Magnesium oxide melting point

This week, Simon Cotton is lighting things up.

© Science Photo Library

Simon Cotton

Many British people thinking back to their schooldays will remember seeing the burning of magnesium ribbon, possibly even doing the experiment themselves. You may recall the instruction of your teacher to ‘look away’ or ‘not to stare at the bright light’ or ‘put your hands over your eyes’. Then the piece of magnesium would be lowered into the Bunsen flame, and would soon ignite. You’d be looking away by now, but would be conscious of a blinding white light – which is why magnesium is used in signal flares. After it finished burning, you’d be left with a white ash, a solid that easily broke up to a powder. This powder is mainly magnesium oxide, MgO.

The hot magnesium supplies enough energy for oxygen molecules to split up, breaking the very strong O=O double bond. Each magnesium atom gives two electrons to an oxygen atom, so magnesium oxide is made of magnesium ions, Mg2+, and oxide, O2– ions.

You’ll notice I said mainly MgO. Magnesium burns so vigorously that there is even enough energy to break the even stronger N–N triple bond in molecules of nitrogen and form magnesium nitride, formula Mg3N2, which is made of magnesium ions, Mg2+, and nitride, N3– ions.

To make pure MgO, one way is to burn magnesium ribbon in pure oxygen, but metallic magnesium is quite expensive. A better and cheaper way is to heat magnesium carbonate or magnesium nitrate until they decompose.

‘Many of you will recognise the blue glass bottles bearing the label ‘milk of magnesia’, which is a suspension of magnesium hydroxide in water’

Magnesium oxide is a compound of a metal with a non-metal, so – as you would expect – it is held together with ionic bonds. It has the same structure as NaCl but because it is made of Mg2+ and O2– ions, rather than the singly charged Na+ and Cl– ions, there is a much stronger attraction between the ions. This means it takes a lot more energy to pull them apart, so MgO has a melting point of 3125K, nearly three times the melting point of 1074K for sodium chloride. This strong attraction between the ions is also responsible for magnesium oxide being much less soluble than sodium chloride.

Because of its high melting point, magnesium oxide finds many uses which apply its refractory properties, so it can be used as a furnace lining and in crucibles. It is also used for fireproofing boarding for panelling walls.