Question

A reaction of direct combination, i.e. synthesis in which two gases combine to give another gas, turns lime-water milky.

Answer 1

Combination reaction

A combination reaction is a reaction where two or more elements or compounds (reactants) combine to form a single compound (product). It is also known as a synthesis reaction.

Example: $$\begin{gathered} 2\mathrm{CO}\left(\mathrm{g}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)\to 2{\mathrm{CO}}_2\left(\mathrm{g}\right) \\ \mathrm{Carbon}\mathrm{monoxide}\mathrm{Oxygen}\mathrm{Carbon}\mathrm{dioxide} \end{gathered}$$

When carbon monoxide reacts with oxygen in the air, it forms carbon dioxide gas which turns lime water milky.