Question

Which of the following is not a water absorber and dehydrating?
A. Silica gel
B. Potassium pentoxide
C. Concentrated sulfuric acid ​
D. Aqueous calcium chloride

Answer 1

Hint: When a liquid or solid passes the molecules of a substance through its surface it is called absorption. The substance gets uniformly scattered throughout the liquid or solid. It is a bulk phenomenon.

Complete Step by Step Solution:
Absorption is a bulk phenomenon as it occurs throughout the body of the material. In this process, a solid or liquid substance passes another substance through it. The concentration of the substance that gets absorbed is the same throughout the material. The rate of absorption also remains the same throughout the process. Dehydration means loss of water.
A substance is called a dehydrating agent if it can eliminate water from a material.

In a reaction or simple process in which dehydration happens, the substance undergoing dehydration or reactant loses a molecule of water. The dehydrating agent absorbs water helping in drying a substance thus also called a drying agent. Examples include sulfuric acid, concentrated phosphoric acid, hot aluminium oxide etc.

From the given options, we have to find an option which does not act as a dehydrating agent.
A. Silica gel
It is non-reactive, toxic-free, and insoluble in water.
It is a good water absorber.
If silica gel is placed in a vessel containing water vapours, it absorbs the water vapours on its surface.
So, A is incorrect.

B. Phosphorus pentoxide
It is used as a strong dehydrating agent.
It removes water from many inorganic compounds like sulphuric acid, nitric acid and several organic compounds.
So, B is incorrect.

C. Concentrated sulfuric acid
It is also effective in absorbing water.
For example, in the charring of sugar, concentrated sulfuric acid is used as a catalyst.
In this reaction, sugar is heated. Sulfuric acid abstracts water molecules from sugar giving carbon and hydrogen in the process.
So, C is incorrect.

D. Aqueous calcium chloride
It does not absorb water and hence, does not act as a dehydrating agent.
So, option D is correct.

Note: Anhydrous calcium chloride absorbs water molecules and reacts with them to form hydrated calcium chloride. Water vapours get uniformly distributed on the surface of calcium chloride.