat causes particles to move faster.
Gases such as oxygen, carbon dioxide and air are made up of molecules. The molecules of such gases are very far apart, so that 99.9% of air is actually empty space.
Solids are made up of molecules or ions, if they are compounds, or atoms if they are elements. So the particles of a solid can be molecules, ions or atoms. The particles of a solid are not free to move in the same way as the particles of a gas or a liquid. They are held together by forces that keep them close to each other. However, even the particles of a solid exhibit constant movement. They vibrate back or forth as if they were attached to little springs. When heat is added to a solid the particles speed up and exhibit bigger vibrations.
The only time that the molecules, ions or atoms do not move at all is when the temperature is absolute zero. Absolute zero is
-273 degrees Celsius. Absolute zero is the coldest possible temperature. Absolute zero represents the absence of all heat. At absolute zero there is no kinetic energy and all particle movement stops.
Liquids can be thought of as a cross between a solid and a gas. Like a solid, the particles of a liquid remain close together. Like a gas, the particles can move freely. If heat is added the particles of a liquid move faster. If enough heat is added liquids can change in phase and become gases. If heat is removed the particles of a liquid slow down. If enough heat is removed the liquid can change in phase and become a solid. This transformation can be observed in water as water becomes water vapor (a gas) or ice (a solid).
Heat is conducted through a substance because of a chain
reaction of particle collisions. If heat is added to one end of a
metal wire, the particles at that end will begin to expand and move
faster. These expanding and fast moving particles will collide
with neighbor particles and cause them to move faster. It is by
this chain reaction of particle collision that heat is conducted in
a metal wire.