Air is a mixture of different gases. The gases that make up the air include nitrogen, oxygen, carbon dioxide, noble gases (argon, helium, neon, krypton and xenon) and a little water vapour. Air may also contain traces of impurities such as carbon monoxide (CO), sulphur dioxide (SO₂), hydrogen sulphide (H₂S) and other gases. The presence of these gases in air results in air pollution. Table bellow shows the composition of air by volume. The proportion of water vapour and impurities in air is very variable.

The composition of air is not exactly the same everywhere. It changes slightly from day to day and from place to place. There is more water vapour in the air on a damp day and in air above water bodies such as oceans, seas, lakes, rivers, etc. Over busy cities and industrial areas there is more carbon dioxide. But the uneven heating of the earth's surface by the sun causes the air to move continually, resulting in winds. The resultant winds spread the pollutants around.

The composition of air is not exactly the same everywhere. It changes slightly from day to day and from place to place. There is more water vapour in the air on a damp day and in the air above water bodies such as oceans, seas, lakes, and rivers. Over busy cities and industrial areas, there is more carbon dioxide. But the uneven heating of the Earth's surface by the sun causes the air to move continually, resulting in winds. The resultant winds spread the pollutants around

1. Experiment. Determination of the presence and proportion of oxygen in air by combustion of a candle

Method

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The oxygen in air enclosed in the measuring cylinder is used to burn the candle to produce carbon dioxide gas. The carbon dioxide so produced dissolves in sodium hydroxide solution. The dissolved carbon dioxide causes the level of sodium hydroxide solution to rise up. The oxygen gas used to burn the candle is practically equal to the amount of carbon dioxide produced. This fact is, therefore, used to calculate the percentage of oxygen in air.

In the experiment, the initial volume of air was found to be 70.5 cm³ and the final volume was 55 cm³. The percentage of oxygen in the air is calculated in two steps:

1. To find the volume of oxygen used up to burn the candle (which is practically equal to the volume of carbon dioxide produced and then absorbed by sodium hydroxide), we subtract the final volume of air from the initial volume

Volume or oxygen used = Initial volume of air – final volume of air

Therefore, the volume of oxygen used for combustion of the candle = 14.7 cm.

Alternatively, the volume of oxygen used up can be calculated by subtracting the initial volume of sodium hydroxide solution from the final volume. That is: Volume of oxygen used = final volume of sodium hydroxide – initial volume of sodium hydroxide = Volume of carbon dioxide dissolved in sodium hydroxide.

Therefore, the percentage of oxygen = 20.8%

In practice, it is difficult to get an accurate result with the above experiment.

This is due to a number of reasons such as:

The experiment in Activity 6.3 involves analysis of oxygen in air by reacting copper with oxygen to give copper(II) oxide. In this experiment;

2. Experiment . Determination of the presence and proportion of oxygen in air by the combustion of copper in air

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2. The final volume of air in the syringe, at the end of the experiment, is less than that of the original volume. This is because oxygen in the original air has combined with copper

The volume of air in the syringe at different heating and cooling is as shown below:

Initial volume before heating = 100 cm³

Volume after first heating and cooling = 82 cm³

Volume after second heating and cooling = 79 cm³

Volume after third heating and cooling =79 cm³

The volume of oxygen used up = Initial volume of air before cooling - volume of air after the last heating and cooling

= 100 - 79

= 21%

Carbon dioxide is present in air to the extent of 0.03% by volume. The gas is formed during the combustion of all common fuels – wood, coal, coke, natural gas, petrol, diesel, paraffin oil, etc, all of which contain carbon.

It is breathed out as a waste product of respiration by all animals. All sorts of combustion and burning produce carbon dioxide. The gas produced by all these processes accumulates in air. However, the amount of carbon dioxide in air remains constant instead of the tremendous quantities released into the atmosphere. This is because plants take up carbon dioxide. They then convert it into complex starchy compounds during photosynthesis. The gas also dissolves in ocean water and other water bodies.

The presence of carbon dioxide in air can be shown by passing air through a test tube containing some limewater (figure 6.5). After a time, the limewater turns milky. This shows the presence of carbon dioxide.

Combustion refers to a reaction of a substance with oxygen which is accompanied by the release of energy in the form of heat and light. Combustion can take place in the open air such as the bonfire, or in a closed system, such as the combustion of fuel in a car engine. Oxygen in air supports the combustion. It is called a supporter of combustion. The gas surrounds any burning material. However, oxygen can sometimes combine chemically with the burning substance to produce a new substance. For example, when copper turnings are heated in air, the oxygen in air combines with copper to form copper(II) oxide.

Combustion in which a flame is produced is described as burning. Many substances can burn in air. During burning, energy is given out in the form of heat, light, and sound. Substances such as coal, wood, kerosene, and petrol burn in air. Any substance that burns is called a combustible material. Various combustible materials burn in air to give different products. For example, the kerosene in a tin lamp burns to produce light and dark smoke (carbon). Magnesium burns to produce whitish magnesium oxide. The items such as paper, piece of cloth, charcoal, wood, dry grass burn up to produce ash

1. The combustion of natural gas is an important source of energy for homes and industries. Natural gas is mainly methane. Complete combustion of methane produces carbon dioxide and water vapour. This reaction is accompanied by the production of heat

2. Burning fuel produces heat, which is used to heat homes, and cook food.

3. Combustion of fuels in automobile engines produces power (energy). The running of automobiles could be impossible without combustion of fuels that produce energy to make them work

4. Combustion of fuel in different burners produces heat and light used for different purposes in a chemistry laboratory

5. Smelting of metals: Smelting is a metallurgy technique to extract base metals from their ores with the use of heat and chemical reducing agents. Moderately reactive metals such as zinc, iron, and lead are roasted in a special furnace (kiln) to form oxides. The resulting oxides are then reduced with carbon to get pure metal. This process of extracting a metal from its ore by heating is called smelting

6. In the metallurgical industries, combustion is used during welding. Welding is the process of joining metals by melting the parts and then using a filler to form a joint. It can be done using different energy sources, including a gas flame

Firefighting is the act of extinguishing destructive fires. A firefighter is a rescuer extensively trained in firefighting, primarily to extinguish hazardous fires that threaten life, property, and the environment. Firefighting is a highly technical profession that requires training and education in order to become skilled

Before starting to fight the fire, it is important to know the size and type of the fire that you are going to put off. The kind of firefighting material you are going to use will also depend on the type of fire in question. Fires are classified based on the type of burning materials.

Before choosing the best fire extinguishers for fighting different types of fires it is crucial to identify the type of burning materials first, and hence the type of fire such as:

Class A: Solids such as paper, wood, clothing, rubber, etc

Class B: Flammable liquids such as paraffin, petrol, oil, spirit, alcohol, etc.

Class C: Flammable gases such as propane, butane, methane, hydrogen, etc

Class D: Metals such as aluminium, magnesium, titanium, etc

Class E: Fires involving electrical equipment such as appliances, circuit breakers and outlets, etc.

To extinguish fire, it is necessary to remove one or more of the three components of combustion. Any fire needs a fuel, oxygen (air) and heat to keep it going. Remove any one of them and the fire will go out. These components are as shown in the fire triangle below.

A fire will continue or start to burn if these components are present:

(i) Fuel: This refers to any combustible material be it solid, liquid or gaseous material provided it can catch fire and burn. You can stop fire by removing the combustible material from the path of fire.

(ii) Oxygen (air): Oxygen supports combustion. A fuel will only burn if there is sufficient supply of oxygen. You can extinguish fire by displacing, or taking away oxygen supply from the fire or by blocking the gas supply to the fire.

(iii) Heat: The temperature should be at the kindling point of that fuel or above it. Every fuel has its own kindling point. Below the kindling point, the fuel will not catch fire. You can put out fire by lowering the temperature below the kindling point of a particular fuel. Water may be used to cool down the fuel. The vapourization of water absorbs the heat; it cools the smoke, air, walls, objects etc, which could be used as further fuel.

Fire extinguishers are classified according to the type of chemicals they contain

Table bellow summarizes the types of fire extinguishers, indicating the chemicals they contain and the classes of fire they are suitable or unsuitable for.

Rusting is the name given to the oxidation of iron or steel in damp air. It is also called corrosion. Rust is hydrated iron (III) oxide. It is a soft, crumbly solid and hence weakens the structure of iron and steel. During rusting, iron reacts with oxygen to form brown iron (III) oxide

At the same time the iron (III) oxide reacts with water to form hydrated iron (III) oxide (or rust):

Note: The x in the equation indicates that the number of water molecules in the hydrated iron (III) oxide can vary. So, both oxygen and water are needed to cause rusting of iron.

Rusting is a serious economic problem. Large sums of money are spent each year to replace damaged iron and steel structures, or protecting structures from such damages. Rusting of bridges, corrugated iron sheets on house roofs, containers, articles, etc. require an expenditure of big sums of money as well as labour for replacement. Rust weakens structures such as car bodies, iron railings, and ships’ hulls, and shortens their useful life. Preventing it can cost a lot of money. All efforts must be made to stop iron or steel items from rusting. This can be achieved if we know the conditions necessary for iron to rust.

When iron is left in contact with both water and oxygen (or air), it reacts to form hydrated iron(III) oxide. Iron will not rust on exposure to dry air or air-free water (water that has been boiled to expel all dissolved air) only. However, iron will easily and readily rust in water containing dissolved air. Therefore, rusting only occurs in the presence of both water and oxygen. If one of the two conditions is excluded, in one way or another, rusting will not take place at all. Consider Figure 6.5; In tube 1, nails are in contact with both water and air (oxygen). Therefore, nails in tube 1 will rust. In tube 2, the water has been boiled to expel the dissolved air. In addition, any air above the water is prevented from dissolving in boiled water by a layer of oil. So, the nails are completely shielded away from air. Therefore, rusting is impossible. In tube 3, nails are in contact with air only. The moisture present in air is absorbed by anhydrous calcium chloride. Any moisture that might have been absorbed by the anhydrous calcium chloride is prevented from reaching the nails by a tuft of cotton wool. The cotton wool also absorbs some moisture directly from the air. Therefore, tube 3 will always carry dry air (moisture-free air). Hence, no rusting of iron nails occurs. This experiment demonstrates that for iron to rust, both water and air (oxygen) must be present. If one of these conditions is controlled, no rusting can take place.

Testing

We have learned that for iron to rust there must be direct contact between the iron and both water and oxygen from the air. Therefore, in order to stop rusting we must protect iron from either water (moisture) or oxygen (air) or both. The following are some of the methods used to prevent iron from rusting: