C1/2/3 resit

gas percentages in the air: 

nitrogen 78%

oxygen 21% argon  1%

The atmosphere also contains a small but important amount of carbon dioxide, approximately 0.04 per cent, and tiny amounts of a few other gases + water vapour

Human activities produce a number of gases that are released into the atmosphere. Most of these pollutant gases are produced by the burning of fuels.

- The burning of fuels releases a large amount of carbon dioxide into the atmosphere. This is thought to cause global warming

- Other pollutant gases that are released when fuels are burned include carbon monoxide, nitrogen oxides and sulfur dioxide.

Some of these pollutants are directly harmful to humans. For example, carbon monoxide is toxic and, if breathed in, can cause death.

Others pollutants are harmful to the environment, so cause harm to humans indirectly. For example, sulfur dioxide causes acid rain that can damage or kill trees and crops.

The atmosphere of the early Earth was largely made up of carbon dioxide and water vapour, probably coming from volcanoes. When the Earth cooled the water vapour condensed to form the oceans.

Carbon dioxide dissolved in the oceans and began to form sedimentary rocks. A lot of carbon dioxide was trapped underground and eventually formed fossil fuels.

Indirect evidence

The early atmosphere cannot be measured directly so scientists look for indirect evidence. This includes the chemical make-up of rocks, examining air bubbles in ice cores and looking at fossils.

When measuring the composition of gases in the atmosphere, or the concentrations of atmospheric pollutants, it is important that the data obtained is accurate and reliable

= The accuracy of each measurement depends on the quality of the measuring apparatus and the skill of the scientists taking the measurement. If the apparatus is faulty or the scientists make a mistake, the measurement may be inaccurate.

=For the data to be reliable, the variation within the values must be small. There is always some variation in any set of measurements, whatever is being measured.

Range - The highest and lowest values in a set of measurements show the range

mean:The results from all of the samples can be added together, and then the total divided by the number of samples. This gives a value called the 'mean'

+ remember outleirs