- Define and explain Testing with Indicators in your own words
- Use key terms such as acid accurately
- Apply what you have learned to new examples and questions
- Avoid the common mistakes learners make with this topic
This lesson focuses on Testing with Indicators: use litmus and universal indicator to test for acids and alkalis.
Use litmus and universal indicator to test for acids and alkalis.
Key ideas
Indicators change colour
An indicator is a dye that changes colour in acids and alkalis: blue litmus turns red in acid, red litmus turns blue in alkali. Universal indicator goes further, showing a rainbow of colours from red (strong acid) through green (neutral) to purple (strong alkali). This lets you estimate pH just by matching the colour.
The pH scale measures hydrogen ions
Acids release hydrogen ions (H⁺) in water, and the more hydrogen ions, the lower the pH and the stronger the acid. A solution of pH 3 is more acidic than one of pH 5 — each whole step on the scale is a tenfold change. Pure water sits at pH 7, exactly neutral.
Key term — acid: A substance with a pH below 7, such as lemon juice or vinegar; acids turn blue litmus red.
Four unlabelled solutions A, B, C and D give these results with universal indicator: A turns red, B turns green, C turns purple, D turns orange. Identify each as acidic, neutral or alkaline, and put the acids in order of strength.
Red means strongly acidic, so A is a strong acid. Orange means weakly acidic, so D is a weak acid. Green means neutral, so B is neutral. Purple means strongly alkaline, so C is a strong alkali. Lower pH means stronger acid, so A (red) is stronger than D (orange).
Answer: A: strong acid; D: weak acid; B: neutral; C: strong alkali. Acids in order of strength: A then D.
- Saying pH 7 is an acid pH 7 is neutral — neither acidic nor alkaline. Acids are below 7 and alkalis are above 7; pure water is the classic example of pH 7.
- Thinking all acids are dangerous Strength matters more than the word acid: citric acid in oranges and ethanoic acid in vinegar are weak acids we eat safely. It is concentrated strong acids, like concentrated sulfuric acid, that are hazardous.
Practice
Litmus only tells you acid or alkali (red or blue), while universal indicator shows a range of colours matching each pH value, so you can estimate the actual pH.
(a) Red — vinegar is acidic. (b) It stays blue — soap solution is alkaline.
Add an alkali such as lime, which neutralises the acid and raises the pH towards 7.
Lemon juice is more acidic — about ten thousand times more acidic (four pH steps of tenfold change each).
Quick check
Which of these best defines "acid"?
Hydrochloric acid and sodium hydroxide are mixed in exactly the right amounts. What is in the beaker at the end?
Write the word equation for the neutralisation of hydrochloric acid by sodium hydroxide.
- Testing with Indicators: use litmus and universal indicator to test for acids and alkalis.
- Indicators change colour: An indicator is a dye that changes colour in acids and alkalis: blue litmus turns red in acid, red litmus turns blue in alkali.
- alkali: A soluble base with a pH above 7, such as soap solution; alkalis turn red litmus blue.
- Watch out for: saying pH 7 is an acid