When two substances are put together in a container, one of three processes will probably occur:

• They will either stay separated and not interact with each other,
  
• They may mix together, or,
  
• They may react with each other and form a new substance or substances.

The science of chemistry is all about investigating and predicting the possible changes that can occur when substances are placed together. Apart from nothing happening, the second option of a mixture forming is an example of a physical change, while reacting together to form new substances is a chemical change.

Physical change - mixing
Physical change occurs when substances mix and either form a solution, a colloidal suspension or a mixture. The only difference between each is the size of the particles in the mixture.

• Solutions like salt water are mixtures on the atomic or molecular scale.
  
• Colloids like milk have larger particles capable of scattering light, mixed with smaller molecules.
  
• Mixtures like muddy water tend to have huge or even visible bits mixed together.
  
• The major characteristic of all physical changes is that they are usually relatively easy to reverse. That is, you should be able to unmix the mixtures easily.

Chemical change - reacting
Chemical change occurs when substances combine on an atomic or molecular scale to form new substances. Chemical changes are marked by the formation of a new substance or substances with different properties from the original substances.

The old molecules are broken apart and new arrangements of atoms and molecules created, thus chemical changes are not usually easy to reverse, unlike physical changes.

In this picture of a burning log, molecules in the wood and air are broken down to their atoms and recombined to form new gases, water and ash. These are virtually impossible to put back together again.

Chemical change can be categorised according to the substances involved and the new substances formed. These reaction types are discussed in the topic "Chemical reactions".

Undoing physical changes
Although sometimes it might not look like it, in mixtures, the original substances still exist; for example, in salt water the original salt and water are still present.

Chemists have developed a number of simple ways of separating mixtures to obtain the original substances. These physical separation techniques are based on differences in physical properties, like size of the particles, density, melting and boiling points, solubility in a solvent, and weight.

Some common techniques are:

• Filtration - separation by size
  
• Flotation - separation by density
  
• Recrystallisation - separation by evaporation
  
• Distillation - separation by boiling
  
• Chromatography - separation by absorption
  
• Centrifuge - separation by density