The proportions of different gases in the atmosphere

What you'll learn

The main gases in today’s atmosphere, and their approximate proportions.
What “four-fifths” and “one-fifth” mean as fractions, percentages and ratios.
How to calculate the volume of a gas in a sample of air.
The key wording examiners expect for this GCSE Chemistry point.

The atmosphere is a mixture of gases

The atmosphere is the layer of gases surrounding the Earth. It is held around the planet by gravity.

A gas is a state of matter where the particles are far apart and move randomly. Gases spread out to fill the space available.

Definition

Atmosphere

The atmosphere is the mixture of gases surrounding the Earth.

Air is not a single substance. It is a mixture: different gases are together in the same space, but they are not chemically bonded to each other. That matters because you can talk about the proportion of each gas in the mixture.

Definition

Mixture

A mixture contains two or more substances that are not chemically bonded together.

Proportions: parts of a whole

A proportion tells you how much of a whole is made up by one part.

For example, if 100 particles of air are represented in a diagram, and 80 of them represent nitrogen, then nitrogen makes up 80 out of 100 parts of the air.

You can describe the same idea using:

a fraction, such as four-fifths
a percentage, such as 80%
a ratio, such as nitrogen:oxygen = 4:1

Definition

Proportion

A proportion is the amount of one part compared with the total amount of the whole.

Fractions and percentages you need here

For this topic, the two key approximations are:

four-fifths = 80%
one-fifth = 20%

That means if you split air into 5 equal “chunks”, about 4 chunks are nitrogen and about 1 chunk is oxygen.

Example

Converting percentages into fractions and a ratio

Air is often approximated as 80% nitrogen and 20% oxygen. Convert this into fractions and a nitrogen:oxygen ratio.

Treat the percentages as parts out of 100: nitrogen is 80 parts and oxygen is 20 parts.
Convert each percentage to a fraction of the whole:

\frac{80}{100}=\frac{4}{5}

\frac{20}{100}=\frac{1}{5}

Compare nitrogen to oxygen as a ratio and simplify:

80 : 20 = 4 : 1

So there are about 4 parts nitrogen for every 1 part oxygen.

The main gases in today’s atmosphere

For about 200 million years, the proportions of different gases in the atmosphere have been much the same as they are today.

Key Idea

Modern atmosphere composition

Today’s atmosphere is approximately:

four-fifths nitrogen — about 80%
one-fifth oxygen — about 20%
small proportions of other gases, including carbon dioxide, water vapour and noble gases

Nitrogen is the most abundant gas in the atmosphere. Oxygen is the second most abundant gas. The other gases make up only a small proportion, but they are still important.

The diagram below shows the composition using 100 equal parts. It also shows slightly more precise values for dry air, while keeping the GCSE memory version: about four-fifths nitrogen and about one-fifth oxygen.

Diagram showing the approximate composition of dry air: 78 parts nitrogen, 21 parts oxygen and 1 part other gases, with GCSE approximation as four-fifths nitrogen and one-fifth oxygen

The “other gases”

The small proportion of other gases includes:

carbon dioxide, CO₂
water vapour, H₂O as a gas
noble gases, such as argon

Definition

Noble gases

Noble gases are Group 0 elements. They are very unreactive gases, such as helium, neon and argon.

Argon is the most common noble gas in air, but at GCSE you usually only need to say “noble gases” unless the question asks for an example.

Common Mistake

Water vapour varies

The amount of water vapour in the air changes with weather, temperature and location. In simple GCSE composition questions, treat it as part of the small proportion of “other gases” unless data is given.

“About” is important

The specification uses the word about because the numbers are rounded.

You should remember:

nitrogen is about 80%, not exactly 80%
oxygen is about 20%, not exactly 20%
other gases are present in small proportions

The more precise values for dry air are close to 78% nitrogen, 21% oxygen and about 1% other gases. But for this specification point, the expected simple answer is the approximate four-fifths / one-fifth description.

Common Mistake

Forcing the percentages to be exact

Do not write “80% nitrogen, 20% oxygen and 1% carbon dioxide” as if all of those exact values belong together. The 80% and 20% figures are rounded approximations, and carbon dioxide is only one of the small-proportion gases.

Calculating volumes from percentages

If you know the total volume of air, you can estimate the volume of a gas using its percentage.

The key idea is:

\text{volume of gas}=\frac{\text{percentage of gas}}{100}\times\text{total volume of air}

The volume unit stays the same. If the total volume is in cm³, your answer will be in cm³. If the total volume is in dm³, your answer will be in dm³.

Example

Finding the volume of oxygen and nitrogen in air

A sample contains 250 cm³ of air. Estimate the volumes of oxygen and nitrogen using the GCSE approximations.

Use the approximate percentages: oxygen is 20% and nitrogen is 80%.
Calculate the oxygen volume:

\frac{20}{100}\times250\ \text{cm}^3=50\ \text{cm}^3

Calculate the nitrogen volume:

\frac{80}{100}\times250\ \text{cm}^3=200\ \text{cm}^3

Check the estimate: 50 cm³ oxygen plus 200 cm³ nitrogen gives 250 cm³ in total, because the small amount of other gases has been ignored in this rounded model.

Tip

Quick mental method

Using the GCSE approximation, oxygen is about one-fifth of air. To find one-fifth, divide the total amount by 5. Nitrogen is about four-fifths, so multiply that one-fifth by 4.

The 200 million year statement

The atmosphere has not always had this composition. Earlier in Earth’s history, the atmosphere was very different.

However, for the last 200 million years, the proportions of the main gases have been much the same as today. In this topic, that is the key time statement to remember.

So if an exam question asks about the atmosphere over the last 200 million years, you should be ready to say:

about 80% nitrogen
about 20% oxygen
small proportions of carbon dioxide, water vapour and noble gases

Common Mistake

Saying the atmosphere has always been the same

The modern 80% nitrogen and 20% oxygen pattern applies to roughly the last 200 million years, not to the whole history of the Earth.

What exam answers often need

For a short answer question, you do not need a long explanation. A strong GCSE answer could be:

“Today’s atmosphere is about four-fifths nitrogen and about one-fifth oxygen, with small proportions of other gases such as carbon dioxide, water vapour and noble gases.”

That answer includes the main gases, approximate proportions and examples of other gases.

Exam technique

In the exam

If asked for the composition of the atmosphere, give nitrogen first, then oxygen, then mention small proportions of other gases.
Use approximate wording: write “about 80%” or “about four-fifths” for nitrogen, and “about 20%” or “about one-fifth” for oxygen.
For calculation questions, convert the percentage into a fraction of 100, then multiply by the total volume and keep the same volume unit.

Self review

Check yourself

What are the approximate percentages of nitrogen and oxygen in today’s atmosphere?
Name two gases found in the small “other gases” proportion.
A sample contains 40 dm³ of air. About what volume is oxygen?

The proportions of different gases in the atmosphere

What you'll learn

The main gases in today’s atmosphere, and their approximate proportions.
What “four-fifths” and “one-fifth” mean as fractions, percentages and ratios.
How to calculate the volume of a gas in a sample of air.
The key wording examiners expect for this GCSE Chemistry point.

The atmosphere is a mixture of gases

The atmosphere is the layer of gases surrounding the Earth. It is held around the planet by gravity.

A gas is a state of matter where the particles are far apart and move randomly. Gases spread out to fill the space available.

Definition

Atmosphere

The atmosphere is the mixture of gases surrounding the Earth.

Definition

Mixture

A mixture contains two or more substances that are not chemically bonded together.

Proportions: parts of a whole

A proportion tells you how much of a whole is made up by one part.

For example, if 100 particles of air are represented in a diagram, and 80 of them represent nitrogen, then nitrogen makes up 80 out of 100 parts of the air.

You can describe the same idea using:

a fraction, such as four-fifths
a percentage, such as 80%
a ratio, such as nitrogen:oxygen = 4:1

Definition

Proportion

A proportion is the amount of one part compared with the total amount of the whole.

Fractions and percentages you need here

For this topic, the two key approximations are:

four-fifths = 80%
one-fifth = 20%

That means if you split air into 5 equal “chunks”, about 4 chunks are nitrogen and about 1 chunk is oxygen.

Example

Converting percentages into fractions and a ratio

Air is often approximated as 80% nitrogen and 20% oxygen. Convert this into fractions and a nitrogen:oxygen ratio.

Treat the percentages as parts out of 100: nitrogen is 80 parts and oxygen is 20 parts.
Convert each percentage to a fraction of the whole:

\frac{80}{100}=\frac{4}{5}

\frac{20}{100}=\frac{1}{5}

Compare nitrogen to oxygen as a ratio and simplify:

80 : 20 = 4 : 1

So there are about 4 parts nitrogen for every 1 part oxygen.

The main gases in today’s atmosphere

For about 200 million years, the proportions of different gases in the atmosphere have been much the same as they are today.

Key Idea

Modern atmosphere composition

Today’s atmosphere is approximately:

four-fifths nitrogen — about 80%
one-fifth oxygen — about 20%
small proportions of other gases, including carbon dioxide, water vapour and noble gases

Nitrogen is the most abundant gas in the atmosphere. Oxygen is the second most abundant gas. The other gases make up only a small proportion, but they are still important.

Diagram showing the approximate composition of dry air: 78 parts nitrogen, 21 parts oxygen and 1 part other gases, with GCSE approximation as four-fifths nitrogen and one-fifth oxygen

The “other gases”

The small proportion of other gases includes:

carbon dioxide, CO₂
water vapour, H₂O as a gas
noble gases, such as argon

Definition

Noble gases

Noble gases are Group 0 elements. They are very unreactive gases, such as helium, neon and argon.

Argon is the most common noble gas in air, but at GCSE you usually only need to say “noble gases” unless the question asks for an example.

Common Mistake

Water vapour varies

“About” is important

The specification uses the word about because the numbers are rounded.

You should remember:

nitrogen is about 80%, not exactly 80%
oxygen is about 20%, not exactly 20%
other gases are present in small proportions

Common Mistake

Forcing the percentages to be exact

Calculating volumes from percentages

If you know the total volume of air, you can estimate the volume of a gas using its percentage.

The key idea is:

\text{volume of gas}=\frac{\text{percentage of gas}}{100}\times\text{total volume of air}

The volume unit stays the same. If the total volume is in cm³, your answer will be in cm³. If the total volume is in dm³, your answer will be in dm³.

Example

Finding the volume of oxygen and nitrogen in air

A sample contains 250 cm³ of air. Estimate the volumes of oxygen and nitrogen using the GCSE approximations.

Use the approximate percentages: oxygen is 20% and nitrogen is 80%.
Calculate the oxygen volume:

\frac{20}{100}\times250\ \text{cm}^3=50\ \text{cm}^3

Calculate the nitrogen volume:

\frac{80}{100}\times250\ \text{cm}^3=200\ \text{cm}^3

Check the estimate: 50 cm³ oxygen plus 200 cm³ nitrogen gives 250 cm³ in total, because the small amount of other gases has been ignored in this rounded model.

Tip

Quick mental method

Using the GCSE approximation, oxygen is about one-fifth of air. To find one-fifth, divide the total amount by 5. Nitrogen is about four-fifths, so multiply that one-fifth by 4.

The 200 million year statement

The atmosphere has not always had this composition. Earlier in Earth’s history, the atmosphere was very different.

However, for the last 200 million years, the proportions of the main gases have been much the same as today. In this topic, that is the key time statement to remember.

So if an exam question asks about the atmosphere over the last 200 million years, you should be ready to say:

about 80% nitrogen
about 20% oxygen
small proportions of carbon dioxide, water vapour and noble gases

Common Mistake

Saying the atmosphere has always been the same

The modern 80% nitrogen and 20% oxygen pattern applies to roughly the last 200 million years, not to the whole history of the Earth.

What exam answers often need

For a short answer question, you do not need a long explanation. A strong GCSE answer could be:

“Today’s atmosphere is about four-fifths nitrogen and about one-fifth oxygen, with small proportions of other gases such as carbon dioxide, water vapour and noble gases.”

That answer includes the main gases, approximate proportions and examples of other gases.

Exam technique

In the exam

If asked for the composition of the atmosphere, give nitrogen first, then oxygen, then mention small proportions of other gases.
Use approximate wording: write “about 80%” or “about four-fifths” for nitrogen, and “about 20%” or “about one-fifth” for oxygen.
For calculation questions, convert the percentage into a fraction of 100, then multiply by the total volume and keep the same volume unit.

Self review

Check yourself

What are the approximate percentages of nitrogen and oxygen in today’s atmosphere?
Name two gases found in the small “other gases” proportion.
A sample contains 40 dm³ of air. About what volume is oxygen?

The proportions of different gases in the atmosphere

What you'll learn

The atmosphere is a mixture of gases

Proportions: parts of a whole

Fractions and percentages you need here

The main gases in today’s atmosphere

The “other gases”

“About” is important

Calculating volumes from percentages

The 200 million year statement

What exam answers often need

You've reached the end

The proportions of different gases in the atmosphere

What you'll learn

The atmosphere is a mixture of gases

Proportions: parts of a whole

Fractions and percentages you need here

The main gases in today’s atmosphere

The “other gases”

“About” is important

Calculating volumes from percentages

The 200 million year statement

What exam answers often need

You've reached the end

The proportions of different gases in the atmosphere

What you'll learn

The atmosphere is a mixture of gases

Proportions: parts of a whole

Fractions and percentages you need here

The main gases in today’s atmosphere

The “other gases”

“About” is important

Calculating volumes from percentages

The 200 million year statement

What exam answers often need

You've reached the end

The proportions of different gases in the atmosphere Revision Guide

The proportions of different gases in the atmosphere

What you'll learn

The atmosphere is a mixture of gases

Proportions: parts of a whole

Fractions and percentages you need here

The main gases in today’s atmosphere

The “other gases”

“About” is important

Calculating volumes from percentages

The 200 million year statement

What exam answers often need

You've reached the end

The proportions of different gases in the atmosphere Revision Guide