Example 1: naming a salt
Name the salt formed from magnesium oxide and hydrochloric acid, and write the word equation.
- Magnesium comes from the base; chloride from hydrochloric acid.
- magnesium oxide + hydrochloric acid → magnesium chloride + water.
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IB DemystifiedMYP Sciences
Neutralisation and salts
Indigestion tablets, toothpaste, farm lime and fertilisers all depend on one type of reaction. When an acid meets a base, they cancel each other out and form a salt, and chemists can use this reaction to make hundreds of useful salts.
Recommended for MYP 3 · About 3 lessons · Criteria A, B, C and D
On this page
- Learning objectives
- Before you start
- Key vocabulary
- Understanding the ideas
- Acid reactions at a glance
- Neutralisation in the real world
- Worked examples
- Assessment tips
- Check your understanding
- Practice questions
- Investigation
- Criterion-linked questions
- Challenge questions
- Topic check
- Review your mistakes
- Your progress
Learning objectives
By the end of this topic you should be able to:
- describe neutralisation, including the reaction between H⁺ and OH⁻ ions
- name salts and write equations for acids with alkalis, metal oxides and carbonates
- describe how to make a pure, dry sample of a soluble salt
- explain everyday uses of neutralisation
- analyse neutralisation data and design experiments
- discuss liming soils and treating acidic waste water
Before you start
You will use these skills. If any feel shaky, review them first.
- acids, alkalis and pH (see Acids, alkalis and indicators)
- word and symbol equations (see Word and symbol equations; conservation of mass)
- filtering and evaporation (see Pure substances, mixtures and separation)
Key vocabulary
- Neutralisation
- A reaction in which an acid and a base react to form a salt and water.
- Base
- A substance that neutralises an acid, such as a metal oxide, hydroxide or carbonate.
- Alkali
- A base that dissolves in water, producing OH⁻ ions.
- Salt
- A compound formed when the hydrogen of an acid is replaced by a metal or ammonium.
- Crystallisation
- Forming crystals by evaporating some water from a solution and letting it cool.
- Indicator
- A substance that changes colour depending on pH.
Understanding the ideas
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Neutralisation
Acids produce hydrogen ions (H⁺) in water, and alkalis produce hydroxide ions (OH⁻). In neutralisation, these ions combine to form water: H⁺ + OH⁻ → H₂O. The other ions form a salt.
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Naming salts
The name of a salt comes from the base and the acid: hydrochloric acid makes chlorides, sulfuric acid makes sulfates and nitric acid makes nitrates. Acids react with alkalis and metal oxides to form a salt and water, and with carbonates to form a salt, water and carbon dioxide.
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Making a salt
To make a pure salt from an insoluble base, add the base to warm acid until no more reacts, filter off the excess, then evaporate some of the water and leave the solution to crystallise.
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Why does it matter?
Neutralisation is used to treat indigestion, protect teeth, correct acidic soils, soothe stings and treat acidic waste water. It is also how fertilisers such as ammonium nitrate are made.
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What does it connect to?
This topic links to reactivity, rates of reaction, titration and quantitative chemistry.
Acid reactions at a glance
- acid + alkali → salt + water.
- acid + metal oxide → salt + water.
- acid + carbonate → salt + water + carbon dioxide.
- acid + metal → salt + hydrogen.
- Naming: hydrochloric → chloride; sulfuric → sulfate; nitric → nitrate.
- Ionic equation: H⁺(aq) + OH⁻(aq) → H₂O(l).
Neutralisation in the real world
Pharmacies sell antacid tablets containing magnesium hydroxide or calcium carbonate. Farmers spread lime on acidic fields. Fertiliser factories neutralise nitric acid with ammonia to make ammonium nitrate.
Worked examples
Example 2: a carbonate
What forms when calcium carbonate reacts with nitric acid?
- acid + carbonate → salt + water + carbon dioxide.
- calcium nitrate + water + carbon dioxide.
Assessment tips
Questions on this topic often ask you to name salts, write equations and describe making salts. Expect to:
- Name salts and write word and symbol equations.
- Describe the steps to make pure salt crystals and explain each one.
- Interpret pH curves and neutralisation data.
- Discuss uses and impacts of neutralisation.
Common mistakes: saying neutralisation produces hydrogen; mixing up chlorides, sulfates and nitrates; forgetting carbon dioxide from carbonates; and evaporating all the water when making crystals.
Check your understanding
Quick questions on the ideas above. Try each one before using a hint.
Practice questions
Investigation: making copper sulfate crystals
Partially guided investigation · about 60 minutes plus crystallising time · pairs
- Research question
- Can you make pure, dry crystals of copper sulfate from copper oxide and sulfuric acid?
- Scientific background
- Copper oxide is an insoluble base. It neutralises sulfuric acid to form blue copper sulfate solution and water.
- 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
- Dilute sulfuric acid (0.5 M), copper oxide powder, beaker, spatula, glass rod, Bunsen burner or hot plate, filter funnel and paper, evaporating basin, tripod and gauze.
- Method
- Warm 30 cm³ of dilute sulfuric acid gently (do not boil).
- Add copper oxide a spatula at a time, stirring, until some black powder remains undissolved.
- Filter the mixture to remove the excess copper oxide.
- Heat the blue filtrate in an evaporating basin until about half the water has gone, then leave it to cool and crystallise; dry the crystals on filter paper.
Safety. Wear eye protection. Sulfuric acid is an irritant and copper compounds are harmful; wash hands after use. Take care with hot equipment and do not boil the solution dry.
Then evaluate: why was excess copper oxide added, and why should the solution not be heated until dry?
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.
| Skill | Correct | Status |
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