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John F. McCullagh
The physical and chemical properties of Milk of Magnesia provide examples of chemical concepts and opportunities for investigative practical work for secondary chemistry students of all ages.
Basing the lesson on the contents of that familiar blue bottle, which is probably lurking somewhere in a cupboard in most households, also showcases the role of chemistry in the formulation and analysis of everyday pharmaceutical products. At key stage 3 (age 14), the formation of an insoluble suspension of magnesium hydroxide from the mixing of two clear solutions of soluble salts provides a very visual representation of a chemical reaction and the concept of chemical change. At GCSE level (age 15–16), direct titration of a sample of this product with dilute hydrochloric acid results in titre values consistent with the amount of magnesium hydroxide stated on the label and produces a most visual endpoint as the cloudy mixture simultaneously clarifies and changes colour. Using a back titration provides opportunities for calculations at A-level (age 16–19) and extends students’ understanding of quantitative analytical methods.
