Which property of concentrated sulphuric acid is in action when sugar turns black in its presence?

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Solution

Dehydration Property
The black color of sugar is due to the presence of carbon.
Sugar turned to black means the atom of $H and O$ is released from the sugar molecule.
This can only happen if concentrated Sulphuric acid is added to sugar.
Here $H and O$ atoms are released which is the constituent of water.
So, dehydration of sugar is taking place.
$C_{6} H_{12} O_{6} (aq) \overset{conc . H_{2} {SO}_{4}}{\to} 6 C {(s)}_{} + 6 H_{2} O {(g)}_{} Glucose carbon steam$
So, The dehydration property of concentrated sulphuric acid is responsible for the black color of sugar.

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Give reasons for the following:
(i) In the contact process, sulphur trioxide formed is dissolved in 98% sulphuric acid and not in water.
(ii) When solution of sulphur dioxide is exposed to air, it gets converted to sulphuric acid.
(iii) When diluting concentrated sulphuric acid, the acid should be added to water and not water to the acid.
(iv) When concentrated sulphuric acid is exposed to air, its volume increases and it becomes slightly dilute.
(v) Sulphuric acid can form two kinds of salts with sodium chloride.
(vi) When barium chloride is added to dilute sulphuric acid, a white precipitate is formed.
(vii) When carbon is heated with concentrated sulphuric acid, carbon dioxide is formed.
(viii) Ammonia gas cannot be dried by passing through concentrated sulphuric acid.
(ix) When concentrated sulphuric acid is added to sugar/glucose, a black mass is left behind.
(x) Concentrated sulphuric acid should not be added to oxalic acid or formic acid in the open laboratory.
(xi) When concentrated sulphuric acid is added to blue crystalline copper sulphate, it turns powdery white.
(xii) Concentrated sulphuric acid must be stored in airtight bottles.
(xiii) Cotton clothes get burnt with concentrated sulphuric acid.
(xiv) H₂SO₄ cannot be prepared by heating sodium sulphate with conc. HCl or HNO₃.

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