CHAPTER 5: THE AIR AROUND US

1) Air

(a) Composition of air:

- Air is matter because it has mass, volume and occupies space.

Percentage by volume Component of air78.1% Nitrogen20.9% Oxygen0.97% Inert gases such as argon, helium, neon, xenon, krypton0.03% Carbon dioxide

Variable amount Water vapour, carbon monoxide, sulphur dioxide, oxides of nitrogen, bacteria, dust, smoke, grit, chlorofluorocarbons and compounds of lead

- Air is mixture because:→ It can be obtained easily by mixing correct gases in right proportions.→ The components of air can be separated by using physical methods such as fractional distillation.

2) Properties of oxygen and carbon dioxide

Property Oxygen Carbon dioxideColour, smell Colourless and odourless Colourless and odourlessDensity Slightly denser than air Denser than oxygen and airSolubility in water Slightly soluble in water Slightly soluble in water to

form a weak acidBurning ability Supports burning Does not support burningReaction with damp litmus paper

Neutral Changes the colour of damp litmus paper from blue to pink

Reaction with limewater

Does not react with limewater Turns limewater chalky

Reaction with sodium hydroxide solution

Slightly soluble in sodium hydroxide solution

Very soluble in sodium hydroxide solution

(a) Identification tests:

i) Oxygen- use a glowing splinter

* Procedure- Light a wooden splinter and then blow off the flame to obtain a glowing splinter.- Quickly lower the glowing splinter into the gas to be tested.

* Conclusion- If the glowing splinter bursts into flames, then the gas is oxygen.

ii) Carbon dioxide- use clear limewater

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* Procedure- Pass gas to be tested through a delivery tube into a test tube of limewater.

* Conclusion- If the limewater turns chalky and white solid particles are formed, then the gas is carbon dioxide.

3) Oxygen and Respiration

- Oxygen needed for respiration.- During respiration, a chemical reaction occurs between food and oxygen to produce carbon dioxide, water and energy.

Glucose + Oxygen Carbon dioxide + Water + Energy

* Differences between inhaled air and exhaled air:

Inhaled air Exhaled airContains more oxygen Contains less oxygenContains less carbon dioxide Contains more carbon dioxideContains less water vapour Contains more water vapourIs cooler Is warmer

4) Oxygen and Combustion

- Combustion is a chemical reaction between a substance and oxygen which produces an oxide, light and heat.

Carbon + Oxygen Carbon dioxide + Energy (heat and light)

5) Air pollution

- Addition of unwanted and harmful substances into the air is known as air pollution.

(a) Gaseous air pollutants:

Gaseous air pollutant

Carbon monoxide Sulphur dioxide Oxides of nitrogen

Properties A very poisonous, colourless and odourless gas

A harmful, colourless and poisonous gas with irritating odour

Poisonous gaseous compounds

Sources The incomplete burning of carbon (charcoal, petrol and oil)

- The burning of fossil fuels that contain sulphur (coal and oil)- Active volcanoes

Burning of oil and coal at very high temperatures in power stations, factories and motor vehicles

Harmful effects

- Leads to anoxia (lack of oxygen in the body) which causes damage to the nervous system, brain damage & death- Kills unborn babies

- Forms acid rain- Destroys plants- Causes respiratory problems such as breathing difficulties, bronchitis & lung cancer

- Forms acid rain- Destroy plants- Damages our lungs

(b) Solid air pollutants:

Solid air pollutant

Chlorofluorocarbon (CFC)

Dust, smoke, grit or asbestos

Compounds of lead

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Properties - They are broken down into chlorine atoms when exposed to light. The chlorine atoms decompose ozone into oxygen.- They are greenhouse gases.

They can remain suspended in high altitudes for long periods of time.

They are poisonous; fine particles suspended in the air.

Sources - They are released into the air from aerosols that contain CFCs such as deodorants, pesticides, cleaning solvents and foam.- They escape from leaked pipes in cooling systems that use CFCs

- Natural emissions come from volcanoes.- Man-made emissions from cement manufacturing factories, paper making factories and construction sites add these pollutants to the air.

The burning of leaded petrol or substances containing lead produces lead compounds.

Harmful effects

- CFCs cause the thinning of the ozone layer; Eventually the ozone layer may not be able to protect us from the sun’s harmful ultraviolet rays.- Overexposure to ultraviolet rays causes skin cancer and cataracts.- CFCs trap heat in the atmosphere thereby increasing the earth’s temperature.

- They cause respiratory problems and damage to the lungs.- They cause haze that reduces the rate of photosynthesis as sunlight is blocked by the particles from reaching the plants.- They cause lung cancer.

- They cause lead poisoning.- They damage the brain.

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