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K326 / K328 / 2027
Acid-Base Chemistry

Full chapter

Acid-Base Chemistry

Use ions and reaction patterns to explain acids, bases and salts.

O-Level 5086 / 5088 (2026) / SEC G3 K326 / K328 (2027)

01

Acids, alkalis and pH

Read acidity as an ion balance, not just a colour.

Make a guess. You are not marked.

Hydrochloric acid of pH 1 is diluted ten times with water. What is the pH now?

  1. Exactly pH 7, because water is neutral
  2. Still pH 1, as the acid is the same
  3. Higher than 1 but still below 7
Show the answer

Higher than 1 but still below 7

There are fewer hydrogen ions in each cm3, so the solution is less acidic. Adding water never makes an acid neutral or alkaline.

An acid produces H+ ions in aqueous solution. An alkali is a soluble base that produces OH- ions in water. A base neutralises an acid; insoluble copper(II) oxide is a base but is not an alkali. Acidic and alkaline solutions both contain H+ and OH-; their relative concentrations differ.

Universal Indicator and the pH scale
Solution at room temperatureRelative ionspH and typical indicator colour
AcidicMore H+ than OH-Below 7; red through orange/yellow as pH increases
NeutralEqual H+ and OH-7; green
AlkalineMore OH- than H+Above 7; blue through purple

Compare colours with the supplied indicator chart. Universal Indicator estimates a pH range; a suitable calibrated pH probe gives a numerical reading. We use qualitative comparisons here, not calculations of pH from hydrogen-ion concentration.

Soil pH affects nutrient availability and plant growth. Calcium hydroxide can treat excessively acidic soil because it neutralises acid. Adding too much creates a different problem: overly alkaline soil. Determine the starting pH and add an appropriate amount rather than assuming more is better.

Check your understanding

Does a neutral solution contain no H+ ions?

  1. It has no H+ ions, because H+ ions are what make a solution acidic.
  2. It has H+ ions, balanced by an equal concentration of OH- ions.
  3. It has H+ ions if a little acid was added to it beforehand, and none otherwise.
Show the answer

It has H+ ions, balanced by an equal concentration of OH- ions.

Water itself ionises slightly, so any aqueous solution has some H+ and OH-. Neutral means the two concentrations are equal, not that the ions are absent.

02

Recognise the reaction pattern

Predict the salt and the other products.

Acid and base reactions
ReactantsProductsBalanced example
Acid + suitable metalSalt + hydrogenMg(s) + 2HCl(aq) -> MgCl2(aq) + H2(g)
Acid + baseSalt + waterCuO(s) + H2SO4(aq) -> CuSO4(aq) + H2O(l)
Acid + carbonateSalt + water + carbon dioxideCaCO3(s) + 2HCl(aq) -> CaCl2(aq) + H2O(l) + CO2(g)
Alkali + ammonium salt, warmSalt + water + ammoniaNH4Cl(aq) + NaOH(aq) -> NaCl(aq) + H2O(l) + NH3(g)

The acid supplies the salt anion: hydrochloric acid gives chlorides, sulfuric acid sulfates, and nitric acid nitrates. The metal or base supplies the cation. Not every metal reacts with dilute acid: copper, for example, does not release hydrogen from dilute hydrochloric acid.

For an acid reacting with an alkali, the reacting ions are H+(aq) + OH-(aq) -> H2O(l). This is neutralisation. The other dissolved ions are spectators. For an ammonium salt and alkali, NH4+(aq) + OH-(aq) -> NH3(g) + H2O(l); warming helps ammonia escape.

Worked examplePredict and confirm the gas

Dilute acid is added to a carbonate and bubbles appear. How should the result be described?

Try it first. Then show the worked stepsHide the worked steps
  1. Predict carbon dioxide from the carbonate reaction pattern.
  2. Report effervescence as the observation.
  3. Pass the gas into limewater: a white precipitate supports carbon dioxide.
Answer

Effervescence alone does not identify a gas. Carbon dioxide turns limewater milky; hydrogen instead gives a pop with a lighted splint.

Check your understanding

What salt forms when zinc reacts with dilute sulfuric acid?

  1. Zinc sulfite, ZnSO3, with oxygen gas given off
  2. Zinc sulfide, ZnS, with water as the other product
  3. Zinc sulfate, ZnSO4, with hydrogen gas given off
Show the answer

Zinc sulfate, ZnSO4, with hydrogen gas given off

Metal + acid -> salt + hydrogen. The salt takes its first name from the metal and its second from the acid: Zn(s) + H2SO4(aq) -> ZnSO4(aq) + H2(g).

03

Classify an oxide by its reactions

Metal/non-metal character suggests a pattern, but exceptions matter.

Four oxide classes
ClassBehaviourExamples
BasicReacts with acids to form salt and waterCuO, MgO: generally metal oxides
AcidicReacts with bases to form salt and waterCO2, SO2: generally non-metal oxides
AmphotericReacts with both acids and strong alkalisAl2O3, ZnO
NeutralDoes not show typical acidic or basic oxide reactionsCO, NO; being a non-metal oxide does not guarantee acidity

For example, CO2(g) + 2NaOH(aq) -> Na2CO3(aq) + H2O(l) with excess alkali shows acidic-oxide behaviour. CuO reacting with an acid shows basic behaviour. An insoluble oxide can still be acidic or basic: classification is not determined by whether it dissolves in water.

Check your understanding

An oxide reacts with hydrochloric acid and also with aqueous sodium hydroxide. How is it classified?

  1. Amphoteric, as it reacts with acids and bases
  2. Neutral, because its acidic and basic sides cancel out
  3. Basic, because it reacts with hydrochloric acid
Show the answer

Amphoteric, as it reacts with acids and bases

A basic oxide reacts with acids only, and an acidic oxide with bases only. Reacting with both is the evidence for an amphoteric oxide, such as zinc oxide or aluminium oxide.

Sort these

Sort the oxides by how they behave with acids and alkalis.

Show the answer

Acidic: SO2 / CO2

Basic: CaO / CuO

Amphoteric: ZnO / Al2O3

Neutral: NO / CO

Quick revision

Revisit the essentials, then return to an explanation when you need it.

Exam definitions: learn these in mark-scheme words
TermDefinition to write
AcidA substance that produces hydrogen ions, H+(aq), when dissolved in water.
AlkaliA soluble base that produces hydroxide ions, OH-(aq), when dissolved in water.
BaseA substance that reacts with an acid to form a salt and water only. Most bases are metal oxides or hydroxides.
NeutralisationThe reaction between hydrogen ions and hydroxide ions to form water: H+(aq) + OH-(aq) → H2O(l).
SaltThe compound formed when the hydrogen ions of an acid are replaced by metal ions or ammonium ions.

Acids supply H+; alkalis supply OH-. Acid + base gives salt and water; carbonate also gives CO2; suitable metal gives H2. Alkali and ammonium salt release NH3 on warming.

Scope

Learning outcomes

5. Acid-Base Chemistry (5086 / 5088 / K326 / K328). Use the outcome map to find the explanation for a particular syllabus requirement.

See the learning outcome map

  1. 5(a) Define acids and alkalis

    • Aqueous H+ and OH-
    • Universal Indicator

    Acids, alkalis and pH

  2. 5(b) Compare acidity and alkalinity

    • Relative ion concentrations
    • Indicator colours
    • pH scale, no logarithmic calculation

    Acids, alkalis and pH

  3. 5(c) Describe acid reaction patterns

    • Metals, bases and carbonates
    • Salt products

    Recognise the reaction pattern

  4. 5(d) Explain neutralisation

    • H+ + OH- -> water

    Recognise the reaction pattern

  5. 5(e) Explain control of soil acidity

    • Importance of soil pH
    • Calcium hydroxide treatment

    Acids, alkalis and pH

  6. 5(f) Describe base reactions

    • With acids
    • With ammonium salts on warming

    Recognise the reaction pattern

  7. 5(g) Classify oxides

    • Acidic, basic, amphoteric, neutral
    • Metal/non-metal character and exceptions

    Classify an oxide by its reactions