When conc. sulphuric acid comes in contact with sugar it becomes black due to:

Dehydration

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Hydrolysis

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Hydration

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none of these

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Solution

The correct option is A Dehydration
Sugar is a carbohydrate that is fructose, $H_{2} S O_{4}$ is a dehydrating agent so if you remove the water from the molecule, you are left with elemental carbon.

$C_{1} 2 H_{2} 2 O_{1} 1 (sugar) + H_{2} S O_{4} (sulfuric acid) \to 12 C (carbon) + 11 H_{2} O (water) + mixture of water and acid$
Hence option A is correct.

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Q. Write balanced chemical equations to show:
The dehydrating property of conc. Sulphuric acid with sugar.

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.

Q. When sugar crystals are added to a hard glass test tube containing conc. sulphuric acid produce ____________.

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