Atmosphere

Level 1

What is air made of?

The air around us seems empty, but it is actually matter: it is a mixture of several gases. We cannot see it, but we can feel it (the wind) and it occupies all the available space.

  The composition of air:

  • About 78 % nitrogen (N2)
  • About 21 % oxygen (O2), the gas we need to breathe
  • About 1 % other gases: argon, water vapour, carbon dioxide (CO2)...

Air and the greenhouse effect

Certain gases present in the air are called greenhouse gases. They trap part of the Sun's heat near the Earth's surface, much like a blanket. Without them, it would be far too cold to live.

The main greenhouse gases are:

  • Water vapour
  • Carbon dioxide (CO2)
  • Methane

  Human activities (cars, factories, heating...) release a lot of CO2. This excess strengthens the greenhouse effect and contributes to global warming. Protecting the air means protecting the planet.

What is combustion?

A combustion is a chemical transformation during which a fuel (wood, methane, carbon, petrol...) reacts with an oxidiser: oxygen (O2) present in the air. This reaction releases energy in the form of heat, and often light (the flame).

  Example: the combustion of methane (mains gas).

Methane + Oxygen → Carbon dioxide + Water

Note: you learned how to write the equation for this reaction (CH4 + 2 O2 → CO2 + 2 H2O) in the lesson Reactions.

The fire triangle

For combustion to occur, three elements must be present at the same time. This is the fire triangle:

  • a fuel (what burns);
  • an oxidiser (the oxygen in the air);
  • an activation energy (a heat source: spark, flame...).

  If you remove just one of these three elements, combustion stops. This is the principle used to extinguish a fire: deprive it of oxygen (lid, fire blanket) or fuel.

Complete or incomplete combustion

The way combustion takes place depends on the amount of dioxygen available:

  • Complete combustion (a lot of dioxygen): it produces only carbon dioxide (CO2) and water. The flame is blue and bright.
  • Incomplete combustion (lack of dioxygen): it also produces carbon monoxide (CO) and soot particles. The flame is yellow and smoky.

  Danger: carbon monoxide (CO) is a toxic, invisible, and odourless gas. It is responsible for serious domestic accidents (badly adjusted heaters or water heaters). You must always ventilate and have combustion appliances serviced.

  We can identify the products of a combustion: carbon dioxide turns limewater cloudy, and water turns anhydrous copper sulfate blue (see the lesson Characteristic tests).

Combustion and climate

Burning fossil fuels (coal, oil, gas) releases large quantities of carbon dioxide (CO2) into the air. However, CO2 is a greenhouse gas: this excess strengthens the greenhouse effect and contributes to global warming (see level 1 of this lesson).