Which of the following will dissolve in water to produce an acidic solution?

Question

A. CO₂

B. CaO

C. MgO

D. Na₂O

Correct answer is A, but how would you go about to solve this? And how would you do it if it asked for basic solution with different answers? Thank you so much! :)

Answer 1

So the best way to approach this is to write out what each of those reactions would like with water.

Na2O + H2O -----> 2NaOH

This is a basic solution when reacted with water, as NaOH will disassociate in water to form Na+ and OH-.

CaO + H2O -----> Ca(OH)₂ (Same as above).

MgO + H2O -----> Mg(OH)₂ (Same as above).

These molecules will form an X-OH molecule which is also an ionic compound. When dissolved in a polar solvent such as water, the atoms disassociate as the only thing keeping them together are the electromagnetic forces between the two atoms, which is broken by the polar water molecules.

Lastly, we have CO2, which is a covalent compound, so for this reaction to occur the bonds must break creating carbonic acid, which will dissolve AND react with water to form an acidic solution.

CO2 + H2O -------> H2CO3

Carbonic acid will disassociate to form H+ and HCO3- in water, lowering the pH of the solution (increasing acidity).

I hope this helps! I know this was a pretty dense answer, let me know if you need any clarification!