IGCSE Chemistry Guide: Acids, Bases & Salts Revision
Talimat Academic Team
Education Specialist
Acids, bases and salts is Topic 7 of Cambridge IGCSE Chemistry (0620/0971) and shows up in every paper. Master three blocks: acid and base reactions, the pH scale and indicators, and choosing the right salt preparation method from solubility rules.
This topic looks easy. It quietly loses marks through the wrong salt method and the strong versus weak acid mix-up. It is guaranteed content across multiple choice, theory and practical papers.
This IGCSE Chemistry acids, bases and salts revision guide gives you confident recall plus a clear salt preparation method you can trust in the exam. No filler, just the parts that score.
How do you revise IGCSE acids, bases and salts?
Split Topic 7 into three learnable blocks: properties and reactions, the pH scale with indicators, and salt preparation. Then drill each block with topical past papers rather than re-reading notes.
This sits in the Cambridge IGCSE Chemistry syllabus (0620/0971) and appears in every paper. Focused review beats passive reading every time.
Use these three blocks as your revision roadmap for Topic 7:
- Block 1: acid, base and carbonate reactions with equations.
- Block 2: the pH scale, indicators and neutralisation.
- Block 3: salt preparation driven by solubility rules.
According to Cambridge International, the 0620 syllabus groups this topic into properties of acids and bases, types of oxides, and preparation of salts. Strong versus weak acids sits in the Extended tier, so confirm your paper first.
New to the course? Skim the full Cambridge IGCSE subject list, then come back and work these three blocks in order.
What are the properties of acids and bases?
Acids are proton (H+) donors. Bases are proton (H+) acceptors, and a soluble base is called an alkali. Acids react with metals to give a salt plus hydrogen.
Acids react with bases to give a salt plus water, and with carbonates to give a salt, water and carbon dioxide. Neutralisation is simply H+ plus OH- forming water.
This table compares the core features of acids and bases that examiners test most often, from the ions they form to how each one turns litmus paper red or blue.
| Feature | Acid | Base |
|---|---|---|
| Definition | Donates hydrogen ions, a proton donor | Accepts hydrogen ions, a proton acceptor |
| Ion in water | Forms H+ ions in solution | Forms OH- ions if it dissolves |
| Litmus colour | Turns blue litmus paper red | Turns red litmus paper blue |
| Reacts with | Metals, bases and carbonates | Acids and ammonium salts |
| pH value | Less than 7 on the scale | More than 7 on the scale |
Notice the mirror pattern: what an acid does, a base or alkali often reverses. That symmetry makes recall faster under exam pressure.
Learn these three acid reaction patterns as word equations you can recall fast:
- Acid plus metal gives salt plus hydrogen.
- Acid plus base gives salt plus water.
- Acid plus carbonate gives salt, water and carbon dioxide.
These same reactions appear in IGCSE Combined Science, so they are worth over-learning if you sit that route too.
How do the pH scale and indicators work?
The pH scale runs from 0 to 14. Below 7 is acidic, exactly 7 is neutral, and above 7 is alkaline. Universal indicator shows this as a colour gradient.
Single indicators give sharper results. Litmus turns red in acid and blue in alkali. Methyl orange and thymolphthalein change at set points, which helps identify solutions in the lab.
Keep these indicator colour changes ready for identification questions:
- Litmus: red in acid, blue in alkali.
- Methyl orange: red in acid, yellow in alkali.
- Thymolphthalein: colourless in acid, blue in alkali.
A lower pH means a higher concentration of hydrogen ions. That single idea explains why acidity rises as pH falls.
What is the difference between strong and weak acids?
Strength describes dissociation, not concentration. A strong acid like hydrochloric acid fully ionises in water. A weak acid like ethanoic acid only partly ionises, shown with a reversible arrow.
Here is the classic trap: a concentrated weak acid can have a higher pH than a dilute strong acid. Concentration is how much acid is dissolved. Strength is how fully it splits into ions.
This also changes reaction rate. A strong acid reacts faster with the same metal because it has more free hydrogen ions at any moment. This distinction is Extended tier content and a frequent examiner favourite.
What are acidic, basic and amphoteric oxides?
Oxides split into four types. Non-metal oxides are usually acidic. Metal oxides are usually basic. Some behave in both ways, and a few react with nothing at all.
Amphoteric oxides, such as aluminium oxide and zinc oxide, react with both acids and bases. Neutral oxides, such as water, carbon monoxide and nitrogen monoxide, react with neither.
This table sorts the four oxide types, each with one clear example, so you can quickly classify an unfamiliar oxide from where its element sits on the periodic table in the exam.
| Oxide type | Example | Reacts with |
|---|---|---|
| Acidic | Carbon dioxide, CO2 | Reacts with bases to form salts |
| Basic | Copper(II) oxide, CuO | Reacts with acids to form salts |
| Amphoteric | Aluminium oxide, Al2O3 | Reacts with both acids and bases |
| Amphoteric | Zinc oxide, ZnO | Also reacts with acids and bases |
| Neutral | Water, H2O | Reacts with neither acids nor bases |
The rule of thumb: metals give basic oxides, non-metals give acidic oxides. Amphoteric oxides are the exam favourites, so learn aluminium oxide and zinc oxide by heart.
How do you prepare salts in IGCSE Chemistry?
The method depends on solubility. For a soluble salt from an alkali, use titration. For a soluble salt from a metal, insoluble base or carbonate, add excess solid, filter, then crystallise.
For an insoluble salt, use precipitation. Mix two solutions, then filter, wash and dry the solid that forms. Choosing the wrong method is where marks quietly disappear.
This method selector links each salt type to the right preparation route and its key steps, which is the single most examined decision in Topic 7 salt questions every year.
| Salt type | Method | Key steps |
|---|---|---|
| Soluble salt from an alkali | Titration with indicator | Titrate with indicator, repeat without it, then crystallise the solution |
| Soluble salt from a solid | Excess solid, then filter | Add excess metal, base or carbonate, filter, then crystallise |
| Insoluble salt | Precipitation reaction | Mix two soluble solutions, then filter, wash and dry |
Read the reactants first. Two solutions point to titration or precipitation. A solid reactant points to the excess method. That single check answers when to titrate versus use excess reactant.
How do you make a soluble salt by titration?
Add acid to alkali using a burette and an indicator to find the exact volume. Repeat the titration without indicator using that volume. Evaporate and crystallise to get pure salt crystals.
How do you make a salt using excess solid?
Add excess metal, insoluble base or carbonate to warm acid until no more reacts. Filter off the leftover solid. Then evaporate and crystallise the filtrate to collect your salt.
How do you make an insoluble salt by precipitation?
Mix two soluble solutions that contain the right ions. The insoluble salt drops out as a precipitate. Filter it, wash it with distilled water, then leave it to dry.
Memorise these solubility rules, since they drive every method choice:
- All sodium, potassium and ammonium salts are soluble.
- All nitrates are soluble.
- Common chlorides are soluble, except silver and lead.
- Common sulfates are soluble, except barium, lead and calcium.
Method selection is examined almost every year. If your method answers keep losing marks, a specialist IGCSE Chemistry tutor can mark them against the real scheme.
How do you prepare for the IGCSE Chemistry exam?
Work topical past papers under timed conditions, then mark against the official mark scheme. Log every recurring mistake, such as the wrong salt method, strong versus weak mix-ups, or missing state symbols.
Confirm whether you sit Core or Extended, then target command words. State, describe and explain each demand a different depth of answer, and mark schemes reward the exact wording.
Watch for these acids, bases and salts traps that cost easy marks:
- Confusing acid strength with concentration.
- Skipping filtration or crystallisation steps.
- Choosing the wrong salt preparation method.
- Picking an indicator that does not fit.
Cambridge sets different papers for Core and Extended, so practise the right tier. Build a Cambridge chemistry revision guide around timed past papers and their mark schemes.
What should you revise next?
IGCSE Chemistry acids, bases and salts revision comes down to three mastered blocks: acid and base reactions, the pH scale with indicators, and solubility driven salt preparation. Remove the strong versus weak and method traps first.
Take one action today. Download a Topic 7 past paper, mark your salt method answers, and book a session if the same mistakes keep returning.
Frequently Asked Questions
A base is any substance that neutralises an acid. An alkali is a base that dissolves in water to release hydroxide ions. So every alkali is a base, but not every base is an alkali.
For strong acid and strong alkali titrations, use methyl orange or an indicator with a sharp end point. Thymolphthalein also works. Litmus is avoided because its colour change is too gradual to read clearly.
Yes. Solubility rules decide which salt preparation method to use, and that choice is examined almost every year. Learn that all nitrates and all sodium, potassium and ammonium salts are soluble, then learn the main exceptions.
Not really. Most students find the facts manageable but lose marks on method choice and the strong versus weak acid trap. Drill salt preparation and that one distinction, and this becomes a reliable Topic 7 scorer.
Very important. Acids, bases and salts appears in multiple choice, theory and practical papers, so it rewards focused revision. Because it links to ions, reactions and practical skills, strong answers lift marks across the whole paper.
About the author
Talimat Academic Team
Education Specialist
The Talimat Academic Team are Cambridge-trained British educators with extensive experience teaching IGCSE and A-Level across the GCC.
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