Separation of The Gases of The Air

Air is a homogeneous mixture of gases like nitrogen, oxygen, argon, carbon dioxide, helium, neon, etc.

Air is a homogeneous mixture and can be separated into its components by fractional distillation of liquid air.

Method

(a) The air is first filtered to remove dust, then water vapour and carbon dioxide are removed. If water vapour and carbon dioxide are not removed, they would become solid in the cooling process and block the pipes.

(b) Air is compressed to a high pressure and then cooled. This cooled air is then allowed to expand quickly into a chamber through a jet. This expansion cools the air even more.

(c) The process of compression, cooling and rapid expansion of air is repeated again and again. This makes the air more and more cool. Ultimately the air gets so cooled that it turns into a liquid. In this way, liquid air is obtained.

Separation of components of air

The liquid air is fed into a tall fractional distillation column from near its bottom and warmed up slowly.

1. Liquid nitrogen which is present in liquid air has the lowest boiling point of – 196°C. So, on heating, liquid nitrogen boils off first to form nitrogen gas.

(b) Liquid argon which is present in liquid air has a slightly higher boiling point of –186°C, so liquid argon boils off next and is collected as argon gas in the middle part of the fractional distillation column.

(c) Liquid oxygen also present in liquid air has the highest boiling point of – 183°C in comparison. So, liquid oxygen boils off last and is collected as oxygen gas at the bottom of the fractional distillation column.

Separation of gases from air

1.0How Can You Prove That Air is A Mixture? 

(a)The composition of air is variable. The composition varies from place to place and with altitude. For instance, at higher altitudes, there is less oxygen in the air. In industrial areas, due to the poison coming out of industrial chimneys in the form of smoke, more impurities are added in the air.

(b) Air has no definite set of properties. Its properties are an average of its constituents. For example, vapour density of oxygen is 16, vapour density of nitrogen is 14 and vapour density of air is 14.4 

(Vapour density = molecular weight/2). 

(c) The components of air can be separated by physical means. Fractional distillation of liquid air can separate N₂ and O₂.

Boiling point of liquid N₂ = –196°C, Boiling point of liquid O₂ = –183°C.

(d) The formation of air does not involve any energy change. No energy is released or absorbed when the constituents of air are mixed in the right proportion. 

(e) Air cannot be assigned a fixed chemical formula.

2.0Obtaining Pure Copper Sulphate From An Impure Sample - Crystallisation 

The process of cooling a hot, concentrated solution of a substance to obtain crystals is called crystallisation. Crystallisation is a process that separates a pure solid in the form of its crystals from its solution.

Method

(a) Take some (approximately 5 g) impure sample of copper sulphate in a china dish.

(b) Dissolve it in a minimum amount of water.

(c) Filter the impurities out.

(d) Evaporate water from the copper sulphate solution so as to get a saturated solution.

(e) Cover the solution with a filter paper and leave it undisturbed at room temperature to cool slowly for a day.

(f) You will obtain the crystals of copper sulphate in the china dish.

This process is called crystallisation.

When a hot saturated solution of copper sulphate is allowed to cool, crystals of copper sulphate separate out. All these crystals have a definite geometrical shape and hence look alike. We can separate these crystals from the liquid (called mother liquor) by the process of filtration.

Crystallisation is a better technique than evaporation to dryness because of the following reasons:

(i) Some solids decompose or get charred- on heating to dryness during evaporation.

(ii) The soluble impurities do not get removed in the process of evaporation. But such impurities get removed in crystallisation.

Application

• Purification of salt that we get from sea water.

• Separation of crystals of alum (phitkari) from impure samples.

3.0Also Read