Acids, Bases and Salts notes: ICSE Class 10 Chemistry (Science Paper 2)

1. Acids, bases and alkalis in terms of ions

Acid: gives H₃O⁺ as the only positive ion in water (ionisation of covalent molecules). Monobasic: HCl, HNO₃; dibasic: H₂SO₄ (two steps). Strong acids ionise almost completely; weak acids (CH₃COOH, H₂CO₃) only partly.

Base: reacts with an acid to give salt and water only (metal oxides and hydroxides). Alkali: a soluble base giving OH⁻ as the only negative ion (dissociation of an ionic solid). All alkalis are bases; not all bases are alkalis.

Table contrasting ionisation and dissociation
Process	Happens to	Example
Ionisation	Covalent molecules	HCl in water
Dissociation	Ionic solids	NaOH in water

Formulas

Overall ionisation of sulphuric acid
$\ce{H2SO4 + 2H2O -> 2H3O+ + SO4^2-}$
Dissociation of potassium hydroxide
$\ce{KOH -> K+ + OH-}$

Key terms

Acid: A compound that gives hydronium ions as the only positive ions in aqueous solution.
Alkali: A base that dissolves in water and gives hydroxide ions as the only negative ions.
Basicity: The number of hydronium ions produced by one molecule of an acid in water.

2. Indicators and pH

Indicator	Acid	Neutral	Alkali
Litmus	Red	Purple	Blue
Methyl orange	Pink	Orange	Yellow
Phenolphthalein	Colourless	Colourless	Pink

pH scale 0–14: 7 neutral; below 7 acidic (0–3 strongly, 4–6 weakly); above 7 alkaline (8–10 weakly, 11–14 strongly). Universal indicator / pH paper gives a colour for each pH (red → green → violet).

Table of pH values and the nature of the solution
pH	Nature
1	Strongly acidic
5	Weakly acidic
7	Neutral
13	Strongly alkaline

Key terms

pH: A number from 0 to 14 expressing how acidic or alkaline a solution is; 7 is neutral.
Universal indicator: A mixture of indicators that shows a different colour at each pH value.

3. Kinds of salts

A salt forms when the hydrogen ion of an acid is replaced, partly or completely, by a metal or ammonium ion.

Normal salt: all replaceable H replaced — Na₂SO₄, NaCl, KNO₃.
Acid salt: part of the replaceable H remains — NaHSO₄, NaHCO₃ (only from acids with more than one replaceable H).
Basic salt: contains hydroxide from the base — Pb(OH)Cl, basic copper carbonate.
Complex salt: contains a complex ion that does not break up in water — K₄Fe(CN)₆, sodium argentocyanide.

Table of four salts and their kinds
Salt	Kind
KNO₃	Normal
NaHCO₃	Acid
Pb(OH)Cl	Basic
K₄Fe(CN)₆	Complex

Formulas

An acid salt
$\ce{NaOH + H2SO4 -> NaHSO4 + H2O}$

Key terms

Acid salt: A salt formed by partial replacement of the replaceable hydrogen of an acid; it still contains replaceable hydrogen.
Basic salt: A salt formed by partial replacement of the hydroxide of a base by an acid radical.

4. Dilute acids on carbonates, sulphites and sulphides

Salt + dilute acid	Gas	Test
Carbonate / hydrogen carbonate	CO₂	Lime water milky; no effect on acidified K₂Cr₂O₇
Sulphite / hydrogen sulphite	SO₂	Acidified K₂Cr₂O₇ orange → green; acidified KMnO₄ pink → colourless
Sulphide	H₂S	Rotten-egg smell; lead acetate paper turns silvery black

$\ce{CaCO3 + 2HCl -> CaCl2 + H2O + CO2}$ ; $\ce{Na2SO3 + 2HCl -> 2NaCl + H2O + SO2}$ ; $\ce{FeS + H2SO4 -> FeSO4 + H2S}$ .

Table of tests for three gases
Gas	Test
CO₂	Lime water milky
SO₂	Dichromate green
H₂S	Lead acetate black

Formulas

Hydrogen sulphite with dilute acid
$\ce{NaHSO3 + HCl -> NaCl + H2O + SO2}$

Key terms

Effervescence: Brisk bubbling caused by a gas being given off from a liquid.

5. Preparing salts: direct combination, displacement, precipitation

Choose the method by solubility. Insoluble salts (AgCl, PbCl₂, BaSO₄, PbSO₄, most carbonates) → precipitation (double decomposition). Soluble salts → other methods.

Direct combination: $\ce{2Fe + 3Cl2 -> 2FeCl3}$
Displacement: $\ce{Zn + H2SO4 -> ZnSO4 + H2}$
Precipitation: $\ce{Pb(NO3)2 + 2NaCl -> PbCl2 + 2NaNO3}$ ; $\ce{BaCl2 + Na2SO4 -> BaSO4 + 2NaCl}$

Table of three salts with solubility and method
Salt	Soluble?	Method
BaSO₄	No	Precipitation
ZnSO₄	Yes	Displacement
FeCl₃	Yes	Direct combination

Key terms

Precipitation (double decomposition): Preparation of an insoluble salt by mixing solutions of two soluble salts that exchange ions.

6. Preparing salts: insoluble base, titration, carbonates

Excess insoluble base + dilute acid, filter, crystallise: $\ce{CuO + H2SO4 -> CuSO4 + H2O}$ .
Titration (salts of Na, K, NH₄): alkali and salt both soluble, so excess cannot be filtered off; find exact volumes with an indicator, repeat without it: $\ce{KOH + HNO3 -> KNO3 + H2O}$ .
Acid + carbonate or hydrogen carbonate: $\ce{ZnCO3 + H2SO4 -> ZnSO4 + H2O + CO2}$ ; $\ce{NaHCO3 + HCl -> NaCl + H2O + CO2}$ .

Table explaining when titration is needed
Base or salt used	Soluble?	Excess removed by
CuO	No	Filtration
NaOH	Yes	Cannot — use titration

Key terms

Titration: Adding one solution to another from a burette until the reaction is exactly complete, judged by an indicator.

Exam tips

Common mistakes

Leaving out “only” in the definitions of acid and alkali.
Phenolphthalein colours reversed; methyl orange called red in alkali.
Choosing titration for copper sulphate or displacement for lead chloride.
Forgetting that SO₂ also turns lime water milky.

How marks are usually awarded

Equations (1 mark): correct formulae and balancing; state the gas.
Methods (2 marks): name the method and justify it by solubility.
Tests (1 mark): reagent and colour change.

1. Acids, bases and alkalis in terms of ions

Formulas

Key terms

2. Indicators and pH

Key terms

3. Kinds of salts

Formulas

Key terms

4. Dilute acids on carbonates, sulphites and sulphides

Formulas

Key terms

5. Preparing salts: direct combination, displacement, precipitation

Key terms

6. Preparing salts: insoluble base, titration, carbonates

Key terms

Exam tips

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