Question

Which of the solutions will remain colourless with phenolphthalein indicator?
(A) Soap solution
(B) $Ca{\left( {OH} \right)_2}$ solution
(C) Vinegar solution
(D) Baking soda solution

Answer 1

A substance will be known as an acid if it gives out positive hydrogen ions that is ${H^ + }$ ions when dissolved in water. A substance will be known as a base if it gives out hydroxide ions that is $O{H^ - }$ ions when dissolved in water. In order to find whether a solution is acidic or basic, indicators are used. An acidic solution will remain colorless in phenolphthalein.

Complete Step by step solution
Indicator is a substance which changes its colour when it comes in contact with an acid or a base. An acidic substance is a substance which includes acid within itself. A basic substance is a substance which includes base within itself. An indicator is used to differentiate between an acidic substance and a basic substance. There is a range of different indicators. Among all, the common indicators are as follows; litmus, china rose, turmeric and phenolphthalein.
The chemical formula of phenolphthalein is ${C_{20}}{H_{14}}{O_4}$ . When phenolphthalein comes in contact with an acid solution, it turns colourless and in basic or alkaline solution, it turns pink in colour.
From the given options; soap solution, baking soda solution and $Ca{\left( {OH} \right)_2}$ solution are all alkaline solutions. Hence phenolphthalein will turn pink in colour. Vinegar is an acidic solution and hence phenolphthalein will remain colourless in it.
Therefore, the correct answer is option C.

Note
Indicators are subdivided into two types which are as follows; natural indicators and synthetic indicators. The examples of natural indicators are litmus, china rose and turmeric. The example of a synthetic indicator is phenolphthalein.