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Acids, alkalis and indicators

Lemon juice, vinegar and your stomach contain acids; soap, baking soda and drain cleaner are alkalis. The pH scale tells us how acidic or alkaline something is, and neutralisation lets us cure indigestion, treat soils and protect our teeth.

Recommended for MYP 2 · About 3 lessons · Criteria A, B, C and D

01234567891011121314← more acidicmore alkaline →neutralstomach acidvinegarpure watersoapdrain cleanerlemon juicebaking sodableachUniversal indicator colours. Each step down the scale is ten times more acidic.
Figure 1. The pH scale runs from 0 (strongly acidic) to 14 (strongly alkaline).
On this page
  1. Learning objectives
  2. Before you start
  3. Key vocabulary
  4. Understanding the ideas
  5. Everyday neutralisation
  6. Acids and alkalis in the real world
  7. Worked examples
  8. Assessment tips
  9. Check your understanding
  10. Practice questions
  11. Investigation
  12. Criterion-linked questions
  13. Challenge questions
  14. Topic check
  15. Review your mistakes
  16. Your progress

Learning objectives

By the end of this topic you should be able to:

  • use the pH scale to describe acids, alkalis and neutral substances
  • use indicators, including universal indicator and plant indicators
  • explain neutralisation and its everyday uses
  • handle acids and alkalis safely
  • analyse pH data and plan simple investigations
  • discuss uses of neutralisation in farming and health

Before you start

You will use these skills. If any feel shaky, review them first.

  • mixtures and solutions (see Pure substances, mixtures and separation)
  • reading scales and tables
  • hazard symbols

Key vocabulary

Acid
A substance with a pH below 7.
Alkali
A soluble base with a pH above 7.
Neutral
Neither acidic nor alkaline: pH 7.
Indicator
A substance that changes colour depending on pH.
Neutralisation
The reaction of an acid with a base, forming a salt and water.
Corrosive
Able to destroy living tissue and materials.

Understanding the ideas

  1. Acids, alkalis and pH

    Acids and alkalis are all around us. Acids have a pH below 7 and alkalis above 7; pure water is neutral at pH 7. The further from 7, the stronger the acid or alkali.

  2. Indicators

    Indicators change colour with pH. Litmus shows only acid (red) or alkali (blue); universal indicator shows a range of colours, giving the approximate pH. Many plants, such as red cabbage, contain natural indicators.

  3. Neutralisation

    When an acid reacts with an alkali, they neutralise each other, forming a salt and water. We can follow neutralisation with an indicator or a pH meter.

  4. Why does it matter?

    Neutralisation is used every day: antacids relieve indigestion, farmers spread lime on acidic soils, and toothpaste neutralises acids that damage teeth. Strong acids and alkalis are corrosive, so they must be handled with care.

  5. What does it connect to?

    This topic links to reactions, salts and acid rain in chemistry, to digestion and teeth in biology, and to soils and the environment.

Everyday neutralisation

  • Indigestion: antacids (bases) neutralise excess stomach acid.
  • Farming: lime neutralises acidic soil.
  • Teeth: slightly alkaline toothpaste neutralises acids made by bacteria.
  • Stings: baking soda (weak alkali) can help an acidic bee sting.
  • Factories: acidic waste is neutralised before it is released into rivers.
  • Word equation: acid + alkali → salt + water.

Acids and alkalis in the real world

Farmers in some regions test their soil pH and add lime or gypsum to improve crops. Dentists advise limiting fizzy drinks because their low pH dissolves enamel. Water treatment works check the pH of drinking water.

Worked examples

Example 1: reading pH

A solution turns universal indicator dark blue. What can you say about it?

  1. Dark blue matches about pH 10–11 on the scale.
  2. It is an alkali, fairly strong.

Example 2: naming a salt

Name the salt formed when hydrochloric acid reacts with potassium hydroxide.

  1. The metal comes from the alkali: potassium.
  2. Hydrochloric acid gives chlorides.
  3. The salt is potassium chloride.

Assessment tips

Questions on acids and alkalis often use the pH scale and indicator colours. Expect to:

  • Use the pH scale and indicator colours to classify substances.
  • Explain neutralisation and its uses.
  • Process pH data and identify anomalies.
  • Plan safe investigations.

Common mistakes: saying a lower pH means a weaker acid; thinking all alkalis are safe; tasting substances to test them; and forgetting that neutralisation also makes water.

Check your understanding

Quick questions on the ideas above. Try each one before using a hint.

Practice questions

Show

Investigation: making a red cabbage indicator

Partially guided investigation · about 45 minutes · pairs

Research question
Can red cabbage juice be used to sort household substances into acids, neutral substances and alkalis?
Scientific background
Red cabbage contains coloured chemicals that change colour with pH: red or pink in acids, purple when neutral, and blue, green or yellow in alkalis.
Hypothesis
Write your own prediction, with a scientific justification.
Variables
Identify your independent, dependent and control variables, and explain how you will control them.
Apparatus
Red cabbage, knife and board, hot (not boiling) water, sieve, beakers, dropping pipettes, spotting tile or test tubes, samples of household substances such as lemon juice, vinegar, baking soda solution, soap and salt water.
Method
  1. Chop the cabbage, soak it in hot water for 10 minutes, then sieve to collect the purple juice.
  2. Put a few drops of each substance in a spotting tile.
  3. Add the same number of drops of cabbage juice to each and record the colour.
  4. Sort the substances into acids, neutral and alkalis, and compare with universal indicator.

Safety. Wear eye protection. Take care with hot water and knives. Your teacher should handle strong cleaners such as drain cleaner; do not taste anything.

Then evaluate: what are the advantages and disadvantages of cabbage juice compared with universal indicator?

Criterion-linked questions

Criterion B: inquiring and designing

Criterion C: processing and evaluating

Criterion D: reflecting on the impacts of science

Challenge questions

Harder problems in unfamiliar contexts. Plan before you calculate.

Topic check

Five questions picked at random from the whole topic. Take a new set whenever you like.

Review your mistakes

Questions you got wrong on this device appear here so you can try them again. Answer one correctly and it leaves the list.

Your progress

Tracked separately for each skill, on this device only.

SkillCorrectStatus

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