A student dips a strip of universal indicator paper into three unknown liquids and gets three different colours: red, green, and purple. What does this range of colours tell the student that a single strip of litmus paper could not?
- The universal indicator shows roughly how acidic or alkaline each liquid is across the whole pH scale, while litmus paper can only show whether a liquid is acidic or alkaline, not its strength
- Universal indicator only works on liquids containing water, while litmus paper works equally well on liquids and gases
- The colours show which liquid conducts electricity best, a property litmus paper is not sensitive to at all
- Universal indicator changes colour permanently, while litmus paper's colour change reverses itself back to its original colour after a few minutes
Why A? And why not the others?
Correct answer: A. The universal indicator shows roughly how acidic or alkaline each liquid is across the whole pH scale, while litmus paper can only show whether a liquid is acidic or alkaline, not its strength
Universal indicator is a mixture of several dyes that produces a whole range of colours corresponding to different pH values, so it can show roughly how strongly acidic or alkaline a liquid is anywhere across the pH scale. Litmus paper contains only a single dye that turns one colour in acids and another in alkalis, so it can only tell you which side of neutral a liquid falls on, not its strength. The option claiming universal indicator only works on liquids is wrong because both indicators are typically used to test liquids and solutions, and this is not the property that explains the difference in colour range. The option about electrical conductivity is wrong because neither indicator measures conductivity at all; that would require a completely different piece of equipment. The option about permanent versus reversible colour change is wrong because neither indicator's colour change works that way; both simply respond to the pH of whatever they are currently dipped in.
Source: RSC Education / BBC Bitesize GCSE Chemistry: Acids, alkalis and the pH scale