4.3 - The Earth's Atmosphere

Today's atmosphere

• The atmosphere is a mixture of gases held around the Earth by gravity.
• Today it's made up of two main gases:
  1. About 78% nitrogen
  2. About 21% oxygen
• The remaining 1% or so is a mixture of other gases in far smaller proportions:
  • Carbon dioxide (about 0.04%)
  • Water vapour
  • The noble gases - an unreactive group that make up a small but constant part of the air
• The most abundant noble gas is argon, at around 1% of the atmosphere. The rest of the noble gases are present only in tiny traces.

Measuring the percentage of oxygen in air

The percentage of oxygen in air can be found by reacting a substance with the oxygen present and measuring how much the volume decreases.

Method 1 - Iron wool

• Iron reacts with oxygen to form iron oxide (rust).
• A known volume of air is trapped with iron wool inside an inverted measuring cylinder standing in water.
• As the iron rusts, it takes up the oxygen, and water rises into the cylinder to fill the space the oxygen occupied.

Method 2 - Phosphorus

1. Phosphorus is sealed inside a tube between two gas syringes holding a known volume of air.
2. The air is passed back and forth over the heated phosphorus, which reacts with the oxygen to form phosphorus oxide.

Finding the percentage

In both methods, the reaction is left until complete, and any gas that was heated is left to cool to room temperature, because a gas expands when it is hot. The final volume is then compared with the starting volume to find the decrease. The starting volume is all the air in the apparatus, including any in a flask or connecting tube, not only the air in a syringe.

Worked example - Percentage of oxygen in air

A student traps 100 cm³ of air with iron wool inside an inverted measuring cylinder standing in water, and leaves it until the water level stops rising. The final volume of air remaining is 79 cm³. Calculate the percentage of oxygen in the air.