Indicators like litmus only say acid or alkali. Universal indicator and the pH scale say how strongly, from 1 to 14. Add acid to alkali a drop at a time and you can watch the pH fall to 7 as neutralisation happens.
The pH scale runs from 1 to 14. pH 7 is neutral; below 7 is acidic and above 7 is alkaline. Universal indicator gives a different colour for each pH, from red (strongly acidic) through green (neutral) to purple (strongly alkaline). The lower the pH, the higher the concentration of H+ ions. In neutralisation, H+ and OH− ions react to form water, so the pH moves towards 7.
Fig. 1: the pH scale and universal indicator
Section 01
The pH scale.
Use the colour chart to read a pH. Each pH unit lower means a much higher concentration of hydrogen ions.
pH 1 to 3: strongly acidic, red or orange. Hydrochloric acid, stomach acid.
pH 4 to 6: weakly acidic, orange or yellow. Vinegar, rainwater.
pH 7: neutral, green. Pure water, sodium chloride solution.
Measuring: add universal indicator (solution or paper) and match the colour to the chart, or use a pH meter for a more precise reading.
Examiner trap
A lower pH means a higher concentration of H+ ions, not a lower one. And pH 7 means neutral only if the solution contains no excess acid or alkali; "more acidic" means a smaller pH number.
Stage 1 · Learn
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What is the pH of a neutral solution?
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A solution turns universal indicator purple. It is:
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Which pH is the most acidic?
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Why do farmers spread calcium hydroxide (slaked lime) on some fields?
Section 02
Following neutralisation.
Run the titration: add acid from the burette to the alkali in the flask and watch the indicator change at the end point.
Start: alkali in the flask, high pH. Thymolphthalein is blue.
During: each drop of acid adds H+ ions that react with OH− ions, so the pH falls.
End point: the indicator changes colour (thymolphthalein turns colourless) when the alkali has just been neutralised.
After: extra acid makes the solution acidic and the pH drops below 7.
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Solution P has pH 2 and solution Q has pH 5. Which statement is correct?
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Dilute hydrochloric acid is added gradually to aqueous sodium hydroxide containing universal indicator until the acid is in excess. How does the colour change?
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Why is a pH meter sometimes used instead of universal indicator?
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An indigestion tablet contains magnesium hydroxide. How does it relieve indigestion?
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