(a) The blue colour of crystals of a substance changed on heating in a closed test tube but the colour was regained after sometime on cooling. Name the substance and write its chemical formula. Explain the phenomenon involved. |
(b) Write name and chemical formula of two such compounds whose formula unit is associated with 10 and 2 water molecules respectively. |
Answer:
(a) The substance is Copper sulphate crystal?s \[(CuS{{O}_{4}}.5{{H}_{2}}O)\] which are blue in colour. When copper sulphate crystals are heated strongly, they lose all the water of crystallisation and forms anhydrous copper sulphate (which is white): \[\underset{\begin{smallmatrix} \text{Hydrated copper} \\ \text{ sulphate(Blue)} \end{smallmatrix}}{\mathop{CuS{{O}_{4}}.{{H}_{2}}O}}\,\xrightarrow{\text{Heat}}\underset{\begin{smallmatrix} \text{Anhydrous} \\ \text{ Copper} \\ \text{sulphate(White)} \end{smallmatrix}}{\mathop{CuS{{O}_{4}}}}\,+\mathrm{ }\underset{\begin{smallmatrix} \text{Water} \\ \text{(goes} \\ \text{away)} \end{smallmatrix}}{\mathop{\mathrm{ }5{{H}_{2}}O}}\,\] Thus, on strong heating, blue copper sulphate crystals turn white due to the loss of water of crystallisation. The dehydration of copper sulphate crystals is a reversible process. So when water is added to anhydrous copper sulphate, it gets hydrated and turns blue due to the formation of hydrated copper sulphate. \[\underset{\begin{align} & \text{Anhydrous} \\ & \text{Copper} \\ & \text{Sulphate} \\ & \text{(White)} \\ \end{align}}{\mathop{CuS{{O}_{4}}}}\,+\underset{\text{Water}}{\mathop{5{{H}_{2}}O\mathrm{ }}}\,\mathrm{ }\to \mathrm{ }\underset{\begin{smallmatrix} \text{Hydrated copper} \\ \text{sulphate (Blue)} \end{smallmatrix}}{\mathop{CuS{{O}_{4}}.5{{H}_{2}}O}}\,\] (b) Chemical Formula of two such compounds whose one formula unit is associated with 10 and 2 water molecules respectively- ? Washing soda - \[N{{a}_{2}}C{{O}_{3}}.10{{H}_{2}}O\] (Sodium Carbonate Decahydrate) ? Gypsum - \[CaS{{O}_{4}}2{{H}_{2}}O\] (Calcium Sulphate Dihydrate)
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