Letters - November 2006
Concrete chemistry, propagating a myth and numbering groups in the periodic table
Concrete chemistry
From Peter Borrows
I note that several of the publications supporting the new GCSE science specifications suggest pupils should make concrete using various mixes of sand and cement and then test its strength. If doing this, it would be a pity not to take the opportunity to show quite a spectacular demonstration involving neutralisation and rates of reaction.
Take a fist-sized lump of concrete. If you look, you will find them lying around all over the place. Pour phenolphthalein solution (highly flammable) over the lump. Nothing happens. Cries of 'Boring!' from the class. Place the concrete lump on the floor and, wearing eye protection, hit it hard with a hammer. (The bang will wake up the bored class.) Quickly pour phenolphthalein over the newly-exposed surface. The concrete goes bright pink. The pink colour fades in a few minutes...
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