Q26 of 79 Page 3

Explain
i) Why sugar chars (turns black) when conc. H2SO4 is added to it?
ii) What is the blue colour of crystalline copper sulphate due to?

i) Conc.H2SO4 is a dehydrating agent. Sugar is C12H22O11. When conc. H2SO4 reacts with sugar, it removes hydrogen and oxygen atoms from the sugar molecules in the form of water, leaving behind carbon. So sugar becomes black or get charred with conc. H2SO4. C12H22O11 12C(s) + 11H2O(l)ii) Blue colour of crystalline copper sulphate is due to five water molecules of hydration in CuSO4 5H2O.

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24 The copper vessels used in homes are found to be coated with green colour. Answer the following.
a) What is the chemical composition of the green coating?
b) Why do copper vessels form such a green coating?
c) What is the name of the phenomenon, responsible for the green coating?
d) Copper vessels are best cleaned with tamarind or lemon juice, but not with soap. Why? 25 Give reasons for the following
i. Silicon counts among metalloids
ii. Carbon is not used for making aluminium from aluminium oxide.
iii. For making hydrogen by reaction with hydrochloric acid, granulated zinc is preferred to a block of zinc. 27 Write chemical equations for reactions taking place when
i. Zinc carbonate is calcined.
ii. Ammonia gas is passed over heated copper (II) oxide. 28 (i) What is the colour of potassium dichromate solution?
(ii) What is the colour of Cr3+ ion?
(iii) What property of SO2 is responsible for bringing change of colour of potassium dichromate?