Which one of the following figures correctly describes the process of electrolytic refining?

Brass is an alloy of

During which reaction is dihydrogen gas not produced under normal conditions?

When finely powdered ore is placed on a rotating grooved table and rotated rapidly,the lighter impurities are removed. What is this method of concentration called?

Iqbal treated a lustrous,divalent element $M$ with sodium hydroxide. He observed the formation of bubbles in the reaction mixture. He made the same observations when this element was treated with hydrochloric acid. Suggest how can he identify the produced gas. Write chemical equations for both the reactions.

Which of the following gases is of brown colour?