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Chemistry

Acids, Bases and Indicators

Acids and bases are found in foods, cleaning products, laboratories and the human body. Indicators make their differences visible through colour changes.

JSS 3–SS 130 minInteractive quizOffline-ready
Acids, Bases and Indicators lesson illustration
Chemistry • JSS 3–SS 130 min • self-paced
Learning objectives

By the end of this lesson, you should be able to:

  • Define acids, bases and alkalis.
  • Interpret the pH scale from 0 to 14.
  • Predict indicator colour changes.
  • Explain neutralisation.
  • Apply basic laboratory safety rules.
Core idea

Acids and bases

Acid: a substance that produces hydrogen ions in water. Many acids taste sour, but laboratory chemicals must never be tasted.
Base: a substance that neutralises an acid. A base that dissolves in water is called an alkali.
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Acid examples

Lemon juice, vinegar and dilute hydrochloric acid.

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Base examples

Soap solution, baking soda and sodium hydroxide.

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Neutral example

Pure water is approximately neutral.

Measure acidity

The pH scale

0 Strong acid7 Neutral14 Strong base
  • pH below 7The solution is acidic.
  • pH exactly 7The solution is neutral.
  • pH above 7The solution is basic or alkaline.
Colour evidence

Common indicators

IndicatorIn acidIn neutralIn base
Blue litmusTurns redStays blueStays blue
Red litmusStays redStays redTurns blue
Universal indicatorRed–yellowGreenBlue–purple
Local investigation: red-cabbage extract can act as a natural indicator, but use only teacher-approved household samples.
Reaction

Neutralisation

Word equationAcid + Base → Salt + Water
Real-life link

Reducing excess soil acidity

Farmers may add lime to acidic soil. The base reacts with acids in the soil and raises the pH towards a range suitable for crops.

Laboratory safety

Handle chemicals responsibly

  • Never taste chemicalsAppearance and smell cannot confirm safety.
  • Wear eye protectionUse goggles and follow the teacher’s instructions.
  • Report spills immediatelyDo not touch or clean an unknown spill without guidance.
  • Add acid to waterWhen dilution is required, follow the correct supervised procedure.
Mother-tongue reinforcement

Review the main idea

English summary

Acids have pH values below 7, neutral substances have pH 7, and bases have pH values above 7. Indicators change colour, while neutralisation produces salt and water.

Question 1 of 5

Assignment

Apply your understanding

  1. Classify pH 2, 7, 9 and 13 as acidic, neutral or basic.
  2. Create a table showing the effect of acids and bases on red and blue litmus.
  3. Explain one agricultural use of neutralisation.
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