The modern atmosphere

Heat can be transferred from place to place by conduction, convection and radiation. Dark matt surfaces are better at absorbing heat energy than light shiny surfaces. Heat energy can be lost from homes in many different ways and there are ways of reducing these heat losses.

You need to know the proportions of the main gases in the atmosphere.

The Earth’s atmosphere has remained much the same for the past 200 million years. The pie chart shows the proportions of the main gases in the atmosphere.

The composition of air

The two main gases are both elements and account for about 99 percent of the gases in the atmosphere. They are:

4 about /5 or 80 percent nitrogen (a relatively unreactive gas)

1 about /5 or 20 percent oxygen (the gas that allows animals and plants to respire and for fuels to burn)

The remaining gases, such as carbon dioxide, water vapour and noble gases such as argon, are found in much smaller proportions.

The percentage of oxygen in the air can be measured by passing a known volume of air over hot copper and measuring the decrease in volume as the oxygen reacts with it. Here are the equations for this reaction: copper + oxygen → copper oxide 2Cu + O2 → 2CuO

Gas syringes are used to measure the volume of gas in the experiment. The starting volume of air is often 100 cm3 to make the analysis of the results easy, but it could be any convenient volume. In the simulation, there is 100 cm3 of air at the start.

Note that there is some air in the tube with the copper turnings. The oxygen in this air will also react with the hot copper, causing a small error in the final volume recorded. It is also important to let the apparatus cool down at the end of the experiment, otherwise the final reading will be too high.

Scientists believe that the Earth was formed about 4.5 billion years ago. Its early atmosphere was probably formed from the gases given out by volcanoes. It is believed that there was intense volcanic activity for the first billion years of the Earth's existence.

The early atmosphere was probably mostly carbon dioxide with little or no oxygen. There were smaller proportions of water vapour, ammonia and methane. As the Earth cooled down, most of the water vapour condensed and formed the oceans.

Mars and Venus today

It is thought that the atmospheres of Mars and Venus today, which contain mostly carbon dioxide, are similar to the early atmosphere of the Earth.

The table shows the proportions of the main gases in their atmospheres.

Gas Mars todayVenus today carbon dioxide 95.3 96.5 nitrogen 2.7 3.5 argon 1.6 trace oxygen, water vapour and other gasestrace trace

There is evidence that the first living things appeared on Earth billions of years ago. There are many scientific theories to explain how life began. One theory involves the interaction between hydrocarbons, ammonia and lightning.

The Miller-Urey experiment - Higher tier

Stanley Miller and Harold Urey carried out some experiments in 1952 and published their results in 1953. The aim was to see if substances now made by living things could be formed in the conditions thought to have existed on the early Earth.

The two scientists sealed a mixture of water, ammonia, methane and hydrogen in a sterile flask. The mixture was heated to evaporate water to produce water vapour. Electric sparks were passed through the mixture of water vapour and gases, simulating lightning. After a week, contents were analysed. Amino acids, the building blocks for proteins, were found.

The Miller-Urey experiment

The Miller-Urey experiment supported the theory of a ‘primordial soup’, the idea that complex chemicals needed for living things to develop could be produced naturally on the early Earth.

The Earth’s early atmosphere is believed to have been mainly carbon dioxide with little or no oxygen gas. The Earth’s atmosphere today contains around 21 percent oxygen and about 0.04 percent carbon dioxide. So how did the proportion of carbon dioxide in the atmosphere go down, and the proportion of oxygen go up?

Increasing oxygen

Plants and algae can carry out photosynthesis. This process uses carbon dioxide from the atmosphere (with water and sunlight) to produce oxygen (and glucose). The appearance of plants and algae caused the production of oxygen, which is why the proportion of oxygen went up.

Decreasing carbon dioxide Photosynthesis by plants and algae used carbon dioxide from the atmosphere, but this is not the only reason why the proportion of carbon dioxide went down. These processes also absorb carbon dioxide from the atmosphere:

dissolving in the oceans the production of sedimentary rocks such as limestone the production of fossil fuels from the remains of dead plants and animals

Today, the burning of fossil fuels (coal and oil) is adding carbon dioxide to the atmosphere faster than it can be removed. This means that the level of carbon dioxide in the atmosphere is increasing, contributing to global warming. It also means that the oceans are becoming more acidic as they dissolve increasing amounts of carbon dioxide. This has an impact on the marine environment, for example making the shells of sea creatures thinner than normal.

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You will recall that about 78 percent of the air is nitrogen and 21 percent is oxygen. These two gases can be separated by fractional distillation of liquid air.

Liquefying the air

Air is filtered to remove dust, and then cooled in stages until it reaches –200°C. At this temperature it is a liquid. We say that the air has been liquefied.

Here's what happens as the air liquefies (note that you do not need to recall the boiling points of the different gases):

water vapour condenses, and is removed using absorbent filters carbon dioxide freezes at –79ºC, and is removed oxygen liquefies at –183ºC nitrogen liquefies at –196ºC.

The liquid nitrogen and oxygen are then separated by fractional distillation.

Fractional distillation

The liquefied air is passed into the bottom of a fractionating column. Just as in the columns used to separate oil fractions, the column is warmer at the bottom than it is at the top. Fractional distillation

The liquid nitrogen boils at the bottom of the column. Gaseous nitrogen rises to the top, where it is piped off and stored. Liquid oxygen collects at the bottom of the column. The boiling point of argon - the noble gas that forms 0.9 percent of the air - is close to the boiling point of oxygen, so a second fractionating column is often used to separate the argon from the oxygen.

Uses of nitrogen and oxygen

liquid nitrogen is used to freeze food food is packaged in gaseous nitrogen to increase its shelf life oil tankers are flushed with gaseous nitrogen to reduce the chance of explosion oxygen is used in the manufacture of steel and in medicine.

Now try a Test Bite - higher tier.