Question

Which one of the following bases is insoluble in water?
A) $NaOH$
B) $KOH$
C) $MgO$
D) $N{H_3}$

Answer 1

Hint: Ionic compounds dissolve in water if the energy released by the ions as they interact with water molecules is sufficient to compensate for the energy necessary to sever the ionic bonds in the solid and separate the water molecules so that the ions can be injected into solution.

Complete answer:
Let us have a look at each option:
-Option A) $NaOH$
As sodium hydroxide (NaOH) dissolves in water, it splits into positively and negatively charged sodium ions (cations) and hydroxide ions (anions). It is highly soluble in water, and its solubility, like that of all gases, is proportional to the partial pressure of the gas.
Hence, this option is incorrect.
-Option B) $KOH$
In water, KOH dissociates fully into its constituent ions, K+ and OH-. Any of these ions is then totally solvated (i.e. surrounded by water molecules) in order to balance their charges. When KOH is combined with a small amount of water, the solution may become hot .
Hence, this option is incorrect.
-Option C) $MgO$
The doubly charged magnesium and oxygen have a heavy attraction ( unlike the singly charged sodium and chloride). MgO needs a lot of energy to separate the ions, making it insoluble in water.
Hence, this option is correct.
-Option D) $N{H_3}$
Water is very soluble in ammonia gas. The hydrogen bonding that occurs between the ammonia and water molecules is responsible for the comparatively high solubility. A small amount of dissolved ammonia reacts with water to form ammonium hydroxide, which then breaks down into ammonium and hydroxide ions.
Hence, this option is incorrect.
Therefore, the correct option is C) $MgO$.

Note:
Adding sugar to hot chocolate, stirring a bouillon packet into hot broth, and taking drugs that easily absorb into the bloodstream are all examples of solubility. The dissolution of radioactive metals and contaminants into a water source is a poor indication of solubility.