Old samples of magnesium oxide become contaminated with magnesium carbonate. Suggest how this contamination takes place Old samples of magnesium oxide become contaminated with magnesium carbonate. Suggest how this contamination takes place.

I don't understand what I'm meant to put for this.
Carbonate contamination in oxides compounds often happens after a certain time and air exposure because of the $\ce {CO2}$ in the air.  

For your magnesium oxide it follows the reaction  

$\ce {MgO + CO2 -&gt; MgCO3}$  

with $\ce {MgCO3}$ being the magnesium carbonate.

A solution to prevent this kind of polution is to keep the oxide in a dessicator or to treat them in a furnace at a temperature around to $500 °C$ wich is the decomposition temperature of most of the carbonate compounds.