CIE IGCSE Biology

Revision Notes

11.3 Composition of Air

Differences between Inhaled & Exhaled Air

  • Air that is breathed in and air that is breathed out has different amounts of gases in it due to exchanges that take place in the alveoli
  • Atmospheric air contains around 20 – 21% oxygen, of which we only absorb around 4 – 5%, breathing out air containing around 16% oxygen
  • Normal carbon dioxide content of air is around 0.04% and, as carbon dioxide diffuses into the alveoli from the blood, we breathe out air containing around 4% carbon dioxide
  • The air we breathe out contains more water vapour than when we breathe it in, and the temperature of exhaled air is higher than inhaled air

Composition of air:

Differences between Inhaled & Exhaled Air table, IGCSE & GCSE Biology revision notes

Extended Only

Reasons for Differences in Inhaled & Exhaled Air

Reasons for Differences in Inhaled & Exhaled Air table, IGCSE & GCSE Biology revision notes

Using Limewater to Test for CO2 in Exhaled Air

The limewater test, IGCSE & GCSE Biology revision notesThe limewater test

 

  • When we breathe in, the air is drawn through boiling tube A
  • When we breathe out, the air is blown into boiling tube B
  • Lime water is clear but becomes cloudy (or milky) when carbon dioxide is bubbled through it
  • The lime water in boiling tube A will remain clear, but the limewater in boiling tube B will become cloudy
  • This shows us that the percentage of carbon dioxide in exhaled air is higher than in inhaled air

Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.
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