Assertion - Oxygen is collected by the downward displacement of water and not over air.

Reason - Downward displacement of water is used to collect oxygen. Any gas can be collected by this method and the gas pushes the water down as it is lighter and doesn’t react with water.

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Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.

Both Assertion and Reason are correct and Reason is not the correct explanation for Assertion.

Assertion is correct but Reason is incorrect.

Both Assertion and Reason are incorrect.

answer is A.

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Detailed Solution

Concept- Water displacement method is useful in collecting gases that are not soluble in water. The water insoluble gases are oxygen, hydrogen, etc.
Now, oxygen produced in a container is collected in a different collecting bottle which is filled with water. The gas created now replaces the water, since no two bodies can occupy the same space at the same time. As oxygen gas is only slightly soluble in water and has a lower density than water, it is collected by pushing the water downwards.
Therefore, it can be collected over water and it does not undergo excessive dilution. It cannot be collected by forcing air downwards because oxygen is slightly heavier than air and also mixes with other gases in the air.
Thus, both the assertion and reason are correct and reason is the correct explanation for assertion.
Hence, the correct answer is 1.

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