6 min readConcept sheet1-mark MCQ, and 2 or 3-mark questions on how ozone forms, why it matters and why it is thinning

The ozone layer and its depletion

Ozone (O₃) high in the atmosphere shields the Earth from ultraviolet rays. Man-made chemicals called CFCs have thinned this protective layer.

OzoneO₃, a molecule of three oxygen atoms

FormationUV splits O₂ into O atoms; then O + O₂ → O₃

RoleAbsorbs harmful ultraviolet (UV) radiation from the sun

Main cause of depletionChlorofluorocarbons (CFCs), used as refrigerants and in fire extinguishers

How ozone is formed

This happens high up in the atmosphere.

1
O₂ → O + O
Ultraviolet (UV) radiation splits an oxygen molecule into two free oxygen atoms.
2
O + O₂ → O₃
A free oxygen atom joins an oxygen molecule to form ozone.

Why the ozone layer matters

Ozone (O₃) is a molecule made of three oxygen atoms; the oxygen we breathe (O₂) has two. At the higher levels of the atmosphere, ozone forms a layer that absorbs most of the ultraviolet (UV) radiation from the sun. UV rays are harmful: they can cause skin cancer in human beings and damage other living things. So the ozone layer works as a protective shield for life on Earth.

Ozone depletion and what was done

In the 1980s, the amount of ozone in the atmosphere began to fall sharply. The main cause is a group of man-made chemicals called chlorofluorocarbons (CFCs). CFCs were used as refrigerants in fridges and air conditioners and in fire extinguishers. In 1987, the United Nations Environment Programme (UNEP) got countries to agree to freeze CFC production at 1986 levels. This agreement is known as the Montreal Protocol. Companies must now make CFC-free refrigerators.

Ozone high up vs ozone near the ground

High in the atmosphereUseful. It forms a layer that absorbs harmful UV rays and protects life on Earth.

Near the groundHarmful. Ozone is a poisonous gas that can damage living things.

In real life

When a family buys a fridge or an AC, the CFC-free label is part of protecting the ozone layer. A cap and shade on a hot afternoon also protect your skin from strong UV rays.

Exam angle

2 marks: “How is ozone formed in the upper atmosphere? Write the equations.” UV radiation splits oxygen: O₂ → O + O. A free oxygen atom joins an oxygen molecule: O + O₂ → O₃.

3 marks: “Explain the importance of the ozone layer. Name the chemicals mainly responsible for its depletion and the step taken in 1987 to control them.” It absorbs harmful UV radiation, which causes skin cancer. CFCs, used as refrigerants and in fire extinguishers, deplete it. In 1987 UNEP got countries to freeze CFC production at 1986 levels.

1 marks: “Name the chemicals mainly responsible for the depletion of the ozone layer.” Chlorofluorocarbons (CFCs).

Remember it

O₃ is the Earth's sunscreen, and CFCs scrape it away.