Section 2 d) Summary

The Earth's atmosphere is made up of many different gases, including

• Nitrogen (78%)
• Oxygen (21%)
• Argon (0.9%)
• Carbon dioxide, Water vapour, etc. (>0.1%)

The last category includes greenhouse gases: Gases that trap the sun's energy inside the atmosphere, causing the greenhouse effect. This is usually a good thing; the greenhouse effect keeps us alive. But excess greenhouse gases are being released into the atmosphere, increasing the volume constantly (due to burning fossil fuels, etc.). This causes the enhanced greenhouse effect, where too much of the sun's energy is trapped and causes a heating effect called climate change. This is a dangerous process that is threatening the world we live in, and must be stopped. 

We can work out the percentage volume of oxygen in air with a simple experiment. 

1. Place a tube containing copper in the middle of two gas syringes (containing a known volume of air), attached so the ends are sealed. 
2. Gently heat the copper with a bunsen burner, while slowly pressing the syringes, alternating, and keeping an eye on the volume. 
3. Once the volume stops changing when you press the syringe through, turn off the bunsen burner and wait for it to cool. 
4. Compare the volume you started with, and the volume you ended up with, and calculate the percentage. This is how much oxygen was in there. 

Oxides can be formed by burning elements, for example:

• Burning magnesium forms magnesium oxide, a basic compound
• Burning sulphur forms sulphur dioxide, an acidic compound that can be dissolved to form sulphuric acid
• Burning carbon to form carbon dioxide, an acidic compound which has many uses but also contributes greatly to the enhanced greenhouse effect. 

Carbon dioxide is water-soluble, making it useful for carbonating drinks. The carbon dioxide is dissolved under high pressure, but as this is a reversible reaction when the pressure is released bubbles form.
It is also denser than air, making it useful for smothering fires. CO2 is used in many fire extinguishers for this reason.

Carbon dioxide can be formed by reacting hydrochloric acid and calcium carbonate:
Hydrochloric acid + Calcium carbonate --> Carbon dioxide + Calcium chloride + Water

HCl(aq) + CaCO3(s) --> CO2(g) +CaCl2(s) + H2O(l)

Calcium carbonate could be used as marble or limestone, and dropped into a sealed flask of dilute hydrochloric acid in small pieces. A delivery tube could be placed in the end of the bung to allow for the gas to be collected in the downwards displacement method.

Carbon dioxide can also be formed through the thermal decomposition of a metal carbonate, for example:
Copper (II) Carbonate --> Carbon Dioxide + Copper Oxide
CuCO3 --> CO2 + CuO

Another decomposition reaction is hydrogen peroxide heated with manganese (IV) oxide.
Hydrogen peroxide --> Water + Oxygen

Section 2 d) Specification

2.16 recall the gases present in air and their approximate percentage by volume

Oxygen - 21%
Nitrogen - 78%
Argon - 0.9%
Other (carbon dioxide, water vapour, etc.) - >0.1%

2.17 explain how experiments involving the reactions of elements such as copper, iron and phosphorus with air can be used to investigate the percentage by volume of oxygen in air

If you have a known volume of air, then remove the oxygen by reacting it with an excess of another element (copper, iron, phosphorus) to create a solid, the volume of gas will change. This change can then be used to calculate the percentage of oxygen in the air. This can be done multiple times, and an average found, to increase the accuracy of the results.

2.18 describe the laboratory preparation of oxygen from hydrogen peroxide, using manganese(IV) oxide as a catalyst

Hydrogen peroxide can be decomposed simply by heating with a catalyst - Manganese oxide
Hydrogen peroxide --> Oxygen + Water
The oxygen bubbles created can be collected using the downwards displacement method.

2.19 describe the reactions of magnesium, carbon and sulfur with oxygen in air, and the acid-base character of the oxides produced

Magnesium burns in air, reacting with oxygen, to form magnesium oxide, a basic substance, as it is a metal oxide.

Magnesium + Oxygen --> Magnesium Oxide

Carbon and sulphur are both non-metals, that react with air, giving out heat and light, to form acidic non-metal oxides.

Carbon + Oxygen --> Carbon dioxide

Sulphur + Oxygen --> Sulphur dioxide

2.20 describe the laboratory preparation of carbon dioxide from calcium carbonate and dilute hydrochloric acid

Hydrochloric acid + Calcium carbonate --> Carbon dioxide + Calcium chloride + Water

HCl(aq) + CaCO3(s) --> CO2(g) +CaCl2(s) + H2O(l)

Calcium carbonate could be used as marble or limestone, and dropped into a sealed flask of dilute hydrochloric acid in small pieces. A delivery tube could be placed in the end of the bung to allow for the gas to be collected in the downwards displacement method. 

2.21 describe the formation of carbon dioxide from the thermal decomposition of metal carbonates such as copper(II) carbonate

Copper (II) Carbonate --> Carbon Dioxide + Copper Oxide
CuCO3 --> CO2 + CuO

2.22 describe the properties of carbon dioxide, limited to its solubility and density

Carbon dioxide is a relatively dense gas, it is denser than air. It is water-soluble at high pressure, so when bubbled through water carbonic acid can be formed, and it turns lime water cloudy when dissolved.

2.23 explain the use of carbon dioxide in carbonating drinks and in fire extinguishers, in terms of its solubility and density

When carbon dioxide is dissolved into a liquid at a high pressure, it carbonates it. This reaction is reversible, so this means that when it is returned to atmospheric pressure, bubbles of carbon dioxide form again, and this is how we get fizzy drinks.

Carbon dioxide is used in fire extinguishers because it is denser than air, and is therefore good at smothering fires and preventing oxygen from reaching them. The carbon dioxide sinks over the fire in a blanket and stops oxygen from reaching it, which is necessary for combustion.

2.24 understand that carbon dioxide is a greenhouse gas and may contribute to climate change.

Carbon dioxide is a greenhouse gas, meaning that it traps the sun's energy in the atmosphere, creating the enhanced greenhouse effect with excess. This excess heat that is not able to escape the Earth is a major cause of climate change.