Full chapter
Acid-Base Chemistry
Use ions and reaction patterns to explain acids, bases and salts.
SEC G2 Science K223 / K225 (2027)
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?
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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.
| Solution at room temperature | Relative ions | pH and typical indicator colour |
|---|---|---|
| Acidic | More H+ than OH- | Below 7; red through orange/yellow as pH increases |
| Neutral | Equal H+ and OH- | 7; green |
| Alkaline | More 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?
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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.
Recognise the reaction pattern
Predict the salt and the other products.
| Reactants | Products | Balanced example |
|---|---|---|
| Acid + suitable metal | Salt + hydrogen | Mg(s) + 2HCl(aq) -> MgCl2(aq) + H2(g) |
| Acid + base | Salt + water | CuO(s) + H2SO4(aq) -> CuSO4(aq) + H2O(l) |
| Acid + carbonate | Salt + water + carbon dioxide | CaCO3(s) + 2HCl(aq) -> CaCl2(aq) + H2O(l) + CO2(g) |
| Alkali + ammonium salt, warm | Salt + water + ammonia | NH4Cl(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
- Predict carbon dioxide from the carbonate reaction pattern.
- Report effervescence as the observation.
- Pass the gas into limewater: a white precipitate supports carbon dioxide.
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?
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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).
Classify an oxide by its reactions
Metal/non-metal character suggests a pattern, but exceptions matter.
| Class | Behaviour | Examples |
|---|---|---|
| Basic | Reacts with acids to form salt and water | CuO, MgO: generally metal oxides |
| Acidic | Reacts with bases to form salt and water | CO2, SO2: generally non-metal oxides |
| Amphoteric | Reacts with both acids and strong alkalis | Al2O3, ZnO |
| Neutral | Does not show typical acidic or basic oxide reactions | CO, 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?
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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.
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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.
| Term | Definition to write |
|---|---|
| Acid | A substance that produces hydrogen ions, H+(aq), when dissolved in water. |
| Alkali | A soluble base that produces hydroxide ions, OH-(aq), when dissolved in water. |
| Base | A substance that reacts with an acid to form a salt and water only. Most bases are metal oxides or hydroxides. |
| Neutralisation | The reaction between hydrogen ions and hydroxide ions to form water: H+(aq) + OH-(aq) → H2O(l). |
| Salt | The 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 (K223 / K225). Use the outcome map to find the explanation for a particular syllabus requirement.
See the learning outcome map
5(a) Define acids and alkalis
- Aqueous H+ and OH-
- Universal Indicator
5(b) Compare acidity and alkalinity
- Relative ion concentrations
- Indicator colours
- pH scale, no logarithmic calculation
5(c) Describe acid reaction patterns
- Metals, bases and carbonates
- Salt products
5(d) Explain neutralisation
- H+ + OH- -> water
5(e) Explain control of soil acidity
- Importance of soil pH
- Calcium hydroxide treatment
5(f) Describe base reactions
- With acids
- With ammonium salts on warming
5(g) Classify oxides
- Acidic, basic, amphoteric, neutral
- Metal/non-metal character and exceptions