This mini-lesson walks you through the key ideas of the CCEA Acids, Bases & Salts unit (1.8): the pH scale and indicators, what makes a solution acidic or alkaline, neutralisation, the main reactions of acids, how to name a salt, and how to make soluble and insoluble salts.
Make a guess: why do only some acid reactions fizz?
Mission 1Why do only some acid reactions fizz?
Mission 2Why add too much on purpose?
Work through each screen, answer the questions as you go (some are wordy, some are calculations) and collect ⭐ stars. Press Start when you're ready.
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The pH scale
The pH scale measures how acidic or alkaline a solution is. It runs from 0 to 14.
pH less than 7 → acidic. The lower the pH, the more acidic.
pH exactly 7 → neutral (e.g. pure water).
pH greater than 7 → alkaline. The higher the pH, the more alkaline.
The pH scale with universal indicator colours. Green is neutral, pH 7.
⚠️ Watch out: pH 7 is neutral, not "no pH". Pure water is pH 7. A pH of 6 is still slightly acidic — it does not have to be close to 0 to count as an acid.
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Indicators
Indicators change colour depending on the pH. CCEA expects you to know four:
The four CCEA indicators and their colours in acid vs alkali.
Litmus: red in acid, blue in alkali.
Methyl orange: red in acid, yellow in alkali.
Phenolphthalein: colourless in acid, pink in alkali.
Universal indicator: a mixture of dyes giving a range of colours, so it tells you the actual pH — not just "acid or alkali".
Quick check
Read the indicator
?A few drops of phenolphthalein are added to a colourless solution and it turns pink. What can you conclude about the solution?
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What makes acids and alkalis
The behaviour comes down to the ions released in water:
An acid produces hydrogen ions, H⁺, when dissolved in water.
An alkali produces hydroxide ions, OH⁻, when dissolved in water. (An alkali is simply a soluble base.)
When an acid and an alkali react, the H⁺ and OH⁻ ions join to form water. This is neutralisation:
H⁺(aq) + OH⁻(aq) → H₂O(l)The same ionic equation describes every acid + alkali neutralisation.
💡 Bases vs alkalis: a base is a metal oxide or hydroxide that neutralises an acid to produce a salt and water. An alkali is a base that dissolves in water, such as sodium hydroxide. All alkalis are bases; not all bases are alkalis.
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The main reactions of acids
CCEA expects you to know what an acid makes with metals, bases, carbonates, hydrogencarbonates and ammonia. Hydrogencarbonates react just like carbonates, and ammonia gives an ammonium salt only (NH₃ + HCl → NH₄Cl). The three core patterns:
acid + metal → salt + hydrogene.g. 2HCl + Zn → ZnCl₂ + H₂ (metal must be above hydrogen in reactivity)
acid + base → salt + waterbase = metal oxide or hydroxide. e.g. H₂SO₄ + CuO → CuSO₄ + H₂O
acid + carbonate → salt + water + carbon dioxidee.g. 2HCl + CaCO₃ → CaCl₂ + H₂O + CO₂
⚠️ Watch out: only the carbonate (or hydrogencarbonate) reaction fizzes off CO₂; only the metal reaction gives off hydrogen. The base (oxide/hydroxide) reaction makes no gas — just salt and water. Test the CO₂ with limewater (turns milky); test hydrogen with a lit splint (squeaky pop).
Quick check
Name the products
?Dilute hydrochloric acid is added to calcium carbonate and the mixture fizzes. What are the products?
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Naming the salt
A salt's name has two parts. The first part comes from the metal (or ammonium) in the base/metal/carbonate. The second part comes from the acid:
The acid decides the ending: chloride, sulfate or nitrate.
⚠️ Watch out: the salt's ending always comes from the acid, never from the base. Hydrochloric → chloride, sulfuric → sulfate, nitric → nitrate.
Sort it
Which salt does each reaction make?
Tap a reaction on the left, then tap the salt it makes on the right.
Mission 1 check
Why do only some acid reactions fizz?
?Dilute sulfuric acid is added to three tubes. One holds zinc, one copper(II) oxide and one copper(II) carbonate. Which statement explains which tubes fizz?
Mission 2 of 2
Why add too much on purpose?
Make a guess: why add more copper oxide than can react?
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Making a soluble salt (insoluble base method)
To make a soluble salt from an acid and an insoluble base, metal or carbonate, you use excess solid so all the acid reacts:
Add the solid (e.g. copper oxide) to warm acid until no more dissolves — the excess tells you all the acid is used up.
Filter off the leftover unreacted solid. The filtrate is the salt solution.
Evaporate some water, then leave to crystallise. Pat the crystals dry.
💡 Why excess? Using excess insoluble solid guarantees the acid is fully neutralised, and the leftover solid simply gets filtered out — so the salt solution is left pure.
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Making a soluble salt (titration method)
When both the acid and the alkali are soluble (e.g. HCl + NaOH) you can't use excess solid — there is nothing to filter off. Instead use a titration:
Acid runs from the burette into a measured volume of alkali plus indicator.
Pipette a known volume of alkali into a flask with an indicator.
Run acid from the burette until the indicator just changes — the end-point, where the solution is neutral. Note the volume.
Repeat using the same volumes but no indicator, then evaporate and crystallise to get the pure salt.
Quick check
Pick the method
?You want pure crystals of sodium chloride from hydrochloric acid and sodium hydroxide (both soluble). Which preparation should you use?
Mission 2 check
Why add too much on purpose?
?A student makes zinc sulfate from dilute sulfuric acid and zinc carbonate, which is insoluble. She adds zinc carbonate until some is left over, then filters. Why add too much on purpose?
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Insoluble salts by precipitation
If the salt you want is insoluble, you can't crystallise it from solution. Instead, mix two soluble solutions so the insoluble salt drops out as a solid — a precipitate:
⚠️ Watch out: precipitation is the route for insoluble salts only. Don't try to make an insoluble salt by evaporating a solution, and don't try to make a soluble salt by precipitation.
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Solubility rules
You decide which method to use by knowing which salts are soluble. The usual rules:
All sodium, potassium and ammonium salts are soluble.
All nitrates are soluble.
Most chlorides are soluble — except silver chloride and lead chloride.
Most sulfates are soluble — except barium, lead and calcium sulfate.
Most carbonates and hydroxides are insoluble — except those of sodium, potassium and ammonium.
Tap a salt, then tap the box it belongs in. Use the solubility rules.
💧 Soluble
🧱 Insoluble
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Strong vs weak acids (Higher)
Strength is about how fully an acid ionises (splits into ions) in water — it is not the same as concentration:
A strong acid (e.g. hydrochloric, sulfuric, nitric) is fully ionised — every molecule releases its H⁺. This gives a high H⁺ concentration and a low pH.
A weak acid (e.g. ethanoic acid, carbonic acid) only partially ionises — most molecules stay whole. Fewer H⁺ ions, so a higher pH for the same concentration.
Same concentration, different ionisation: the strong acid releases far more H⁺.
⚠️ Watch out:strong ≠ concentrated. A dilute strong acid can have more free H⁺ than a concentrated weak acid. Strength = degree of ionisation; concentration = how much acid per volume.
Quick check · Higher tier
Strong or weak? (Higher)
?Why does ethanoic acid have a higher pH than hydrochloric acid of the same concentration?