Let us now, study the laboratory preparation of some gases, namely: hydrogen, ammonia and carbon dioxide. The focus will primarily be on their preparation, drying, and mode of collection. The choice of method of collection and drying depend upon some of physical characteristics of these gases. In general,
• Gases which are lighter than air, i.e., which have molecular mass less than average molecular mass of air 28.8 can rise upward and can be collected by downward displacement of air.
• Gases which have molecular mass greater than 28.8 i.e., heavier than air tend to move downward and can be collected by upward displacement of air.
• The drying agent employed for drying of the gas should be such which does not react chemically with the gas. The common drying agents are anhydrous calcium chloride (CaC2),,phosphorus (V) oxide (P4 O10), cone. tetraoxosulphate (VI) acid, H2S04), silica gel, etc., because all such substances have the property to absorb moisture.
The students are advised to carry out the preparation of these gases either in fume cupboard or in well ventilated room. Let us now proceed to study the preparation of these gases through activities.