5

Chemistry

The early atmosphere

The evidence includes:• the gases produced by volcanoes

today which tells us what gases

would have been produced by

volcanoes in the past• the atmospheres of other planets

and moons in the solar system,

where the atmosphere has not been

changed by living things

• iron compounds found in very old

rocks that could only form if there

was no oxygen.

1. (a) Name the two gases present in the largest amounts in the Earth’s early atmosphere. (2 marks) (b) State where scientists think these gases came from. (1 mark)

2. Describe how the Earth’s oceans formed. (2 marks)

You don’t need to remember these different sources of information. You DO need to remember that there ARE different sources of information, some of which tell us different things.

The Earth’s first atmosphere was formed by gases produced from volcanoes.

The Earth was very hot to start with, and there were no oceans. As the Earth cooled down the water vapour in the atmosphere condensed to form liquid water. This liquid water became the oceans.

The Earth’s first atmosphere

Explain why scientists cannot be certain about the composition of the early atmosphere.

There were no humans around to measure the atmosphere, so they have to use clues. There are different sources of information, but not all the evidence leads to the same conclusion. This means that it is difficult to know exactly what the early atmosphere was like and how it has changed.

Volcanoes gave out...

...lots of water vapour

...lots of carbon dioxide

...small amounts of other gases

The early atmosphere did not contain any oxygen.

Target

D −G

6

Chemistry

A changing atmosphere

The atmosphere of the early Earth was mainly carbon dioxide, with little or no oxygen. Today the atmosphere has much less than 1% carbon dioxide and has 21% oxygen.This means that:• carbon dioxide has been removed

from the atmosphere• oxygen has been added to the

atmosphere.

Adding oxygenThe first organisms that evolved on Earth did not use or release oxygen. About 1 billion years ago some forms of life evolved in the oceans that used photosynthesis. Photosynthesis uses carbon dioxide and produces oxygen as a waste product. Eventually plants evolved and some of them adapted to life on land, so gradually more and more oxygen was added to the atmosphere.

There is a lot of limestone in the Earth’s crust. Explain how the formation of limestone removes carbon dioxide from the Earth’s atmosphere.Some carbon dioxide in the atmosphere dissolves in the oceans. Some marine organisms use dissolved carbon dioxide to make calcium carbonate for their shells. When they die, these marine organisms sink to the sea bed and their shells eventually form limestone. The carbon locked up in the limestone originally came from the atmosphere.

You need to explain the link between limestone and carbon by saying that limestone is made from shells which are mostly calcium carbonate. In an exam you could present the answer to this kind of question as a flow chart.

Adding oxygen

Removing carbon dioxide

Over millions of years the layers of sediment become squashed and form sedimentary rocks.

The shells of dead marine organisms fall to the sea bed and become part of the sediment.

Marine organisms used the dissolved carbon dioxide to make

calcium carbonate for shells.

Carbon dioxide dissolved in the oceans.

Oxygen builds up in the atmosphere.

Oxygen builds up in the oceans.

Forms of life evolve in the oceans and photosynthesis begins.

Earth cools and oceans form.

1. State how the percentage of carbon dioxide changed from the Earth’s early atmosphere to the amount in today’s atmosphere. (1 mark)

2. (a) State the main substance in the shells of marine organisms. (1 mark)

(b) Describe how the shells change into limestone. (3 marks)

3. Explain why the amount of oxygen in the atmosphere only started to increase about 1 billion years ago.

(2 marks)

Target

D −GTarget

C −D

7

Chemistry

Changes in the atmosphereThe composition of the atmosphere today is not constant. It can be changed by human activities.• Burning fossil fuels releases carbon

dioxide into the atmosphere. Sulfur dioxide can also be released from burning coal.

• Farming affects the atmosphere, as cattle and rice fields release methane.

• Deforestation adds carbon dioxide to the atmosphere if the trees are burnt. There are also fewer trees to remove carbon dioxide from the atmosphere.

The atmosphere todayThe amounts of different gases in the atmosphere are shown in the pie chart and table. The atmosphere also contains water vapour, but this is not usually included because the amount changes depending on the weather.

Gas % in dry air

nitrogen 78

oxygen 21

argon 0.9

carbon dioxide 0.04

other gases traces

Remember that sulfur dioxide is

also added to the atmosphere when

fossil fuels such as coal are burnt.

The composition of the atmosphere

Explain one natural cause of changes in the atmosphere.When volcanoes erupt they release a lot of sulfur dioxide into the atmosphere. They also release carbon dioxide.

1. Name the gas that is present in the largest amount in air.

(1 mark)

2. (a) State two human activities that add carbon dioxide to the atmosphere.

(2 marks) (b) Name one other gas that human

activities add to the atmosphere. (1 mark)

3. What changes to the atmosphere can occur naturally?

(2 marks)

Target

D −G

There are two marks for this question, so you need to mention two different natural changes.

Exam questions similar to this have proved to be especialy tricky - be prepared!

oxygen

nitrogen

other gases