Acids, Bases and Salts

Slide 1 · Learning Outcomes

What you will be able to do

In Form 1 you met acids and bases as substances that change indicator colours. In Form 4 we explain why they behave that way, and use that to identify ions and handle salts.

Define acids and bases using the Arrhenius and Brønsted–Lowry theories, and pick out conjugate acid–base pairs.
Explain the role of water (the solvent) in the properties of acids and bases.
Tell strong and weak acids and bases apart using pH, conductivity and rate of reaction.
Write equations showing amphoteric oxides and hydroxides reacting with acids and alkalis.
Use NaOH(aq) and NH₃(aq) to identify cations by their precipitates and complex ions.
Read solubility curves and calculate the mass of crystals formed on cooling.
Explain the causes, effects and removal of hardness in water.

How to use this lesson: work through slides 2–8, try every activity, then do the Practice (slide 9) before the Quiz (slide 10). Use the arrows or dots at the bottom to move.

Slide 2 · Theories of acids and bases

What makes a substance an acid?

Arrhenius theory

An acid is a substance that dissolves in water to produce hydrogen ions, H⁺ (really hydroxonium ions, H₃O⁺) as the only positive ions.

A base reacts with an acid to form a salt and water only. A soluble base is an alkali: it gives OH⁻ ions in water.

HCl(g) + H₂O(l) → H₃O⁺(aq) + Cl⁻(aq)

Brønsted–Lowry theory

An acid is a proton (H⁺) donor.

A base is a proton acceptor.

This theory also covers reactions that do not happen in water, e.g. NH₃(g) + HCl(g) → NH₄Cl(s).

Conjugate pairs: when an acid gives away a proton, what is left is its conjugate base. When a base accepts a proton, it becomes its conjugate acid. Water can act as either, so it is amphiprotic.

Activity: spot the proton donor

Tap the species on the left side of the equation that acts as the acid (proton donor).

Slide 3 · Role of water

Acids need water to behave as acids

Dry hydrogen chloride is made of covalent HCl molecules. Only when it dissolves in water do the molecules ionise into H₃O⁺ and Cl⁻ ions. In a non-polar solvent such as methylbenzene (toluene) it stays molecular.

Activity: choose the solvent

Remember: the acidic properties of HCl are due to H₃O⁺ ions, and the alkaline properties of ammonia are due to OH⁻ ions. No water → no ions → no acidic/alkaline properties.

Slide 4 · Strength of acids and bases

Strong vs weak: how much ionises?

A strong acid ionises completely in water. A weak acid ionises only partially, so most of it stays as molecules and an equilibrium (⇌) is set up.

HCl(aq) → H⁺(aq) + Cl⁻(aq)

CH₃COOH(aq) ⇌ CH₃COO⁻(aq) + H⁺(aq)

NaOH(aq) → Na⁺(aq) + OH⁻(aq)

NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq)

Activity: compare 0.1 M solutions

Each dot = a particle. Green dots are ions, grey dots are un-ionised molecules. Same concentration, very different numbers of ions.

Strong ≠ concentrated. Strength is about the degree of ionisation; concentration is about how many moles per litre. You can have a dilute strong acid or a concentrated weak acid.

Slide 5 · Amphoteric oxides and hydroxides

Oxides that react with acids and alkalis

Most metal oxides are basic. A few, like ZnO, Al₂O₃ and PbO (and their hydroxides), react with both acids and alkalis. They are amphoteric.

Activity: build the equations

Pattern: with acid → normal salt + water (the oxide acts as a base). With alkali → a complex salt such as sodium zincate (the oxide acts as an acid).

Slide 6 · Identifying cations

Precipitates and complex ions

Adding NaOH(aq) or NH₃(aq) to a salt solution supplies OH⁻ ions. Most metal hydroxides are insoluble, so a precipitate forms. What happens in excess reagent identifies the ion.

M²⁺(aq) + 2OH⁻(aq) → M(OH)₂(s)

Virtual test tube

Solution containing:

Choose an ion, then add a reagent.

Key complex ions: [Zn(OH)₄]²⁻, [Al(OH)₄]⁻, [Pb(OH)₄]²⁻ form in excess NaOH (all colourless). In excess NH₃, only Zn²⁺ → [Zn(NH₃)₄]²⁺ (colourless) and Cu²⁺ → [Cu(NH₃)₄]²⁺ (deep blue) dissolve.

Slide 7 · Solubility and solubility curves

How much salt will dissolve?

Solubility is the maximum mass of a solute that dissolves in 100 g of water at a given temperature to form a saturated solution. A graph of solubility against temperature is a solubility curve.

KNO₃ NaCl KClO₃

Temperature: 40 °C

Reading the curve: KNO₃ solubility rises steeply with temperature; NaCl hardly changes. That is why NaCl is obtained by evaporation, while KNO₃ is obtained by cooling (crystallisation).

Mass crystallising on cooling = solubility at higher T − solubility at lower T (per 100 g water). Then scale to the actual mass of water.

Fractional crystallisation

A mixture of KNO₃ and NaCl dissolved in hot water is cooled. KNO₃ crystallises out first because its solubility falls sharply; NaCl stays in solution. Filtering separates them.

Slide 8 · Hardness of water

Hard water and how to soften it

Hard water does not lather easily with soap. It contains dissolved Ca²⁺ and/or Mg²⁺ ions, which react with soap to form an insoluble scum.

	Temporary hardness	Permanent hardness
Caused by	Ca(HCO₃)₂, Mg(HCO₃)₂	CaSO₄, MgSO₄, CaCl₂, MgCl₂
How it forms	Rain water with dissolved CO₂ flows over limestone: CaCO₃ + H₂O + CO₂ → Ca(HCO₃)₂	Water dissolves calcium/magnesium sulphates and chlorides from rocks
Removed by boiling?	Yes	No

Boiling: Ca(HCO₃)₂(aq) → CaCO₃(s) + H₂O(l) + CO₂(g)

Washing soda: Ca²⁺(aq) + CO₃²⁻(aq) → CaCO₃(s)

Activity: sort the facts

Tap a card, then tap the box it belongs to.

Removes temporary hardness only

Removes both types

Advantage of hard water

Disadvantages: wastes soap, forms fur/scale in kettles and boilers (blocks pipes, wastes fuel), stains clothes. Ion exchange: Ca²⁺ and Mg²⁺ in the water swap places with Na⁺ on a resin (permutit).

Slide 9 · Practice

Worked example and quick checks

Worked example: crystals on cooling

The solubility of potassium nitrate is 110 g/100 g water at 60 °C and 32 g/100 g water at 20 °C. A saturated solution containing 50 g of water at 60 °C is cooled to 20 °C. Calculate the mass of crystals formed.

Step 1: Mass that crystallises per 100 g water = 110 − 32 = 78 g

Step 2: The solution contains 50 g of water, not 100 g.

Step 3: 100 g water → 78 g crystals, so 50 g water → (78 × 50) ÷ 100

Answer: 39 g of KNO₃ crystals

Your turn

1. 12 g of a salt saturates 30 g of water at 25 °C. Its solubility is g per 100 g water.

2. In NH₄⁺ + H₂O ⇌ NH₃ + H₃O⁺, the conjugate base of NH₄⁺ is .

3. A white precipitate that dissolves in excess NaOH but not in excess NH₃ could be from Al³⁺ or .

4. The fur that forms inside kettles in hard-water areas is mainly calcium .

Slide 10 · Quiz

Check your understanding

Part A: Fill in the blanks (auto-marked, 1 mark each)

1. According to Brønsted–Lowry, a base is a proton .

2. In methylbenzene, hydrogen chloride exists as , so it does not conduct electricity.

3. Ethanoic acid is a acid because it only partially ionises in water.

4. An oxide such as ZnO that reacts with both acids and alkalis is described as .

5. Copper(II) ions in excess aqueous ammonia form a deep solution.

6. Hardness that can be removed by boiling is called hardness.

7. Solubility is the mass of solute that saturates g of water at a given temperature.

8. The insoluble solid formed when soap reacts with hard water is called .

9. The ion H₃O⁺ is called the ion.

10. A precipitate that is green and insoluble in excess NaOH shows the presence of ions.

Part B: Short answers (smart-marked against key points)

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