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The Boring Magnesium Oxide & Water Reaction

posted Nov 1, 2016, 1:27 AM by Grace Ong   [ updated Nov 1, 2016, 1:27 AM ]
The following post was first posted on Blogger on Sunday, 26 January 2014.

I finally have a post on an inorganic chemistry topic!

My school year began with Group 2 elements, a topic which my students were asked to complete on their own during the year-end holidays. During a tutorial discussion, I asked my students what they would observe when water is added to solid magnesium oxide. As expected, given their lack of practical chemistry experience, a good number of them told me that they would observe either all or some of the solid dissolving to give a clear, alkaline solution.

They only got the alkaline solution part right though. I must point out that when one adds water to magnesium oxide, one does not see an observable change. There is a reaction – no doubt about it, but it cannot be simply ‘observed’; you only know that a reaction has occurred if you test the pH of the mixture with litmus or pH paper.

Before water is added ...
solid MgO in a test tube
a white solid of magnesium oxide, MgO

After water is added ...
suspension forms when water is added to solid MgO

magnesium is insoluble in water a suspension of white solid (of magnesium hydroxide) is observed,
i.e. white solid remains insoluble.

Solid magnesium oxide reacts with water to form solid magnesium hydroxide, Mg(OH)2.
MgO(s) + H2O(l) → Mg(OH)2(s)

On standing ...
litmus test on MgO suspension

MgO reacts with H2O to form Mg(OH)2 the white solid (of magnesium hydroxide) settles to the bottom of the test-tube.
Supernatant liquid above solid is tested alkaline, as red litmus paper turns blue.
(Notice that there is no discernible change in quantity of white solid after the reaction.)

Magnesium hydroxide 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.

Interestingly, there is a big jump in ‘observed reactivity’ with water from magnesium oxide to calcium oxide: there is no observable change when magnesium oxide reacts with water, but calcium oxide reacts so vigorously with water that the amount of heat evolved is enough to bring the mixture to a boil, as the home video below demonstrates!

A note of caution!
If you are planning to try this experiment at home using a glass container, make sure it is made of borosilicate glass (e.g. Pyrex®;) rather than silica-glass. The latter is soluble in hot, concentrated alkali solution, or in molten alkali!