Lesson focus: acidA substance with a pH below 7, such as lemon juice or vinegar. and alkaliA base that dissolves in water and has a pH above 7. are aqueousDissolved in water, shown as (aq) in equations. chemical systems with characteristic ions, reactions and hazards.
Classify acids, baseA substance that neutralises an acid to form a salt and water, such as a metal oxide or hydroxide. and alkalis and relate their aqueous behaviour to hydrogen and hydroxide ions.
Build on earlier learning
AtomsYear 7 introduced ions; this lesson uses ions to describe acidic and alkaline solutions.
You will be successful when you can…
- Distinguish acid, base and alkali.
- Relate acidity to H⁺(aq).
- Relate alkalinity to OH⁻(aq).
- Use hazard information rather than taste.
Words that carry the science
- acid
- A substance that produces hydrogen ions, H⁺(aq), in aqueous solution.
- base
- A substance that reacts with and neutralises an acid.
- alkali
- A soluble base that produces hydroxide ions, OH⁻(aq), in water.
- aqueous
- Dissolved in water.
- corrosive
- Able to damage living tissue or materials by chemical action.
Acidity and alkalinity describe solutions
01Acidity and alkalinity describe solutions
Acidic aqueousDissolved in water, shown as (aq) in equations. solutions contain hydrogen ions, H⁺(aq). Alkaline solutions contain hydroxide ions, OH⁻(aq).
Examples include hydrochloric acidA substance with a pH below 7, such as lemon juice or vinegar. and sodium hydroxide solution. Household names do not reliably reveal concentration, composition or hazard.
02Base and alkali are not synonyms
A baseA substance that neutralises an acid to form a salt and water, such as a metal oxide or hydroxide. neutralises an acid. An alkaliA base that dissolves in water and has a pH above 7. is a baseA substance that neutralises an acid to form a salt and water, such as a metal oxide or hydroxide. that dissolves in water and forms an alkaline solution.
Copper oxide is a base but not an alkali because it is insoluble. Sodium hydroxide is both a base and an alkali.
03Safe classification uses evidence
Never taste an unknown substance. Use labels, hazard information and small-scale indicator tests with eye protection.
Dilution lowers concentration but does not change the identity of the acid. Even dilute solutions require appropriate handling.
Classify two bases
Compare copper oxide and sodium hydroxide.
- Both react with acids, so both are bases.
- Copper oxide is insoluble in water.
- Sodium hydroxide dissolves and supplies OH⁻(aq).
- Therefore only sodium hydroxide is an alkali.
How to make the reasoning visible: Every alkali is a base, but not every base is an alkali.
Classify substances from evidence
Use solubility and reaction evidence, not familiar names.
| Substance | Dissolves? | Neutralises acid? | Classification |
|---|---|---|---|
| hydrochloric acid | yes | reacts with base | acid |
| sodium hydroxide | yes | yes | alkali + base |
| copper oxide | no | yes | base |
| sodium chloride | yes | no evidence | neutral salt |
Why is copper oxide not an alkali?
Why is solubility alone insufficient?
Check your understanding
Answer from memory, then check against the suggested answer.
What ion characterises acidic solution?
What ion characterises alkaline solution?
Define alkali.
Is insoluble magnesium oxide a base or alkali?
Why does adding water not turn hydrochloric acid into a weak acid?
Exam Question Practice
Exam Question Practice
Mark points shown after marking
Bases and alkalis are related.
Compare a base and an alkali.
(3)Give one example of an alkali and one example of an insoluble base.
(1)Total for this question: 4 marks
Command word: For compareA command word: say how two or more things are similar and how they are different., state both similarity and difference.