Question

When conc. ${H_2}S{O_4}$ comes in contact with sugar, it becomes black due to ___.
A.Hydrolysis
B.Hydration
C.Decolourisation
D.Dehydration

Answer 1

Dehydration: It is defined as a process in which water molecules from the compounds are eliminated by any reaction, is known as dehydration.
Hydration: It is defined as a process in which water molecules are added to the compounds by any reaction, is known as hydration.

Complete step by step answer:
Let us first talk about sugar.
Sugars are basically carbohydrates. Carbohydrates are defined as the substance or compounds made of hydrogen, carbon and oxygen in which hydrogen and oxygen ratio is as $2:1$. Sugars are made of different carbohydrates. Carbohydrates have general formula ${C_m}{({H_2}O)_n}$ , where $m$ is number of carbon atoms in the compound and $n$ is the number of hydrogen and oxygen in the ratio of $2:1$.
Now let us talk about the processes given in the option of the question.
Decolourisation: Generally when sugars are formed by any reaction then they are coloured. The process of removal of colour of sugars, is known as decolourisation.
Hydrolysis: It is defined as a process in which water molecules are added to the compounds by any reaction, is known as hydrolysis.
Here in the question we are asked that when conc. ${H_2}S{O_4}$ comes in contact with sugar, it becomes black due to dehydration. Because when sulphuric acid reacts with sugar then water is released as a product.
For example: if the sugar is sucrose i.e. ${C_{12}}{H_{22}}{O_{11}}$ then the reaction of sugar with sulphuric acid is as follows: ${{\text{C}}_{{\text{12}}}}{{\text{H}}_{{\text{22}}}}{{\text{O}}_{{\text{11}}}}{\text{ + }}{{\text{H}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{4}}} \to {\text{12C + 11}}{{\text{H}}_{\text{2}}}{\text{O + mixture of acid and water}}$. From the reaction we can say the water is released as a product so it is an example of dehydration.

Hence option D is correct.

Note:
The difference between hydration and hydrolysis is that in the hydration process water reacts with unsaturated compounds to form the new compound while in the hydrolysis process chemical bonds of a compound are broken by the addition of water.