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All raw and untreated gases are mixtures that contain moisture (water vapour)
All gases (air) can hold a certain mass of water vapour per unit volume of air
This is why we measure humidity, also known as a concentration of water vapour of the air
Why do gases we collect in experiments contain water?
In experiments, the gases we collect are often a product of chemical reactions. Some examples that we have learnt include:
acid + metal → salt + hydrogen (gas)
acid + metal carbonate → salt + water + carbon dioxide (gas)
alkali + ammonium salt → salt + water + ammonia (gas)
The reactants and products of these chemical reactions are in the form of solutions, which contain water
Acids and alkalis are solutions
Salts are usually dissolved in the water to form a salt solution
Therefore, it is expected that any gases that are evolved from these chemical reactions must contain water.
Why is raw, untreated gases containing water a problem?
Gases that are not dried and contain water are considered impure. Water itself can act as a reactant that can undergo chemical reactions.
Examples of chemical reactions where water can react include:
Alkali metals (e.g., sodium, potassium) can react with water
Video: Reaction of sodium in water (Link)
If water is not removed from gases collected, this can result in the formation of other impurities when the gases are used as reactants in other chemical reactions.
Purpose of Drying of Gases
Drying of gases are required to remove moisture so as to obtain a pure and dry gas
This is achieved through the use of substances known as drying agents (desiccant). The three classic drying agents used in experiments are:
concentrated sulfuric acid
quicklime (calcium oxide)
fused calcium chloride
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