Acids, Bases and Salts — CXC CSEC Integrated Science Revision Notes

What you'll learn

This topic examines the properties, identification and reactions of acids, bases and salts — a core component of CXC CSEC Integrated Science Paper 02. You must understand pH values, neutralisation reactions, the formation of salts, and how to identify substances using indicators. Questions typically appear as structured response items worth 8-12 marks, requiring balanced equations and explanations of observable changes.

Key terms and definitions

Acid — A substance that produces hydrogen ions (H⁺) when dissolved in water, tastes sour, and has a pH less than 7.

Base — A substance that neutralises acids to form salt and water only; includes metal oxides and hydroxides.

Alkali — A soluble base that produces hydroxide ions (OH⁻) in aqueous solution and has a pH greater than 7.

Salt — An ionic compound formed when the hydrogen ion of an acid is replaced by a metal ion or ammonium ion.

Neutralisation — A chemical reaction between an acid and a base that produces salt and water, with pH moving toward 7.

Indicator — A substance that shows different colours in acidic and alkaline solutions (litmus, methyl orange, phenolphthalein, universal indicator).

pH scale — A numerical scale from 0 to 14 that measures the acidity or alkalinity of a solution, with 7 being neutral.

Corrosive — A property of strong acids and alkalis that describes their ability to damage or destroy living tissue and materials.

Core concepts

Properties of acids

Acids share characteristic properties that appear frequently in CXC CSEC Integrated Science practical questions:

Physical properties:

Sour taste (citric acid in limes and tamarind, found throughout the Caribbean)
Turn blue litmus paper red
pH values between 0 and 6.9
Conduct electricity in aqueous solution due to mobile ions

Chemical properties:

React with metals above hydrogen in the reactivity series to produce hydrogen gas and a salt
React with carbonates and hydrogencarbonates to produce carbon dioxide, water and a salt
React with bases to produce salt and water (neutralisation)

Common acids tested at CSEC level:

Hydrochloric acid (HCl) — stomach acid, used in Caribbean manufacturing
Sulfuric acid (H₂SO₄) — car batteries, fertiliser production in Trinidad
Nitric acid (HNO₃) — fertiliser manufacture
Ethanoic acid (CH₃COOH) — vinegar, weak acid
Citric acid — limes, oranges, guineps

Properties of bases and alkalis

Bases include metal oxides and metal hydroxides. The soluble bases are called alkalis.

Physical properties of alkalis:

Bitter taste and slippery feel (sodium hydroxide solutions)
Turn red litmus paper blue
pH values between 7.1 and 14
Conduct electricity in aqueous solution

Chemical properties:

React with acids in neutralisation reactions
React with ammonium salts when heated to produce ammonia gas
Strong alkalis are corrosive to skin and tissue

Common bases and alkalis in CSEC questions:

Sodium hydroxide (NaOH) — caustic soda, soap making in Jamaica
Calcium hydroxide (Ca(OH)₂) — limewater, used to treat acidic soils
Potassium hydroxide (KOH) — strong alkali
Ammonia solution (NH₃(aq)) — weak alkali, cleaning products
Copper(II) oxide (CuO) — insoluble base
Zinc oxide (ZnO) — insoluble base

The pH scale and indicators

The pH scale measures hydrogen ion concentration on a logarithmic scale from 0 to 14:

pH 0-2: Strong acids (battery acid, stomach acid)
pH 3-6: Weak acids (citrus juice, rainwater, carbonated drinks)
pH 7: Neutral (pure water, blood)
pH 8-10: Weak alkalis (baking soda solution, soap)
pH 11-14: Strong alkalis (bleach, drain cleaners)

Universal indicator provides a range of colours corresponding to pH values:

Red/Pink — pH 1-3 (strong acid)
Orange/Yellow — pH 4-6 (weak acid)
Green — pH 7 (neutral)
Blue — pH 8-10 (weak alkali)
Purple/Violet — pH 11-14 (strong alkali)

Other indicators tested at CSEC:

Indicator	Colour in acid	Colour in alkali
Litmus	Red	Blue
Methyl orange	Red	Yellow
Phenolphthalein	Colourless	Pink

Reactions of acids

1. Acid + Metal → Salt + Hydrogen

When acids react with reactive metals (magnesium, zinc, iron), hydrogen gas is produced:

Magnesium + Sulfuric acid → Magnesium sulfate + Hydrogen
Mg(s) + H₂SO₄(aq) → MgSO₄(aq) + H₂(g)

Zinc + Hydrochloric acid → Zinc chloride + Hydrogen
Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g)

Test for hydrogen: Insert a lighted splint — hydrogen burns with a "pop" sound.

2. Acid + Carbonate → Salt + Water + Carbon dioxide

Carbonates and hydrogencarbonates react with acids producing effervescence (fizzing):

Calcium carbonate + Hydrochloric acid → Calcium chloride + Water + Carbon dioxide
CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)

Sodium hydrogencarbonate + Ethanoic acid → Sodium ethanoate + Water + Carbon dioxide
NaHCO₃(s) + CH₃COOH(aq) → CH₃COONa(aq) + H₂O(l) + CO₂(g)

Test for carbon dioxide: Bubble gas through limewater — it turns milky/cloudy white.

3. Acid + Base/Alkali → Salt + Water (Neutralisation)

This reaction type is heavily tested at CXC CSEC level:

Hydrochloric acid + Sodium hydroxide → Sodium chloride + Water
HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)

Sulfuric acid + Copper(II) oxide → Copper(II) sulfate + Water
H₂SO₄(aq) + CuO(s) → CuSO₄(aq) + H₂O(l)

Nitric acid + Ammonia → Ammonium nitrate + Water
HNO₃(aq) + NH₃(aq) → NH₄NO₃(aq) + H₂O(l)

Observable changes: Temperature increases (exothermic), pH moves toward 7, insoluble base dissolves.

Preparing salts

CXC CSEC questions ask you to describe methods for preparing soluble and insoluble salts.

Method 1: Preparing soluble salts from acids and insoluble bases

Used for most metal sulfates, chlorides and nitrates:

Add excess insoluble base (metal oxide or carbonate) to warm dilute acid
Stir until no more base dissolves (excess ensures all acid reacts)
Filter to remove excess unreacted base
Evaporate the filtrate to reduce volume
Leave to crystallise
Filter and dry crystals between filter paper

Example: Preparing copper(II) sulfate crystals from copper(II) oxide and dilute sulfuric acid.

Method 2: Preparing soluble salts from acid-alkali neutralisation (titration)

Used when both reactants are soluble — requires careful measurement:

Measure alkali into conical flask using pipette
Add indicator (methyl orange or phenolphthalein)
Add acid from burette until neutralisation point (colour change)
Record volume of acid used
Repeat without indicator using exact volumes measured
Evaporate and crystallise

Method 3: Preparing insoluble salts (precipitation)

Mix two soluble salts; the insoluble product precipitates:

Mix solutions of two soluble salts
Filter to collect the precipitate
Wash precipitate with distilled water
Dry between filter paper

Example: Lead(II) nitrate + Sodium chloride → Lead(II) chloride (precipitate) + Sodium nitrate

Pb(NO₃)₂(aq) + 2NaCl(aq) → PbCl₂(s) + 2NaNO₃(aq)

Uses of acids, bases and salts in the Caribbean

Acids:

Sulfuric acid in car batteries throughout the region
Citric acid from citrus industries in Dominica and Jamaica
Nitric acid in fertiliser production for banana and sugar cane cultivation

Bases and alkalis:

Calcium hydroxide (slaked lime) to neutralise acidic soils in Guyana rice fields
Sodium hydroxide in soap manufacture in Trinidad and Barbados
Ammonia in household cleaning products region-wide

Salts:

Sodium chloride from sea salt production in Turks and Caicos
Calcium sulfate (gypsum) in construction materials
Ammonium nitrate fertilisers for Caribbean agriculture

Worked examples

Example 1: Identifying substances using pH (6 marks)

A student tested four colourless liquids, P, Q, R and S, using universal indicator. The results are shown in the table:

Liquid	Universal indicator colour	pH
P	Red	2
Q	Green	7
R	Yellow	5
S	Purple	13

(a) Which liquid is a strong acid? (1 mark)
(b) Which liquid is neutral? (1 mark)
(c) Which liquid is a strong alkali? (1 mark)
(d) Name the ion responsible for the alkaline properties of liquid S. (1 mark)
(e) Liquid P was added to a piece of magnesium ribbon. State TWO observations. (2 marks)

Answers:

(a) Liquid P ✓ (pH 2 indicates strong acid)

(b) Liquid Q ✓ (pH 7 is neutral)

(d) Hydroxide ion / OH⁻ ✓

(e) Any TWO from:

Bubbles/fizzing/effervescence ✓
Magnesium dissolves/disappears ✓
Colourless gas produced ✓
Temperature increases/solution gets warm ✓

Example 2: Neutralisation and salt formation (8 marks)

(a) Define the term 'neutralisation'. (2 marks)
(b) A farmer in St. Vincent found that his soil was too acidic. Suggest a substance he could add to neutralise the soil. (1 mark)
(c) Write a balanced chemical equation for the reaction between sulfuric acid and calcium hydroxide. (3 marks)
(d) Name the salt formed in part (c). (1 mark)
(e) State the pH of the solution at the end of a complete neutralisation reaction. (1 mark)

Answers:

(a) A reaction between an acid and a base ✓ that produces salt and water only ✓

(b) Calcium hydroxide / slaked lime / lime / calcium oxide / calcium carbonate ✓

(c) H₂SO₄ + Ca(OH)₂ → CaSO₄ + 2H₂O
(Correct formulae: 1 mark ✓, correct balancing: 1 mark ✓, state symbols or → : 1 mark ✓)

(d) Calcium sulfate ✓

(e) pH 7 / neutral ✓

Example 3: Preparing a salt (5 marks)

Describe how you would prepare a pure, dry sample of zinc chloride crystals starting from zinc oxide powder and dilute hydrochloric acid. (5 marks)

Answer:

Add excess zinc oxide to dilute hydrochloric acid ✓
Warm/heat gently and stir ✓
Filter to remove excess zinc oxide ✓
Evaporate the filtrate to reduce volume / heat filtrate gently ✓
Leave to crystallise / cool to form crystals ✓
Filter and dry crystals between filter paper ✓

(Any 5 correct steps)

Common mistakes and how to avoid them

Mistake: Confusing bases and alkalis — stating all bases are alkalis.
Correction: All alkalis are bases, but not all bases are alkalis. Alkalis are soluble bases only (e.g., NaOH, KOH, NH₃ solution). Metal oxides like CuO are bases but not alkalis because they are insoluble.
Mistake: Writing "sodium hydroxide + hydrochloric acid → sodium chloride" without water as a product.
Correction: Neutralisation always produces salt AND water. The complete equation is: NaOH + HCl → NaCl + H₂O. Omitting water loses marks.
Mistake: Claiming that adding water to an acid neutralises it.
Correction: Diluting an acid with water makes it less concentrated but does NOT neutralise it. Neutralisation requires a base. The diluted acid remains acidic with pH < 7.
Mistake: Stating that hydrogen gas turns limewater milky.
Correction: Carbon dioxide turns limewater milky. Hydrogen burns with a "pop" sound when a lighted splint is applied. Know the correct gas tests for CXC practicals.
Mistake: Using the term "alkaline solution" when the pH is exactly 7.
Correction: pH 7 is neutral, not alkaline. Alkaline solutions have pH > 7. Pure water at 25°C is neutral with pH 7.
Mistake: Writing unbalanced equations without checking.
Correction: Always count atoms on both sides. For H₂SO₄ + NaOH → Na₂SO₄ + H₂O, you need 2NaOH to balance the sodium and provide enough H and OH for 2H₂O. Balanced: H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O.

Exam technique for "Acids, Bases and Salts: Properties and Reactions"

Command word "Describe" requires a sequence of steps or observations. For salt preparation, give at least 4-5 detailed steps including filtration, evaporation and crystallisation. Each step earns a mark; vague answers like "mix and heat" score poorly.
Equation questions award separate marks for correct formulae (1 mark), correct balancing (1 mark), and sometimes state symbols (1 mark). Write formulae carefully — CaSO₄ not CaSO3, Mg not mg. Check your balancing before moving on.
"State and explain" or "Suggest and explain" questions require both identification and reasoning. Stating "add calcium hydroxide" (1 mark) then "because it is a base that neutralises acids" (1 mark) earns full credit. One without the other loses marks.
Practical questions test your ability to describe procedures with precision. Use technical terms: "filter" not "separate", "effervescence" not "bubbles", "precipitate" not "solid formed". Mention safety (e.g., wear goggles with corrosive substances) when asked about precautions.

Quick revision summary

Acids (pH < 7) produce H⁺ ions, turn litmus red, and react with metals to produce hydrogen, with carbonates to produce CO₂, and with bases in neutralisation. Alkalis (pH > 7) are soluble bases producing OH⁻ ions, turning litmus blue. Neutralisation forms salt and water only. Salts are prepared by reacting acids with metals, bases, or carbonates (for soluble salts) or by precipitation (for insoluble salts). Universal indicator shows pH by colour. Master balanced equations, gas tests (hydrogen pops; CO₂ turns limewater milky), and detailed salt preparation methods for CXC CSEC success.