The rainbow indicator
One indicator, a different colour for every amount of H⁺.
Two acids, one answer
Blue litmus turns red in dilute hydrochloric acid. It also turns red in lemon juice. But the hydrochloric acid has far more H⁺.
How can we tell which has more H⁺?
What this lesson covers
The idea
A universal indicator is a mixture of several indicators that shows different colours at different concentrations of hydrogen ions in a solution.
Two acids, one answer
Blue litmus turns red in dilute hydrochloric acid. It also turns red in lemon juice. But the hydrochloric acid has far more H⁺.
How can we tell which has more H⁺?
- Use an indicator that shows a different colour for each amount of H⁺
- Taste both and compare
- We cannot tell
Dip both indicators
Dip blue litmus and universal indicator in each liquid. Watch which one shows more colours.
A mixture that paints the scale
A universal indicator is a mixture of several indicators. It shows different colours at different concentrations of hydrogen ions in a solution.
The exact shades differ a little between brands. Always compare with the chart that comes with the paper.
- Colour | H⁺ in the solution
- red, orange | many H⁺: strong acid
- yellow | some extra H⁺: weak acid
- green | neutral
- blue, violet | few H⁺: alkali
Notes
A universal indicator is a mixture of several indicators that shows different colours at different concentrations of hydrogen ions in a solution.
Check yourself
What is a universal indicator?
Universal indicator turns green in a liquid. The liquid is …
Liquid X turns universal indicator red. Liquid Y turns it yellow. Which has more H⁺?
Why is universal indicator better than litmus for comparing two acids?
- A single dye, like litmus. It is a mixture, not a single dye.
- A mixture of several indicators — correct. Yes! Each one changes colour at a different H⁺ level.
- A kind of acid. It only shows the nature of a solution. It is not an acid.
- a strong acid. A strong acid turns it red.
- a strong alkali. A strong alkali turns it blue or violet.
- neutral — correct. Yes! Green is the middle of the chart.
- X — correct. Yes! Red means many H⁺.
- Y. Yellow is only a little acidic. Red has more H⁺.
- Both the same. Different colours mean different amounts of H⁺.
- It is cheaper. That is not the reason. It is the range of colours.
- It gives a different colour for each amount of H⁺ — correct. Yes! Litmus is just red in both.
- It tastes better. Never taste an indicator or a chemical.