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GCSE Chemistry - Air and Water

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Air and Water - The Atmosphere

Whilst the earth was forming intense volcanic activity caused a very different composition of the atmosphere than is present today. Carbon dioxide and water vapour made up the majority, as they are the primary products of combustion, with smaller amounts of ammonia and methane.

As time progressed the carbon dioxide declines as the gas dissolved in the oceans, leading to the formation of sedimentary rocks; and was absorbed by plants in photosynthesis, leading to the formation of fossil fuels, such as coal.

The amount of water vapour in the atmosphere declined as the temperature of the Earth dropped, forming the oceans.

Whilst photosynthesis requires carbon dioxide, as well as sunlight and water, it gives out oxygen. Therefore, as the amount of carbon dioxide declined the amount of oxygen increased.

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Air and Water - Oxygen

(1) Percentage of O2 in air :

The volume of oxygen present in air can be found by passing air over hot copper turnings, as shown in the diagram below,

The oxygen will react with the hot copper and the overall volume of gas in the syringes will drop. The copper starts off a pinky-red colour and after the reaction is complete it is black, the classic colour of copper(II) oxide, CuO.

Equations -

copper + oxygen → copper(II) oxide

2Cu(s) + O2(g) → 2CuO(s)

The percentage volume of oxygen in the sample of air can be calculated using the following mathematical formula,

% oxygen = volume of gas before heating-volume of gas after heating x 100

volume before heating

The value should come out to about 21%. The rest of the air is composed of about 78% nitrogen, N2, with about 0.93% argon, 0.03% carbon dioxide and traces of other noble gases making up the remainder.

Test for oxygen -

Oxygen is tested for by placing a glowing splint into the gas. If the splint relights then oxygen is present.

N.B.: Do not confuse the test for oxygen with the test for hydrogen which involves a burning splint.

(2) Reaction with other elements :

Oxygen reacts with both metals and non-metals in what is called an oxidation reaction, although it could also be called simply combustion.

When oxygen reacts with metals it produces basic oxides, i.e. compounds that have a pH greater than 7.

Exemplar equations -

magnesium + oxygen → magnesium oxide

2Mg(s) + O2(g) → 2MgO(s)

Try balancing this equation,

K(s) + O2(g)K2O(s)

When oxygen reacts with non-metals it produces acidic oxides, i.e. compounds that have a pH less than 7.

Exemplar equations -

sulphur + oxygen → sulphur dioxide

S(s) + O2(g) → SO2(g)

Try balancing this equation,

P(s) + O2(g)P4O10(s)
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Air and Water - Water Hardness

Hardness in water is a consequence of the dissolved calcium and magnesium ions. These ions prevent the proper function of soap molecules, by forming insoluble calcium and magnesium salts with the soap molecules, called scum.

There are two types of hardness, resulting from different compounds - permanent and temporary hardness. Permanent hardness is caused by dissolved calcium and magnesium sulphate.

Temporary hardness is caused by calcium and magnesium hydrogencarbonate. These compounds are formed by the action of carbon dioxide and water on solid limestone and dissolved carbonate salts. The carbon dioxide and water form a weak acid, carbonic acid, H2CO3, which reacts with the carbonate as normal acids do.

Equations -

calcium carbonate + carbon dioxide + water → calcium hydrogencarbonate

CaCO3(s) + CO2(g) + H2O(l) → Ca(HCO3)2(aq)

Solubilities -

Compound Solubility in g per 100 cm3 of water
CaSO4 0.0149
CaCO3 0.000693
Ca(HCO3)2(aq) 16.6
MgSO4 33.7
MgCO3 0.039

Removing hardness :

There are two main methods of removing hardness in water. They both rely on the need to remove the calcium and/or magnesium ions from the water.

The simplest method is to boil the water. This only removes temporary hardness (hence the name), by forming a precipitate of calcium carbonate.

Equations -

calcium hydrogencarbonate → calcium carbonate + carbon dioxide + water

Ca(HCO3)2(aq) → CaCO3(s) + CO2(g) + H2O(g)

This provides the fur in kettles, washing machines and central heating pipes. The sulphate ions cannot be decomposed by heating in this manner, so permanent hardness is not removed by boiling.

The other method of removing hardness involves a process called ion exchange. The water is passed through a column full of a gel-like material containing sodium ions. These sodium ions pass into the water as it flows through, and the calcium ions are left behind in the column.

This process removes both temporary and permanent hardness as it targets the metal ions themselves and not the non-metal sulphate or hydrogencarbonate ions. Since all sodium compounds are soluble in water no scum will form when soap is added to the water.

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written by Dr Richard Clarkson : © Saturday, 1 November 1997

updated : Sunday, 15th July, 2012

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