SC F1 CHAPTER 5

5.1 What is Air Made Up of ?

The layer of air surrounding the Earth consists of a mixture of gases.  This layer is called the atmosphere.  The atmosphere is not formed by a compound nor by an element.  It is a mixture of gases we call air and is composed principally of nitrogen and oxygen.  Air is a mixture of nitrogen, oxygen, carbon dioxide, and inert gases.

Nitrogen

- biggest component of air.  

- percentage by volume is 78%.

- an inactive gas. 

- colourless, has no smell and no taste, does not support combustion and is inert (not active) chemically.  

- serves a useful propose in diluting the oxygen in the air and thus slows down its action..

Oxygen

- makes up one-fifth of the air by volume. 

- has no colour, taste or smell. 

- All living things need oxygen for cell respiration. 

- also found in water. Aquatic plants and animals take in dissolved oxygen from the water. 

- Beside respiration oxygen is necessary for burning, rusting and decay. 

- released during the process of photosynthesis. 

- in the air can be compressed into its liquid state and stored in cylinder for the use of firemen, divers, and hospital emergencies.

Carbon Dioxide

- only makes up 0.03% of the air by volume. 

- very important gas for green plants because it is needed for photosynthesis. 

- removed from the air by green plants during photosynthesis. 

- added to the air by processes such as combustion, respiration, and decaying.

Inert gases

- Inert gases in air are helium, neon, argon, krypton, xenon and radon. 

- very inactive.

Water vapour, microorganisms and dust

- present in amounts which vary greatly from place to place and from time to time to make up the remaining components of air.  

- Most microorganisms in air are bacteria and spores from fungi.

The composition of air

Nitrogen-78%

Oxygen -21%

Carbon dioxide -0.003%

Inert gases and others (water vapour, microorganisms and dust – 0.97%

Why is Air a Mixture?

Air is a mixture of various substances. This is explained below.

1.  (a) The proportion of the constituent gases, namely nitrogen, oxygen and carbon dioxide is not constant.

      (b) The contents by (percentage) of air vary from place to place. For example, the air in urban areas contains more dust and carbon dioxide.

      (c) The components are mixed and vary in quantity.

2.  (a) The constituents of air are not chemically combined.

      (b) The constituent gases retain their individual properties.  

      (c) Nitrogen, oxygen and carbon dioxide can be extracted by the physical means of cooling and stored as liquefied gas in gas tanks.

3.  (a) The properties of air are the same as the properties of its constituents.

     (b) Oxygen in air can be used in process such as respiration, burning, decaying and rusting.

     (c) Carbon dioxide in air is used for the photosynthesis in green plants.

4.  (a) There is no heat change when there is a change in the percentage of the constituents of air in any particular area.

5.2 The Properties of Oxygen and Carbon Dioxide

The Properties of Oxygen

- colourless, odourless and tasteless gas. 

- slightly soluble in water. 

- neutral when tested with moist litmus paper. 

- It is slightly denser (heavier) than water.

- extremely active element. 

- supports burning, breathing, decaying and rusting. 

- combines easily with many metals and non-metals. 

- does not burn by itself.

The Properties of Carbon Dioxide

- colourless, odourless gas with a slight acidic taste. 

- denser (heavier) than water. 

- slightly soluble in water. 

- does not burn by itself. 

- does not support combustion. 

- very soluble in sodium hydroxide solution. 

- turns moist blue litmus paper to a faint red; hence, it is a weak acid. It turns lime water cloudy.

Confirmatory test for oxygen

Glowing wooden splinter that bursts into flame shows the existence of oxygen.

Confirmatory test for carbon dioxide

Carbon dioxide turns the limewater cloudy.

Carbon dioxide turns the colour of the bicarbonate indicator from purple to yellow.

Property	Oxygen	Carbon dioxide
Colour, smell and taste	Colourless, odourless, tasteless	Colourless, odourless, tastes a bit sour
Solubility in water	Slightly
Solubility in sodium hydroxide solution	Not soluble	Very soluble
Solubility in alkaline pyrogallol solution	Very soluble	Not soluble
Density	Denser than air
Test with moist litmus paper	No effect (neutral)	The colour of blue litmus paper changes to red (acidic)
Test with bicarbonate indicator		The purple colour of the bicarbonate indicator changes to yellow (acidic)
Test with a glowing wooden splinter	Ignites	Extinguishes
Test with a lighted wooden splinter	Burns brighter
Test with lime water	No change	Turns lime water cloudy

 

5.3 Oxygen is Needed in Respiration

- Living things need food to survive. 

- Food gives us energy. 

- The process where our body obtains energy from food is called respiration. 

- Respiration takes place in the cells of our body.

- During respiration, food in the form of glucose is oxidised by oxygen. 

- The products of respiration are energy, carbon dioxide and water vapour. 

- Living things use the energy to carry out living processes such as breathing, moving and reproduction. 

- Carbon dioxide and water vapour are waste products that need to be removed from the body.

- Living things obtain the oxygen needed for respiration through breathing. 

- During breathing, living things take in oxygen from air and remove carbon dioxide and water vapour.