4.1 Acids and alkalis
Summary: What do vomit, vinegar, and lemons have in common? Tags: science oxford-science student-book-7 chemistry Created: 2026-05-19 Last Updated: 2026-05-19
After this topic, you will be able to:
Think back
- Describe the movement of particles in a liquid.
- Describe how to protect your eyes during an experiment.
- Name a food that is sour.
Key idea A concentrated solution has fewer water Molecules than the same volume of dilute solution. Hazard symbols show the dangers of using different substances.
Key words acid, alkali, corrosive, solute, concentrated, dilute
What do vomit, vinegar, and lemons have in common?
They all taste sour. This is because they contain acids. Vomit includes an acid from the stomach that helps digest foods. Vinegar contains ethanoic acid. Lemons contain citric acid.
Alkalis are the chemical opposite of acids. Soap solutions and toothpaste are alkalis. Most alkalis feel soapy. You must never taste or touch solutions in the lab.
A Write the chemical names of two acids.
Using acids and alkalis safely
It is safe to eat the acid in lemons, and to use alkaline soap. But there are hazards (dangers) linked to some acids and alkalis.
The bottle in Figure 1 has a hazard symbol. The symbol shows that the solution in the bottle is corrosive. It could burn your skin and eyes.
Figure 1This solution is corrosive.
You can control risks from corrosive solutions by:
- wearing eye protection
- keeping the solution off your skin.
If a solution is very corrosive, a teacher might wear gloves to protect their hands when using it.
B Describe two hazards of using a corrosive solution.
Concentrated or dilute?
Pure ethanoic acid causes very bad burns. It catches fire easily. Vinegar contains ethanoic acid. It is safe to eat, and does not catch fire. Why is there a difference? Pure ethanoic acid contains no water. Dissolving in water changes some properties.
The amount of water makes a difference, too. Both the bottles in Figure 2 contain hydrochloric acid. Hydrochloric acid is a solution of hydrogen chloride in water. Hydrogen chloride is the solute – a substance that dissolves in a liquid to make a solution.
Figure 2Two bottles of acid.
- The acid in bottle A has 370 g of hydrogen chloride in 1 litre of solution.
- The acid in bottle B has 3.70 g of hydrogen chloride in 1 litre of solution.
Acid A has more hydrogen chloride per litre than acid B. Acid A is concentrated. Acid B is dilute. The concentrated acid burns skin and eyes. The dilute acid hurts if it gets into a cut, and might make your skin slightly red, but has no other hazards.
Figure 3 shows some particles in concentrated and dilute solutions of the same acid.
Figure 3The acid on the left is more concentrated. It has more acid particles per litre. Particles not to scale. Full text alternative to the content
An alkali may also be concentrated or dilute. The hazards of using acids and alkalis depend on:
- the acid or alkali you are using
- whether the solution is concentrated or dilute.
C Describe one difference between a concentrated solution of an acid and a dilute solution of the same acid.
Working scientifically A teacher has a solution of an alkali. The solution is corrosive – it causes very bad burns to skin and eyes. Describe how to control the risks from these hazards. Do you think the teacher should allow your class to use the alkali? Explain your decision.
Stretch zone
- Copy and complete the sentences. Acids taste sour/feel soapy. Some acid and alkali solutions are corrosive/correlated. A concentrated acid is more/less corrosive than a dilute acid. A concentrated acid has fewer/more acid particles per litre than a dilute acid.
- Acids can be dangerous. Describe two hazards of using the acid in Figure 1.Describe how to control the risks from these hazards.
Stretch zone
- Calculate which is the more concentrated alkali. Show your working. alkali X (20 g of alkali in 250 cm3 of water)alkali Y (10 g of the same alkali in 500 cm3 of water)