Question

Why does dilution of a strong acid or strong base not change its pH colour indication on a pH paper while for weak acids and weak bases after dilution the colour indicator of pH changes?

Answer 1

With the help of the Universal pH indicator paper, strong acids and bases on dilution show the pH colour change.

The colour change would be from red (strong acid) to brown, dark orange, light orange (diluted strong acid). Similarly, the colour change would be from violet (strong base) to indigo, lavender, blue, light blue (diluted strong base).

The pH value can easily be derived with the help of universal indicator pH papers. The colour change in the universal indicator pH papers will help one to identify the pH of acids. One can, thus, identify whether the acid is strong or not with its pH values.

Universal indicator contains a combination of indicators that shows a range of colours indicating the pH of a solution. The pH scale runs from 0 (strongly acidic) to 7 (neutral), and to 14 (strongly alkaline). The colours of the Universal indicator are the same as that of a rainbow:-
Red — Orange — Yellow....Green...Blue — Indigo — Violet
And, chronologically, it goes like, Strong Acid — Weak Acid — Neutral — Weak Alkali — Strong Alkali

So, the universal indicator shows how much strong or weak an acid or alkali is (along with its corresponding pH value).