Question

When a standard solution of NaOH is left in air for few hours:
(A) Precipitation occurs
(B) Strength of solution decreases
(C) Strength of solution increases
(D) The concentration of $N{{a}^{+}}$ions in the solution remains the same

Answer 1

NaOH is hygroscopic in nature. NaOH absorbs the water molecules present in the atmosphere. This phenomenon also affects the pH of the solution.

Complete step by step solution:
NaOH is hygroscopic in nature it means that it has the ability to attract the moisture from the surrounding. It is a very important property of a compound as it signifies the absorptivity of the chemical substance. This is the reason why sodium hydroxide is not used to prepare the primary standard titration solution. Sodium hydroxide can absorb so much water that it can actually dissolve. Sodium hydroxide is also highly delicate, it forms a watery solution as soon as it is exposed to air.
The Strength of a solution is defined as the concentration of solution in gram per litre. NaOH absorbs the molecule present in air and leads to the decrease in strength of the solution. Sodium hydroxide forms precipitate when it is added to a solution containing aluminium ions or calcium ions or magnesium ions etc. sodium hydroxide generally forms a white precipitate with these ions.

Hence the correct option is (B) i.e. when a standard solution of NaOH is left in air for few hours the Strength of solution decreases

Note: Do not get confused between the strength and concentration of an acid or base. Strength of an acid or base refers to the release of ions of acid and base in the solution whereas concentration refers to the total number of moles per volume which is contained within the solution.