The Boring Reaction Between Magnesium Oxide And Water

magnesium oxide

(Source: Wikimedia Commons)

What do you observe when water is added to solid magnesium oxide, MgO? I often ask this of my students and the common answer given is that they expect some, if not all, of the solid MgO to dissolve to give a clear, alkaline solution.

They only got the alkaline solution part right though. I must point out that when water is added to a white solid sample of MgO, no visible change is observed. There IS a reaction, no doubt about it, but it cannot be ‘observed’ with the naked eye; you only know that a reaction has occurred if you test the pH of the mixture with litmus or pH paper.

1 ——

Before water is added, a white solid sample of MgO sits in a test-tube.

2 ——

After water is added, a suspension of white solid is observed.

Explanation:
Solid magnesium oxide reacts with water to form magnesium hydroxide, Mg(OH)₂, which is also a white solid. Since Mg(OH)₂ is sparingly soluble in water, no visible change is observed.

MgO(s) + H₂O(l) → Mg(OH)₂(s)

3 ——

On standing, the white solid of Mg(OH)₂ settles. The supernatant liquid above the solid is tested alkaline when red litmus paper turns blue.

(Notice that there is also no discernible change in quantity of white solid after the ‘reaction’.)

Explanation:
Mg(OH)₂ is only sparingly soluble in water. Thus, the amount of solid does not decrease significantly for a change to be observed.

Mg(OH)₂(s) ⇌ Mg²⁺(aq) + 2OH^–(aq)

The solution above the white solid is tested alkaline due to the presence of a very low concentration of OH^– ions, formed when a small amount of the Mg(OH)₂ dissolves.

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Even though calcium is just below magnesium in Group 2, interestingly, there is a HUGE jump in the ‘observed reactivity’ from MgO to calcium oxide, CaO. While there is no visible change when MgO reacts with water, CaO reacts so vigorously with water that the amount of heat evolved is enough to bring the mixture to a boil, as the following home video below demonstrates!

As with MgO, CaO reacts with water to form Ca(OH)₂, which is more soluble in water than Mg(OH)₂.

CaO(s) + H₂O(l) → Ca(OH)₂(s)

Ca(OH)₂(s) → Ca²⁺(aq) + 2OH^–(aq)

You would therefore expect the resulting Ca(OH)₂ solution to be more alkaline than the supernatant liquid above the solid Mg(OH)₂, due to a higher concentration of OH^– present.