Acids, Bases and Salts

What you'll learn

This topic forms a major component of the CXC CSEC Chemistry syllabus and appears in both Paper 01 (multiple choice) and Paper 02 (structured questions). You will master the properties of acids, bases and salts, their reactions, methods of salt preparation, and the pH scale. Questions typically assess your ability to classify substances, predict products of reactions, and explain preparation methods with appropriate reagents.

Key terms and definitions

Acid — a substance that releases hydrogen ions (H⁺) when dissolved in water, tastes sour, turns blue litmus red, and has a pH below 7.

Base — a substance that reacts with an acid to form a salt and water only; metal oxides and metal hydroxides are bases.

Alkali — a soluble base that releases hydroxide ions (OH⁻) when dissolved in water, feels slippery, turns red litmus blue, and has a pH above 7.

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

Neutralisation — the reaction between an acid and a base to produce a salt and water, where H⁺ ions from the acid combine with OH⁻ ions from the alkali to form water.

pH scale — a numerical scale from 0 to 14 that measures the acidity or alkalinity of a solution; pH 7 is neutral, below 7 is acidic, above 7 is alkaline.

Indicator — a substance that changes colour depending on whether it is in an acidic or alkaline solution.

Basicity of an acid — the number of hydrogen ions that can be replaced by metal ions per molecule of acid; HCl is monobasic, H₂SO₄ is dibasic, H₃PO₄ is tribasic.

Core concepts

Properties and classification of acids

Acids share distinct physical and chemical properties that appear regularly in CXC CSEC examination questions:

• Physical properties: sour taste (never taste laboratory acids), corrosive to skin and metals, conduct electricity in aqueous solution due to presence of ions
• Effect on indicators: turn blue litmus paper red, turn methyl orange from yellow to red, turn phenolphthalein from pink to colourless
• Common laboratory acids: hydrochloric acid (HCl), sulfuric acid (H₂SO₄), nitric acid (HNO₃), ethanoic acid (CH₃COOH)

Strong versus weak acids: Strong acids like HCl, H₂SO₄ and HNO₃ ionise completely in water, releasing all available H⁺ ions. Weak acids like ethanoic acid ionise only partially. At the same concentration, strong acids have lower pH values and react more vigorously.

Dilute versus concentrated acids: Concentration refers to the amount of acid dissolved in a given volume of solution. A dilute strong acid can have the same pH as a concentrated weak acid. The Caribbean soft drinks industry uses dilute ethanoic acid and citric acid as flavourings and preservatives.

Properties and classification of bases and alkalis

All alkalis are bases, but not all bases are alkalis — this distinction appears frequently in exam questions.

Base properties:

• React with acids to form salts and water
• Metal oxides (CuO, ZnO, CaO) and metal hydroxides (Cu(OH)₂, Zn(OH)₂, Ca(OH)₂) are bases
• Most bases are insoluble in water

Alkali properties:

• Soluble bases that dissolve in water to form OH⁻ ions
• Bitter taste, slippery feel (never taste or touch laboratory alkalis)
• Conduct electricity in aqueous solution
• Turn red litmus blue, turn methyl orange yellow, turn phenolphthalein pink
• Common examples: sodium hydroxide (NaOH), potassium hydroxide (KOH), calcium hydroxide (Ca(OH)₂), aqueous ammonia (NH₃(aq))

Ammonia is a special case: the gas NH₃ is not an alkali, but when dissolved in water it forms ammonium hydroxide (NH₄OH), which releases OH⁻ ions, making the solution alkaline. Ammonia solution is used in the manufacture of fertilisers at the Point Lisas Industrial Estate in Trinidad.

The pH scale and indicators

The pH scale ranges from 0 to 14 and measures hydrogen ion concentration:

• pH 0-2: strongly acidic (battery acid, stomach acid)
• pH 3-6: weakly acidic (vinegar pH 3, rainwater pH 5-6)
• pH 7: neutral (pure water, blood)
• pH 8-10: weakly alkaline (baking soda solution, soap)
• pH 11-14: strongly alkaline (ammonia solution, caustic soda)

Universal indicator shows a range of colours across the pH scale: red (pH 1-3), orange (pH 4-5), yellow (pH 6), green (pH 7), blue (pH 8-10), purple (pH 11-14). CXC examiners expect you to interpret universal indicator colours and assign approximate pH values.

Natural indicators: Many Caribbean plants contain compounds that act as indicators. Red cabbage juice turns pink in acid and green in alkali. Hibiscus (sorrel) extract changes from red in acid to blue-green in alkali.

Reactions of acids

CXC CSEC examination papers test four main types of acid reactions. You must write correct word equations and balanced symbol equations.

1. Acid + metal → salt + hydrogen

Magnesium + hydrochloric acid → magnesium chloride + hydrogen

Mg + 2HCl → MgCl₂ + H₂

Test for hydrogen: gas burns with a 'pop' sound when a lighted splint is applied. Not all metals react with acids — copper, silver and gold do not displace hydrogen from acids.

2. Acid + metal oxide (base) → salt + water

Copper(II) oxide + sulfuric acid → copper(II) sulfate + water

CuO + H₂SO₄ → CuSO₄ + H₂O

This is a neutralisation reaction. The black copper(II) oxide dissolves to form a blue solution of copper(II) sulfate.

3. Acid + metal hydroxide (base/alkali) → salt + water

Sodium hydroxide + nitric acid → sodium nitrate + water

NaOH + HNO₃ → NaNO₃ + H₂O

This is also a neutralisation reaction. The ionic equation shows the essential process:

H⁺(aq) + OH⁻(aq) → H₂O(l)

4. Acid + metal carbonate → salt + water + carbon dioxide

Calcium carbonate + hydrochloric acid → calcium chloride + water + carbon dioxide

CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂

Test for carbon dioxide: gas turns limewater milky (cloudy). This reaction explains why limestone (calcium carbonate) buildings deteriorate in industrial areas where acid rain (dilute sulfuric and nitric acids) falls. The Blue Mountains in Jamaica contain limestone caves formed by this type of reaction over geological time.

Salt nomenclature and preparation methods

Naming salts: The salt name has two parts:

1. First part: the metal or ammonium from the base
2. Second part: derived from the acid (hydrochloric → chloride, sulfuric → sulfate, nitric → nitrate, ethanoic → ethanoate)

Examples: potassium + sulfuric acid → potassium sulfate; zinc + nitric acid → zinc nitrate

Preparation methods for salts:

CXC CSEC papers regularly test your knowledge of appropriate preparation methods. The method chosen depends on the solubility of the salt and the reactants available.

Method 1: Titration (for soluble salts from alkali + acid)

Used when both reactants are soluble (e.g., making sodium chloride from NaOH and HCl).

Steps:

1. Titrate acid into alkali using phenolphthalein indicator to find the exact volumes needed for neutralisation
2. Mix the same volumes of acid and alkali without indicator
3. Evaporate the solution to crystallisation point
4. Filter off crystals, wash with distilled water, dry between filter paper

Method 2: Excess solid reactant (for soluble salts from insoluble base/metal + acid)

Used to prepare soluble salts like copper(II) sulfate from copper(II) oxide and sulfuric acid.

Steps:

1. Add excess solid (metal oxide, hydroxide, carbonate, or reactive metal) to warm dilute acid
2. Stir until no more solid dissolves and acid stops fizzing
3. Filter to remove excess unreacted solid
4. Evaporate filtrate to crystallisation point
5. Filter off crystals, wash, dry

The excess solid ensures all acid is used up, so the salt solution is not contaminated with acid.

Method 3: Precipitation (for insoluble salts)

Used to prepare insoluble salts like lead(II) sulfate or barium sulfate.

Example: Lead(II) nitrate solution + sodium sulfate solution → lead(II) sulfate (precipitate) + sodium nitrate solution

Pb(NO₃)₂(aq) + Na₂SO₄(aq) → PbSO₄(s) + 2NaNO₃(aq)

Steps:

1. Mix solutions of two soluble salts
2. Filter to collect the precipitate
3. Wash precipitate with distilled water
4. Dry between filter paper or in a warm oven

The solubility rules determine which method to use. All nitrates, sodium salts, potassium salts and ammonium salts are soluble. Most chlorides are soluble except silver chloride and lead(II) chloride. Most sulfates are soluble except barium sulfate, lead(II) sulfate and calcium sulfate.

Applications and uses

Agricultural applications: Lime (calcium oxide or calcium hydroxide) is added to acidic soils in Jamaican and Trinidadian farming regions to neutralise excess acidity and improve crop yields. The pH of soil affects nutrient availability.

Industrial applications:

• Sulfuric acid is used in car batteries and in petroleum refining
• Hydrochloric acid removes rust and scale from metals
• Sodium hydroxide is used in soap manufacture and paper production
• Calcium carbonate (limestone) is quarried in Barbados and Jamaica for cement production

Domestic applications:

• Vinegar (ethanoic acid) in cooking and food preservation
• Citric acid in soft drinks produced by Caribbean Bottling Company and other regional manufacturers
• Antacid tablets contain bases like magnesium hydroxide or calcium carbonate to neutralise excess stomach acid
• Sodium hydrogencarbonate (baking soda) neutralises acid stings from ants and bees

Worked examples

Example 1: Salt preparation

Question: Describe how you would prepare a pure, dry sample of zinc sulfate crystals starting from zinc oxide powder and dilute sulfuric acid. Your answer should include a balanced symbol equation. [6 marks]

Answer:

Equation: ZnO + H₂SO₄ → ZnSO₄ + H₂O [2 marks for correct formulae and balancing]

Method:

1. Add excess zinc oxide powder to warm dilute sulfuric acid in a beaker [1 mark]
2. Stir until the acid stops reacting (no more fizzing) [½ mark]
3. Filter the mixture to remove unreacted zinc oxide [1 mark]
4. Heat the filtrate gently to evaporate water until crystals start to form at the surface [1 mark]
5. Leave to cool and crystallise, then filter off the crystals and dry them between filter paper [½ mark]

Example 2: pH and indicators

Question: A student tested four solutions labelled A, B, C and D with universal indicator. The colours observed were: A-red, B-yellow, C-green, D-purple.

(a) State which solution is neutral. [1 mark]

(b) Arrange the solutions in order of increasing pH. [1 mark]

(c) Which solution would neutralise solution D most effectively? Explain your answer. [2 marks]

Answer:

(a) Solution C [1 mark] - green indicates pH 7, which is neutral

(b) A, B, C, D [1 mark] - red has lowest pH (most acidic), purple has highest pH (most alkaline)

(c) Solution A [1 mark] because it is the most acidic (lowest pH) and would neutralise the most alkaline solution D most effectively by providing the most H⁺ ions [1 mark]

Example 3: Reactions of acids

Question: Calcium carbonate reacts with hydrochloric acid.

(a) Write a balanced symbol equation for this reaction. [2 marks]

(b) Describe a test to identify the gas produced. [2 marks]

(c) A farmer in St. Elizabeth, Jamaica has acidic soil. Explain why adding crushed limestone (calcium carbonate) would improve the soil. [2 marks]

Answer:

(a) CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂ [2 marks for correct equation with correct balancing; 1 mark if minor error]

(b) Bubble the gas through limewater (calcium hydroxide solution) [1 mark]. If carbon dioxide is present, the limewater turns milky/cloudy [1 mark]

(c) The calcium carbonate is a base and will neutralise the acids in the soil [1 mark], raising the pH to a level more suitable for plant growth / making nutrients more available to plants [1 mark]

Common mistakes and how to avoid them

• Confusing bases and alkalis: Students often describe all bases as producing OH⁻ ions. Correction: Only alkalis (soluble bases) produce OH⁻ ions in solution. Copper(II) oxide is a base but not an alkali because it is insoluble.

• Writing incorrect salt names: Students write "zinc chlorine" instead of "zinc chloride" or "copper sulfur" instead of "copper sulfate". Correction: The second part of the salt name comes from the acid: hydrochloric acid gives chlorides, sulfuric acid gives sulfates, nitric acid gives nitrates.

• Wrong preparation method chosen: Students use titration for preparing copper(II) sulfate (should use excess solid method because copper(II) oxide is insoluble). Correction: Use titration only when both reactants are soluble; use excess solid method when making a soluble salt from an insoluble reactant.

• Forgetting to filter excess reactant: Students evaporate the mixture without filtering first, producing impure salt. Correction: Always filter to remove excess unreacted solid before evaporating to obtain pure salt crystals.

• Mixing up concentrated and strong: Students think concentrated acids are always stronger than dilute acids. Correction: Concentration is about the amount of acid in a given volume; strength is about the degree of ionisation. A concentrated weak acid (ethanoic acid) can have a higher pH than a dilute strong acid (hydrochloric acid).

• Incorrect neutralisation equation: Students write the products as just "salt" or omit water. Correction: Acid + base always gives salt AND water. Both products must be included in word and symbol equations.

Exam technique for Acids, Bases and Salts

• Command words matter: "Describe" requires a sequence of steps for preparation methods (4-5 distinct points for maximum marks). "Explain" requires reasons, not just statements. "State" needs brief answers without elaboration. "Write a balanced equation" requires correct formulae and coefficients.

• Structured answers for salt preparation: CXC mark schemes allocate 1 mark per distinct step. Include: reactants used, heating/stirring, filtering, evaporating to crystallisation point, drying method. Missing the filtering step loses marks. Always mention "excess" solid when using Method 2.

• Show your working in calculations: Even if the question does not explicitly say "show working", write down the equation used and substitute values. Partial marks are awarded for correct method even if the final answer is wrong.

• Use state symbols when required: Questions worth 2-3 marks for equations often require state symbols (s), (l), (g), (aq). Practice writing these automatically: acids and alkalis are (aq), water is (l), gases are (g), precipitates are (s).

Quick revision summary

Acids release H⁺ ions, have pH below 7, turn blue litmus red. Bases react with acids to form salts and water. Alkalis are soluble bases that release OH⁻ ions and have pH above 7. Neutralisation: H⁺ + OH⁻ → H₂O. Acids react with metals (producing H₂), bases, and carbonates (producing CO₂). Salt preparation: use titration for two soluble reactants, excess solid method for insoluble base plus acid, precipitation for insoluble salts. Name salts from the metal and acid used. Learn solubility rules to select correct preparation method.