How do human activities affect the atmosphere?

A reaction in which oxygen combines with another substance is called an oxidation reaction.

Fuels react rapidly with oxygen, transferring energy to the surroundings by heating and by light. This type of oxidation reaction is known as burning or combustion. The products of combustion then enter the atmosphere.

Hydrocarbon fuels

Hydrocarbons are compounds that contain hydrogen and carbon atoms only.

Hydrocarbon fuels can undergo complete combustion or incomplete combustion, depending on the amount of oxygen available.

Complete combustion

Complete combustion of a hydrocarbon fuel happens when there is a good supply of air or oxygen. Carbon and hydrogen atoms in the fuel react with oxygen:

  • carbon dioxide and water are produced
  • energy is given out

In general:

hydrocarbon + oxygen → carbon dioxide + water


Write a balanced chemical equation for the complete combustion of propane (C3H8) with oxygen (O2). Explain why the equation is balanced.

C3H8 + 5O2 → 3CO2 + 4H2O

Reactants: 3 × C, 8 × H, (5 × 2 × O) = 10 × O

Products: 3 × C, (4 × 2 × H) = 8 × H, (3 × 2 × O) + (4 × O) = 10 × O

Incomplete combustion

Incomplete combustion happens when the supply of air or oxygen is poor. Water is still produced, but carbon monoxide and carbon are the other products. Less energy is released than during complete combustion.

Here is one possible equation for the incomplete combustion of propane:

propane + oxygen → carbon monoxide + carbon + water

C3H8 + 3O2 → 2CO + C + 4H2O

Notice that fewer oxygen molecules are needed to balance the equation than are needed for the complete combustion of propane.


The carbon is released as fine black particles, which we can see in smoky flames, and is deposited as soot. Soot can cause breathing problems and it blackens buildings. It may block boilers and other appliances, or cause a fire.

Factory chimneys producing smoke
Incomplete combustion produces smoke and invisible carbon monoxide

Carbon monoxide

Carbon monoxide is a toxic gas. It is absorbed in the lungs and binds with the haemoglobin in the red blood cells. This reduces the capacity of the blood to carry oxygen. Carbon monoxide causes drowsiness, and affected people may fall unconscious or even die.