Why do we collect the hydrogen gas by downward displacement of water?  Why not by air? 

• Hydrogen is collected by downward displacement of water as it is less denser than water. Hence it comes out at the surface of water . It is insoluble in water so it does not dissolve in water.
• In case of displacement with air, it is not possible even if hydrogen is lighter than air (14.5 times lighter than air). This method is dengerous as the air and hydrogen mixture is explosive.
• The hydrogen gas is highly combustible and get oxidised when comes in contact of oxygen it burns with a pale blue flame and produces water.
• So, it is impossible to collect hydrogen gas in air medium.
• Also, if anyhow we prevent it from catching fire, then also it may get mixed with other impurities of air and it becomes difficult to get the pure hydrogen gas (dihydrogen).