Q. 56

Why can dilute solutions of hydrogen peroxide not be concentrated by heating. How can a concentrated solution of hydrogen peroxide be obtained?

Answer :

The dilute solutions of hydrogen peroxide cannot be concentrated by simple heating. This is because hydrogen peroxide readily decomposes below its boiling point.So, to obtain a concentrated solution of hydrogen peroxide, the following steps are needed to done – The diluted solution is carefully heated on a water bath. Doing slow evaporation, helps the release of water molecules and we obtain 50% solution of H2O2.The 50% solution is placed in vacuum desiccator over concentrated sulphuric acid. The dehydrating nature of sulphuric acid absorbs water vapours and we get 90% solution of H2O2. The 90% solution of H2O2 is then subjected to distillation under reduced pressure. Water distils over at 303 – 313 K and we get 99% pure H2O2 solution.


The 99% solution of H2O2 is cooled in a freezing mixture of solid carbon dioxide and ether. As a result, crystals of hydrogen peroxide separate out which are removed. This gives completely pure hydrogen peroxide.

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