# Inhaled and Exhaled Air

In this worksheet, students will explain the main differences between inhaled and exhaled air.

Key stage:  KS 3

Curriculum topic:   Biology: Structure and Function of Living Organisms

Curriculum subtopic:   Gas Exchange Systems (Breathing)

Difficulty level:

### QUESTION 1 of 10

Breathing

Everyone knows that we breathe in oxygen and breathe out carbon dioxide. But that's not technically true. The truth is we breathe in air and breathe out air... so what's the difference?

Inhaled Air

So, we need oxygen to live, but do you know how much of the air is made up of oxygen? 80%? 75%? ....it actually only makes up about 21% of the air.

So what else is in there?

The air that is all around us is made up of mostly nitrogen, about 78% nitrogen in fact. So what about the other 1%? ...The other 1% is made up of 0.04% carbon dioxide, some water vapour and other gases such as argon, helium and neon.

Exhaled Air

The reason we breathe is to get oxygen into our bodies to allow for respiration, so you would think that the air we breathe out would have a lot less oxygen right? Well, there is less oxygen in exhaled air but probably a bit more than you would expect. Exhaled air actually contains around 16% oxygen which means we only use about 5% of the oxygen available!

The percentage of nitrogen stays at nearly 78% (as we do not have any use for nitrogen) but the main difference is the increase in carbon dioxide. The carbon dioxide in exhaled air is more-or-less 4% and the percentage of water vapour is also higher than in inhaled air.

Experiment Time

There is a very simple experiment that we can do to prove that exhaled air contains more carbon dioxide than inhaled air.

Carbon dioxide will turn colourless limewater milky/cloudy.

The CO2 percentage in inhaled air is so small that it doesn't have an effect on the limewater, however bubble some exhaled air through it and it will turn milky/cloudy.

Which gas makes up around 78% of the air around us?

oxygen

carbon dioxide

nitrogen

water vapour

How much of the air we breathe in is oxygen?

16%

21%

50%

78%

How much oxygen would you find in exhaled air?

0.04%

16%

21%

78%

Other than carbon dioxide, which gas would you find more of in exhaled air?

carbon monoxide

oxygen

nitrogen

water vapour

Fill in the correct percentages of these three gases found in inhaled air.

 Percentage (%) Nitrogen Oxygen Carbon Dioxide

What is the chemical test for carbon dioxide?

Relights a glowing splint

Turns limewater milky/cloudy

Burns with a squeaky pop

Turns universal indicator purple

Why does limewater not change when inhaled air is bubbled through it?

The percentage of carbon dioxide is too high

The percentage of carbon dioxide is too low

The percentage of oxygen is too high

The percentage of oxygen is too low

Fill in the correct percentages (by volume) of gases found in exhaled air.

 Percentage (%) Nitrogen Oxygen Carbon Dioxide
• Question 1

Which gas makes up around 78% of the air around us?

nitrogen
EDDIE SAYS
It's well worth getting a really good feel for the make-up of the air. At least nail the #1 gas (Nitrogen, at 78%) and the #2 gas (Oxygen, at 21%). As a bare minimum, know that air is about one fifth oxygen and four-fifths nitrogen.
• Question 2

How much of the air we breathe in is oxygen?

21%
EDDIE SAYS
So, if the percentage of oxygen in the air is 21%, or about one fifth, that's all that's available to sustain life ... but it does!
• Question 3

How much oxygen would you find in exhaled air?

16%
EDDIE SAYS
With ordinary breathing, our bodies use up only about 5% of the available oxygen for our general needs. Do you reckon that this might change with increased activity?
• Question 4

Other than carbon dioxide, which gas would you find more of in exhaled air?

water vapour
EDDIE SAYS
It makes sense that there's an increase in the amount of water vapour in exhaled air; after all, it's coming out of a warm, moist body and, indeed, we need to remove excess water and one of the ways in which we do this is through breathing.
• Question 5

Fill in the correct percentages of these three gases found in inhaled air.

 Percentage (%) Nitrogen Oxygen Carbon Dioxide
EDDIE SAYS
So, remember that the air (by volume) is 78% nitrogen and 21% oxygen. Carbon dioxide is an interesting one: if you look up the percentage in air over the last 50 years, you'll find that it's been increasing. It's now 0.04%. Can you think what might have been causing this increase in overall CO2 in the air?
• Question 6

What is the chemical test for carbon dioxide?

Turns limewater milky/cloudy
EDDIE SAYS
Colourless limewater will turn milky or cloudy in the presence of carbon dioxide. Do you know why? Briefly, there's a reaction between the

CO2 and the calcium hydroxide solution (limewater) which results in tiny insoluble particles of chalk (calcium carbonate) forming. It's these little particles of chalk that give it the 'milky' appearance.

• Question 7

Why does limewater not change when inhaled air is bubbled through it?

The percentage of carbon dioxide is too low
EDDIE SAYS
There is no change in the limewater when it is in the presence of inhaled air because there is not enough carbon dioxide in inhaled air (only 0.04%) to have any effect on the limewater.
• Question 8

Fill in the correct percentages (by volume) of gases found in exhaled air.

 Percentage (%) Nitrogen Oxygen Carbon Dioxide
EDDIE SAYS
Do you remember this from the introduction? We use about 5% of the available 21% of oxygen - so that's 16% being breathed out. Then, we've added lots more CO2 into the air to be breathed out: that's gone up from 0.04% to 4% (can you work out how many times more that is?). Finally: 78% of nitrogen in, 78% out.
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