In the laboratory preparation of oxygen, dried oxygen is usually collected over

Explanation: In the laboratory, oxygen gas is commonly prepared by heating potassium trioxochlorate(V) (\(\ce{KClO3}\)) or hydrogen peroxide (\(\ce{H2O2}\)) with a catalyst. The oxygen produced is dried before collection. To collect dry oxygen, it is passed over a drying agent. A suitable drying agent is concentrated sulphuric acid (H2SO4)</strong), but the question is specifically about the method of collection.

Since oxygen is only slightly soluble in water but reacts with acids, it is not collected over hydrochloric acid or sulphuric acid. Instead, in the lab, the traditional safe method for collecting dried oxygen is by displacement over mercury, because oxygen is insoluble and unreactive with mercury.