WHAT ARE CHEMICAL CHANGES?

Chemical changes occur when one or more substances combine to form a new substance. A chemical change is non-reversible.

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The process in which two or more substances combine chemically is called a chemical reaction.
Some chemical reactions produce thermal energy; these are called exothermic reactions.


Other reactions absorb thermal energy; these are called endothermic reactions.


Combustion, or burning, is the name for a chemical reaction in which oxygen combines with a fuel, such as wood, coal or natural gas. It is an exothermic reaction.


We use combustion in various ways:

• Petrol and diesel are burnt to power vehicles and to heat homes.
• Coal is burnt in power stations to generate electricity.
• We burn natural gas to cook and heat our homes.

Combustion causes problems:

• Most fuels produce carbon dioxide during combustion. Carbon dioxide contributes to global warming.
• Burning coal, petrol or diesel produce particles that pollute the air.
• Burning coal and diesel can produce an acid called sulphur dioxide. If sulphur dioxide mixes with rainwater, it produces acid rain.