I, II, III, and IV represent four processes as shown:

I. Burning of coal

II. Burning of magnesium

III. Heating of chalk powder

IV. Heating of lead nitrate

Which two processes represent combination reactions?

In a combination reaction, two or more substances combine to form only one product.

Burning of coal is a combination reaction. In this reaction, two elements (carbon and oxygen) combine to form carbon dioxide.

$\mathrm{C}\left(\mathrm{s}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)\to {\mathrm{CO}}_2\left(\mathrm{g}\right)$

Burning of magnesium is also a combination reaction. In this reaction, magnesium burns in the presence of air to form magnesium oxide.

$2\mathrm{Mg}\left(\mathrm{s}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)\to 2\mathrm{MgO}\left(\mathrm{s}\right)$

Hence, processes I and II are examples of combination reactions.