Question

Why do we need an indicator???

Answer 1

Chemical indicator. Chemicalindicator, any substance that gives a visible sign, usually by a colour change, of the presence or absence of a threshold concentration of a chemical species, such as an acid or an alkali in a solution. 

Answer 2

Unlike litmus, universal indicator can show us exactly how strongly acidic or alkaline a solution is. This is measured using the pH scale. The pH scale runs from pH 0 to pH 14. Universal indicator has many different colour changes, from red for strong acids to dark purple for strong bases.