Why is concenration of hydrogen peroxide difficult?

The hydrogen peroxide prepared whether in the laboratory or on a commercial scale is quite dilute. Hence, it has to be concentrated. The concentrated of hydrogen peroxide is done very carefully because it decomposes readily on heating to water and oxygen.
  
The explosive decomposition of hydrogen peroxide is catalysed by the presence of organic matter and metallic ions. The concentration of the aqueous dilute solution having H₂O₂ is done carefully by the following methods:
(i) By cooling: The solution is frozen at 273 K. Some of the water in it separates as ice. The remaining liquid is now richer in hydrogen peroxide.
(ii) By evaporation: Hydrogen peroxide obtained above is evaporated on a water bath at about 345K. Water evaporates slowly and hydrogen peroxide of about 50% strength is obtained.

(iii) Vacuum desiccator evaporation: 50% hydrogen peroxide is placed in vacuum desiccator which contains concentrated H₂SO₄ (dehydrating agent). Concentrated sulphuric acid absorbs most of the water from hydrogen peroxide solution and nearly 90% hydrogen peroxide is obtained.
(iv) Vacuum distillation: 90% hydrogen peroxide obtained above is subjected to distillation under reduced pressure when water distills over at about 308-313 K at a pressure of about 15 mm and the hydrogen peroxide left is about 99% pure.
(v) Freezing with solid carbon dioxide : By freezing liquid H₂O₂ (99%) in a freezing mixture of solid carbon dioxide (dry ice) and ether, crystals of H₂O₂ separate out. These are dried and melted to get 100% pure hydrogen peroxide.