A substance is made of very tiny particles. There are three types of particles: atoms, molecules and ions.

In solids, the attraction forces between particles are strong, are regularly and closely packed and can only vibrate slowly in fixed positions. In liquids, the attraction forces between particles are still strong but weaker than those in a solid, are irregularly and closely packed and are free to move about. In gases, the attraction force is very weak, are far apart and irregular packed, and move freely and quickly. Particles have kinetic energy and are in constant and random motion. Particles with higher speed of movement have more kinetic energy. Particles have potential energy which becomes higher when their attraction forces are weakened. Gas particles possesses the greatest amount of energy and solid particles possess the least amount of energy. During melting/boiling, particles gain energy for overcoming the attraction forces between them so as to become more widely spread. During condensation/freezing, particles lose energy when the attraction forces between them become stronger, bringing them closer together. Boiling occurs at a definite temperature called the boiling point and all the particles in the liquid may escape. Sublimation is when substances change directly from a solid to gas without going through the liquid state. Sublimation occurs when particles of a solid vibrate fast enough to overcome the attraction forces between them and become widely separated and able to move freely and quickly.

The Brownian movement refers to the random motion of particles (zig-zag path) in a gas or liquid, caused by the collisions with numerous rapidly moving particles in all directions.

Diffusion is the spreading out of particles in a fluid from a region with higher concentration to a region of lower concentration. Dissolving of potassium permanganate occurs, forming an aqueous solution KMnO4. The particles (potassium ions and permanganate ions) in potassium permanganate leave the crystal, mix with the water particles and diffuse. Lighter particles diffuse faster than heavier ones. Diffusion is faster at higher temperatures. Diffusion is slower in liquids and gases because liquid particles move much slower than gas particles and because liquid particles are much closer so there is less space for them to move without colliding with another one.

Solvent is a liquid that dissolves a solute. Solute is a substance that is dissolved in a solvent. Solution is the liquid formed from dissolving a solute in the solvent. A solid solute can be transferred into a beaker of solvent by using a spatula. By stirring with a glass rod, the resulting mixture can become a uniform solution. During dissolution, the attraction forces between the solute particles are being weakened, and the solute particles mix with the solvent particles as new attraction forces being formed between them. The solubility of a solute in a solvent depends on whether the new attraction forces are strong enough to overcome the old ones. A concentrated solution has more dissolved solute per unit space than a dilute one. A saturated solution is a solution containing the maximum amount of dissolved solute at a given temperature. The solubility of a solid in a solvent at a given temperature.

Most solids have higher solubility at higher temperatures so their mobility curves have positive slopes. All gases have lower mobility at higher temperatures.

When a hot saturated solution is cooled down, less solid can be dissolved.