All changes that occur in matter are either physical or chemical. Physical changes occur when a change in state occurs, like when ice melts. Physical changes are often easy to reverse; cooling down the liquid water to below its freezing point refreezes it.

Chemical changes, on the other hand, form new substances from old and are usually difficult to reverse. Have you ever tried unburning the toast?! This topic and related topics will investigate some common chemical changes and what happens during a chemical change, or reaction.

Biochemical reactions
Chemical reactions are occurring all around us as well as within us all the time. Living is a continuous series of chemical reactions. Too much acid and you have an upset stomach. Eat breakfast, breathe the air, take a headache tablet - all these involve chemical reactions. In fact, the science of biochemistry deals only with the chemistry of life.

Corrosion
The combination of a metal with oxygen or other chemical to produce a metal compound is called corrosion. Cars rusting, aluminium window frames becoming crusty, copper and silver going black, are all forms of corrosion. The most common form of corrosion involves the combination metal with oxygen to form a metal oxide. Rust is iron oxide while the white powder on aluminium is aluminium oxide.

Combustion
Cooking with gas, a bush fire and burning petrol in a car are examples of combustion. Combustion is like corrosion in that it involves combining a substance with oxygen, but this time very quickly to produce heat and light as a flame.

Acid/base reactions
Acid/base reactions involve water and occur everywhere that water is found. Taking bicarbonate for an upset stomach, digesting food, cleaning copper with acid and adjusting the pH in garden soil all involve acid/base reactions.

Reduction
Reduction is the opposite of corrosion or rusting. Refining iron ore and copper ore and extracting aluminium from bauxite are processes involving the removal of oxygen from a compound of the metal.

Chemical reactions crossword puzzle
When you have investigated these everyday examples of chemical reactions, come back and try the online crossword puzzle - Chemical reaction.