Air

Air is a mixture of different gases and water vapour. It covers the earth. The thick layer of air that surrounds the earth is called atmosphere. The atmosphere lies within 16 km from the earth’s surface. It also contains dust particles. The air is also found in the soil, water and porous materials such as sponges, bricks, wood, breads, etc. It cannot be seen but can be felt. Living things cannot survive without air. It is the most important things to all the living things. All the living things take in oxygen from the air and give out carbon dioxide during respiration. All the green plants use carbon dioxide of the air to make their food during photosynthesis.

           

Composition of air

            We learn that air is mixture of gases. The main gases of air are nitrogen and oxygen. Nearly four-fifth of the air is nitrogen and oxygen whereas one-fifth of air contains oxygen. Besides these gases, the other gases present in the air are carbon dioxide, water vapour and the inert gases like neon, argon, helium, krypton, xenon and radon.

            The given figure shows the composition of the air on the earth. Plants use nitrogen in the form of fertilizer. Carbon dioxide and oxygen gases are exchanged between atmosphere and photosynthesis. Carbon dioxide is formed in the air by the combustion of fuel. In this way, the composition of the gases in the atmosphere is generally constant everywhere on the earth. However, the amount of water vapour and carbon dioxide may differ from place to place.

            Various activities have shown that the composition of air on the earth by volume is as given in the table.

S.N	Gases in air	Percentage by volume
1. 2. 3. 4. 5. 6. 7.	Nitrogen Oxygen Carbon dioxide Argon Neon Other inert gases Water vapour	78.07 20.98 0.03 0.85 0.002 0.008 0.06

Rusting

Fig: Rusting of iron

            We may have noticed that when shiny iron rods are exposed in a moist place for few days, their outer surface becomes reddish brown. The reddish brown layer on the surface of the iron rod is called rust. This brown rust is reddish due to the oxides of the iron. Thus, the process of formation of rust by the slow oxidation of iron into oxide due to the atmospheric oxygen in the presence of water vapour is called rusting.

Presence of oxygen in air

White phosphorus powder glows green in the presence of oxygen.

           Oxygen is an important gas found in the air.  It constitutes one fifth of the air by volume. All the living thing uses it during the process of respiration. Oxygen in the air also helps in the process of burning.

Carbon dioxide in air

Fig: Carbon dioxide in air

            Carbon dioxide in the air is used by plants to make their food during  the process of photosynthesis. It is given off during the process of respiration by all living things. It is also produced when fuels are burnt in air.

Properties of air

1.Air is mixture of different gases. It is composed of nitrogen, oxygen, carbon dioxide, inert gases and water vapour.

2.Air occupies spaces and has weight.

3.Pure air is colorless, tasteless and odorless.

4.Air exerts pressure and can be compressed.

5.Air can blow from one place to another.

6.Air expands on heating.

Uses of air

1.Air helps to burn things as it contains oxygen. Oxygen is a supporter of burning.

2.Almost the all animals and plants use oxygen during the process of respiration.

3.All the green plants take carbon dioxide to prepare their food during the process of photosynthesis.

4.All the living things need nitrogen. It is the most essential gas that is used to make protein by all the living things. Proteins are essential for life.

5.The larger amount of nitrogen in the air prevents constant fire as it is neither a combustible nor a burning supporter.

6.Air is used to fill in the tires of vehicles, bladder of football, etc.

7.Air helps in the dispersal of seeds and pollination of flowers.

8.The moving air is used in wind-mills as it is a source of energy.

Solubility of Substance

            Scientists have found that all the substances of same weight cannot be dissolved in the same quantity of water at a particular temperature. “The solubility of a substance at a given temperature is defined as the quantity of the substance that can be dissolved in 100 parts of the solvent at that temperature to give a saturated solution”. It means, solubility of a substance at a particular temperature is fixed. Different substances have different solubility at the same temperature.

            At 30°C, 25gm of copper sulphate (CuSO₄) dissolved in 100gms of water forms a saturated solution. The solubility of copper sulphate is 25gm. Similarly, the solubility of potassium nitrate (KNO₃) is 46 gm, sodium chloride (NaCl) is 36gm, sugar is 220gm and sodium nitrate (NaNO₃) is 95gm. The solubility of different substance differs according to the temperature can be understood from below table:

S.N	Name of substance	Temp°C	Solubility gm/100gm water
1. 2. 3. 4. 5.	Copper sulphate Potassium nitrate Sodium chloride Sugar Sodium nitrate	30 30 30 30 30	25 46 36 220 95

           

            The solubility of a substance can be obtained by using the following relationship:

                                Solubility = 100 × Wt. of solute (gm)

                                                               Wt. of solvent (gm)

Example: Find the solubility of sugar if 5.1 gm of it when completely dissolved in 2.5gm of water at 20°C forms saturated.

Solution:

            Here,

            Wt. of solute = 5.1 gm

            Wt. of solvent = 2.5 gm

            Temperature = 20°C

            Solubility =?

We have,

Solubility = 100 × Wt. of solute (gm)

                                Wt. of solvent (gm)

                        = 100×5.1

                                     2.5

                        = 204

Solubility of sugar at 20°C is 204.

Solubility Curves:

            Scientists have found that the solubility of a particular substance is not same at different temperatures. The capacity of solvent to dissolve a substance depends upon the temperature. Therefore, solubility of a substance also depends upon the temperature. Solubility differs according to the change in temperature. This can be understood from solubility curves.

            The solubility of a substance at different temperature when plotted on a graph, we can get a line or curve called solubility curve. In solubility curve, the temperature is plotted on X-axis and solubility against that temperature on Y-axis.

Fig: solubility curve

            A curve obtained by plotting the solubility of a substance at different temperatures against these temperature is known as solubility curve.

            It is observed that in most cases the solubility rises with the temperature. This rise in solubility varies for different substances. It the curve shows the break, it suggests formation of a new hydrate.

Information from solubility curves:

            The solubility curves give us valuable information. These are listed below:

1. At a glance, we know the solubility at different temperature.

2. Solubility of a substance increases with temperature.

3. Solubility of different substances can be compared.

4. If two or more solutes are present, we know which one will crystallize first.

5. Temperature of formation of hydrate is indicated.