Question

Assertion : Oxygen is collected by downward displacement of water and not over air
Reason : Downward displacement of water is used to collect oxygen. Any gas be collected by this method and the gas pushes the water down as it is lighter and does not react with water.
(A) Both Assertion and Reason are correct and Reason is the correct explanation for Assertion
(B) Both Assertion and Reason are correct and Reason is not the correct explanation for Assertion
(C) Assertion is correct but Reason is incorrect
(D) Both Assertion and Reason are incorrect

Answer 1

Hint: An attempt to this question can be made by understanding the process of downward displacement of water. Determine if oxygen is obtained from the process. Now try to identify the reason if the assertion statement is true and understand if the reason follows with the assertion and make conclusions regarding the 2 statements.

Complete step by step answer:
Oxygen is collected by the downward displacement of water only because it is only slightly soluble in water and relatively less dense than water. Therefore, it can be collected over water without excessive dilution which is done usually for collecting other gases.
On the other hand, oxygen cannot be collected by downward displacement of air. This is because oxygen is slightly heavier than air as well as it will get mixed with other gases present in air.
From the above statements we can conclude that the assertion statement as well as reason statement are true and reason is the correct explanation of the assertion statement.

Therefore, the correct answer is option (A).

Note: It is important to know that hydrogen is lighter than air and yet cannot be collected by downward displacement of air. This is because hydrogen gets oxidised in the presence of oxygen which makes it burn with a pale blue flame and water is obtained as the residue.