Why is Hydrogen although lighter than air, is not collected by the downwards displacement of air?

Hint: Consider the requirements of the method for collection of a gas by downward displacement of air. Identify the problems we will phase if we collect hydrogen gas by downward displacement of air.

Complete answer:
If a gas is lighter than another gas, then we can collect the first gas by the downward displacement of other gas. For this purpose, we must ensure that these two gases do not mix with each other. If two gases mix with each other, then we cannot separate them. So, we cannot use the downward displacement method. We must also ensure that two gases do not react with each other. If two gases react with each other, then they form a different product. So, we will not be able to use the downward displacement method. Third, we must ensure that when two gases come in contact with each other, they should not form explosive mixtures or they should not burn in the air. This is from a safety point of view. This precaution is necessary to avoid accidents.
Hydrogen gas is lighter than air, but we cannot collect the hydrogen gas by the downward displacement of air. This is due to the following reasons.
Hydrogen gas and air mix with each other. Since they mix with each other, we cannot separate them. Thus, we cannot obtain hydrogen gas from the mixture of hydrogen and air. So, we cannot use the downward displacement method.
Hydrogen gas and oxygen gas (from air) react with each other to form water. So, we will not be able to use the downward displacement method.
When hydrogen gas and air come in contact with each other, they form explosive mixtures and hydrogen gas burn in the air with pale blue flame. This is not good from a safety point of view. This may lead to accidents.

Note: Instead of using downward displacement of air for hydrogen gas, we can use downward displacement of water. Hydrogen gas is not miscible in water and does not react with water.
Also, hydrogen gas does not catch fire.