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.

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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

$\ce {MgO + CO2 -> MgCO3}$

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.

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  • $\begingroup$ This is the most likely culprit. It also has nice applications for carbon storage and sequestration. $\endgroup$ – Gimelist Dec 9 '14 at 18:57

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