Characteristic Properties of Acids and Bases
Chemistry· 7.1 2026-2028 Syllabus· 25 min read
1. Core: Physical Properties and Core Definitions★★☆☆☆⏱ 6 min
Acid
A substance that turns blue litmus paper red, has a pH below 7, tastes sour, and neutralises bases. Common lab acids include hydrochloric, sulfuric and nitric acid.
Example:
Vinegar is a household acid containing dilute ethanoic acid.
Base
A substance that turns red litmus paper blue (if soluble, i.e. an alkali), has a pH above 7, and neutralises acids. Most bases are insoluble in water.
Example:
Sodium hydroxide is a soluble base (alkali); copper(II) oxide is an insoluble base.
Core tier learners only need to recall observable properties and definitions for assessment; knowledge of ion dissociation is only required for Extended tier.
The syllabus names three indicators you must be able to use to distinguish acids from alkalis. Learn each indicator's colour in an acid and in an alkali; a neutral solution gives the acid-side colour for thymolphthalein and methyl orange, and stays purple for full-range litmus.
Litmus: red in acids, blue in alkalis
Thymolphthalein: colourless in acids, blue in alkalis
Methyl orange: red in acids, yellow in alkalis
A student tests two unknown solutions with universal indicator: Solution X turns red, Solution Y turns purple. Classify each solution and state their approximate pH ranges.
- 1
Step 1: Recall indicator colour rules: acids produce warm colours (red/orange/yellow) and bases produce cool colours (blue/purple) with universal indicator.
- 2
Step 2: Classify Solution X: red = acid, approximate pH range 1-3.
- 3
Step 3: Classify Solution Y: purple = alkali (soluble base), approximate pH range 12-14.
Exam tip:
Always specify that alkalis are soluble bases in exam answers to gain full marks; describing all bases as soluble is a common mark-losing error.
2. Core: Key Chemical Reactions of Acids★★★☆☆⏱ 7 min
Acid + reactive metal (above H in reactivity series) → salt + hydrogen gas
Acid + base/alkali → salt + water (neutralisation reaction)
Acid + metal carbonate → salt + water + carbon dioxide gas
You must be able to write word equations for these reactions, and recall tests for gaseous products: hydrogen burns with a pop when a lit splint is applied, carbon dioxide turns limewater milky.
Write the word equation for the reaction between dilute sulfuric acid and solid magnesium carbonate, and state the test for the gaseous product.
- 1
Step 1: Identify reactant types: sulfuric acid = acid, magnesium carbonate = metal carbonate.
- 2
Step 2: Apply the general reaction rule for acid + carbonate: products are salt, water and carbon dioxide.
- 3
Step 3: Name the salt: metal cation = magnesium, acid anion = sulfate, so salt = magnesium sulfate.
- 4
Step 4: Full word equation: sulfuric acid + magnesium carbonate → magnesium sulfate + water + carbon dioxide.
- 5
Step 5: Test for carbon dioxide: bubble the gas through limewater; a milky white precipitate forms if carbon dioxide is present.
3. Core: Key Chemical Reactions of Bases/Alkalis★★★☆☆⏱ 6 min
Alkali + ammonium salt → salt + water + ammonia gas
Base + acid → salt + water (neutralisation, same as acid reaction rule)
Ammonia gas has a sharp pungent smell and turns damp red litmus paper blue. This reaction is commonly tested in practical questions to identify ammonium ions in unknown salts.
A student heats a mixture of potassium hydroxide solution and ammonium nitrate. Name the products formed and state the observation for the gaseous product.
- 1
Step 1: Identify reactant types: potassium hydroxide = alkali, ammonium nitrate = ammonium salt.
- 2
Step 2: Apply general reaction rule for alkali + ammonium salt: products are salt, water and ammonia.
- 3
Step 3: Name the salt: cation = potassium, anion = nitrate, so salt = potassium nitrate.
- 4
Step 4: Observation for ammonia: damp red litmus paper held above the mixture turns blue, and a pungent odour is detected.
Exam tip:
Always specify litmus paper is damp for ammonia testing; dry litmus will not change colour as ammonia needs water to form alkaline hydroxide ions.
4. Extended Only: Strong and Weak Acids and Bases★★★★☆Extended only⏱ 6 min
Strong Acid
An acid that completely dissociates into ions when dissolved in water, producing a high concentration of H⁺ ions.
Example:
Hydrochloric, sulfuric and nitric acid are all strong acids.
Weak Acid
An acid that only partially dissociates into ions when dissolved in water, producing a low concentration of H⁺ ions.
Example:
Ethanoic acid, citric acid and carbonic acid are weak acids.
The same rule applies to alkalis: strong alkalis (e.g. sodium hydroxide) completely dissociate to produce OH⁻ ions, while weak alkalis (e.g. ammonia solution) only partially dissociate. Note that strength (degree of dissociation, fixed property) is different from concentration (amount of substance per volume of solution, variable).
Explain why 1 mol/dm³ ethanoic acid has a higher pH than 1 mol/dm³ hydrochloric acid, even though their concentrations are identical.
- 1
Step 1: Classify the acids: hydrochloric acid = strong acid, ethanoic acid = weak acid.
- 2
Step 2: Describe dissociation: Hydrochloric acid completely dissociates in water to produce a high concentration of H⁺ ions. Ethanoic acid only partially dissociates, producing a lower concentration of H⁺ ions.
- 3
Step 3: Link to pH: Lower H⁺ ion concentration = higher pH value, so ethanoic acid is less acidic with a higher pH.
5. Common Pitfalls
Wrong move:
Calling all bases alkalis
Why:
Only soluble bases are alkalis; most bases are insoluble so do not fit the definition of an alkali.
Correct move:
State that alkalis are a subset of soluble bases, and explicitly label a base as an alkali only if it dissolves in water.
Wrong move:
Testing ammonia gas with dry red litmus paper
Why:
Ammonia only forms alkaline OH⁻ ions when dissolved in water, so dry litmus paper will not change colour.
Correct move:
Always use damp red litmus paper to test for ammonia gas.
Wrong move:
Confusing acid strength with acid concentration
Why:
Strength refers to degree of ion dissociation (fixed property of the acid), concentration refers to amount of acid per volume of solution (variable).
Correct move:
Explicitly distinguish between the two terms in extended-tier explanation questions, and use the correct term as required.
Wrong move:
Predicting hydrogen gas production when acid reacts with any metal
Why:
Only metals above hydrogen in the reactivity series react with dilute acids to produce hydrogen; unreactive metals like copper do not react.
Correct move:
Check the reactivity series position of the metal before predicting reaction products.
Wrong move:
Writing incorrect salt names in reaction equations
Why:
Salt names combine the cation from the base/carbonate/metal with the anion from the acid; incorrect anion naming is a common error.
Correct move:
First identify the anion from the acid (hydrochloric = chloride, sulfuric = sulfate, nitric = nitrate) before naming the salt.
6. Quick Reference Cheatsheet
Property | Acid | Base/Alkali |
|---|---|---|
Litmus effect | Turns blue litmus red | Turns red litmus blue (alkalis only) |
Thymolphthalein | Colourless | Blue (alkalis only) |
Methyl orange | Red | Yellow (alkalis only) |
pH range | Below 7 | Above 7 |
Reaction with reactive metal | Salt + hydrogen | No reaction |
Reaction with carbonate | Salt + water + CO₂ | No reaction |
Reaction with ammonium salt | No reaction | Salt + water + ammonia (alkalis only) |
Extended: Strength rule | Strong = full H⁺ dissociation, Weak = partial H⁺ dissociation | Strong = full OH⁻ dissociation, Weak = partial OH⁻ dissociation |
7. Frequently Asked
Are all bases alkalis?
No. Only soluble bases are classified as alkalis. All alkalis are bases, but most bases (e.g. copper oxide) are insoluble so are not alkalis.
How do I test for ammonia gas?
Hold damp red litmus paper above the reaction mixture. Ammonia will turn the damp red litmus paper blue, and has a sharp pungent odour.
Going deeper
- syllabus2026-2028 CIE IGCSE Chemistry 0620 Specification
- worksheetAcid-Base Reaction Word Equations Practice
What's Next
Now that you have mastered the characteristic properties of acids and bases for CIE IGCSE Chemistry 0620, you can progress to learning about the pH scale, neutralisation reactions, and acid-base titrations, which are required for both Core and Extended written and practical assessments. You will also build on this foundational knowledge to study the preparation of soluble and insoluble salts, a high-weightage topic frequently tested in Papers 3 and 4. Be sure to practice writing word and balanced symbol equations for all reaction types covered in this guide to avoid common mark-losing errors.