Answer :
Final answer:
Some gases, like Hydrogen, can be collected by downward displacement of air due to their density being lower than that of air, unlike gases such as Chlorine or Sulphur(IV) oxide.
Explanation:
Hydrogen can be collected by downward displacement of air because it is less dense than air. When collected in this manner, hydrogen displaces the air downward in a collection vessel.
On the other hand, gases like Chlorine and Sulphur(IV) oxide would not be suitable for collection by downward displacement of air due to their density being similar to that of air.
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