# 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.

• Welcome to Chemistry.SE! To acquaint yourself with this page, take the tour and visit the help center. Furthermore this tutorial shows you how math and chemical formulae can be nicely formatted on this site. Finally, we have an important policy: your questions (especially homework questions), should show your own work or thinking that you have already done in an initial attempt to answer the question. – Philipp Dec 8 '14 at 23:31

Carbonate contamination in oxides compounds often happens after a certain time and air exposure because of the $\ce {CO2}$ in the air.
$\ce {MgO + CO2 -> MgCO3}$
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.