Edexcel IGCSE Chemistry · Spec 2.9-2.13
Gases in the Atmosphere
The gases in the atmosphere, the combustion of elements, and carbon dioxide.
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Gases in the Atmosphere
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Explained
What air is made of, and what happens when things burn in it
Clean dry air is about 78 per cent nitrogen and about 21 per cent oxygen. The remaining one per cent is mostly argon, with a small and rising amount of carbon dioxide and traces of other noble gases.
Air is a mixture, not a compound. The gases are not chemically joined, which is why they can be separated physically, by fractional distillation of liquid air.
Combustion
Burning is a reaction with oxygen, and it is always exothermic.
Complete combustion of a hydrocarbon, with plenty of oxygen, gives carbon dioxide and water only. Incomplete combustion, with a limited supply, gives carbon monoxide and carbon as soot as well, and releases less energy.
The two products of incomplete combustion are dangerous in different ways. Carbon monoxide is toxic because it binds to haemoglobin and reduces the blood's capacity to carry oxygen. Soot blackens surfaces, blocks flues and causes breathing problems.
A yellow, smoky flame is the sign of incomplete combustion; a clear blue flame means complete combustion. That is exactly what the air hole on a Bunsen burner controls.
Burning elements
Magnesium burns with a brilliant white flame to give magnesium oxide, a white solid. Sulfur burns with a blue flame to give sulfur dioxide, a colourless acidic gas. Hydrogen burns to give water only, which is why it is attractive as a fuel.
Metal oxides are generally basic and non metal oxides are generally acidic, which is worth carrying into the acids topic.
Why carbon dioxide is a greenhouse gas
Short wavelength radiation from the Sun passes through the atmosphere and warms the Earth's surface. The surface re-radiates energy at longer wavelengths, as infrared.
Carbon dioxide absorbs that outgoing infrared and re-radiates some of it back towards the surface, so less energy escapes to space and the planet stays warmer. Increasing the concentration traps more, which is what drives global warming.
Carbon dioxide also turns limewater milky, which is the standard test for it.
What the mark scheme accepts and rejects
An Edexcel International GCSE Chemistry mark scheme asks candidates to name gases in the atmosphere. For nitrogen it allows N2 but ignores N. For oxygen it allows O2 but ignores O. Argon, being a single atom gas, is allowed as Ar.
The distinction is real chemistry rather than pedantry. N is a nitrogen atom, which does not exist on its own in air; N2 is the molecule the air actually contains. If you write a formula rather than a name, it has to be the formula of the substance.
On combustion, the same series of mark schemes credits carbon as a product of incomplete combustion and allows soot or C, and explains the danger of carbon monoxide as reducing the capacity of the blood to transport oxygen, accepting correct references to haemoglobin and allowing the term carboxyhaemoglobin.
Notice that the equation mark for combustion is split, with one mark for all the formulae being correct and a second for balancing correct formulae. Balancing an equation containing a wrong formula earns neither.
Measuring the oxygen in air
Pass a fixed volume of air repeatedly over heated copper in a sealed pair of syringes. The copper reacts with the oxygen to form black copper oxide, so the oxygen is removed and the volume of gas falls.
Keep going until the volume stops changing, which shows all the oxygen has reacted. The decrease as a percentage of the starting volume gives you the proportion of oxygen, and it should come out near 21 per cent.
Spec 2.9-2.13
What you need to know
- State the composition of clean air
- Describe complete and incomplete combustion
- Explain why carbon dioxide is a greenhouse gas
Active recall
Quick check
Answer each question before opening the answer.
What are the approximate percentages of nitrogen and oxygen in clean air?
About 78% nitrogen and about 21% oxygen.
What is formed when a metal or non-metal burns in oxygen?
An oxide — metals form basic oxides, non-metals form acidic oxides.
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