Neutralisation of acids and salt production — AQA GCSE Combined Science
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Neutralisation of acids and salt production explained
Neutralisation occurs when an acid reacts with a base, removing acidic and basic character.
Read the full explanation
Alkalis are soluble bases, typically soluble metal hydroxides such as sodium hydroxide, NaOH(aq). Insoluble bases include metal oxides such as copper(II) oxide, CuO(s), and insoluble metal hydroxides such as iron(III) hydroxide, Fe(OH)₃(s). Both react with acids to form a salt and water only: acid + base → salt + water. For example, hydrochloric acid with sodium hydroxide gives sodium chloride and water: HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l). Metal carbonates, such as calcium carbonate, CaCO₃(s), also neutralise acids but produce carbon dioxide as well: acid + metal carbonate → salt + water + carbon dioxide. For example, 2HCl(aq) + CaCO₃(s) → CaCl₂(aq) + H₂O(l) + CO₂(g). The carbon dioxide is observed as effervescence and turns limewater milky.
The particular salt produced in any reaction between an acid and a base or alkali depends on:
The salt formed in a neutralisation reaction has two parts: the metal or ammonium ion from the base or alkali, and the anion from the acid. The acid determines the anion: hydrochloric acid produces chloride salts, nitric acid produces nitrate salts, and sulfuric acid produces sulfate salts. The base or alkali determines the metal or ammonium ion: sodium hydroxide gives sodium salts, copper(II) oxide gives copper(II) salts, and ammonia gives ammonium salts. For example, sulfuric acid with copper(II) oxide gives copper(II) sulfate and water: H₂SO₄(aq) + CuO(s) → CuSO₄(aq) + H₂O(l). Note that ammonia reacts with acids to form only an ammonium salt, without producing water. The name must show the metal or ammonium ion and the acid-derived anion, and the formula must balance charges, such as Cu²⁺ with SO₄²⁻ giving CuSO₄.
the acid used (hydrochloric acid produces chlorides, nitric acid produces nitrates, sulfuric acid produces sulfates)
When an acid reacts with a base, alkali or carbonate, the salt formed depends on which acid is used. Hydrochloric acid supplies chloride ions, so its salts are chlorides, such as sodium chloride from sodium hydroxide. Nitric acid supplies nitrate ions, giving nitrates such as potassium nitrate. Sulfuric acid supplies sulfate ions, giving sulfates such as copper(II) sulfate. The metal or ammonium part of the salt comes from the base, alkali or carbonate, while the acid fixes the second part of the salt name. For example, reacting sulfuric acid with copper(II) oxide produces copper(II) sulfate and water. Naming the salt therefore requires identifying both the positive ion from the base, alkali or carbonate and the negative ion from the acid.
the positive ions in the base, alkali or carbonate.
The positive ion in a base, alkali or carbonate determines the first part of the salt name. In sodium hydroxide the positive ion is Na⁺, so salts formed are sodium salts. In copper(II) oxide the positive ion is Cu²⁺, so salts are copper(II) salts. In calcium carbonate the positive ion is Ca²⁺, so salts are calcium salts. The acid supplies the negative ion, but the positive ion comes from the base, alkali or carbonate. For example, magnesium oxide reacting with hydrochloric acid gives magnesium chloride, because Mg²⁺ comes from the oxide and Cl⁻ comes from the acid. Identifying the positive ion correctly, including its charge where relevant, allows the salt to be named and its formula to be deduced.
predict products from given reactants
Predicting products means working out what is formed when named reactants are neutralised. First decide whether the reactants are an acid and a base or alkali. Acids include hydrochloric, sulfuric and nitric acid. Bases include metal oxides, metal hydroxides, metal carbonates and ammonia. An acid with a metal hydroxide or metal oxide gives a salt and water. An acid with a metal carbonate also gives carbon dioxide, seen as fizzing. An acid with ammonia gives only an ammonium salt. The salt name comes from the metal or ammonium ion plus the acid ending: chloride from hydrochloric acid, sulfate from sulfuric acid, and nitrate from nitric acid. For example, nitric acid with calcium carbonate gives calcium nitrate, water and carbon dioxide.
use the formulae of common ions to deduce the formulae of salts.
A salt is an ionic compound formed when the hydrogen of an acid is replaced by a metal or ammonium ion. To deduce a salt formula, split it into its cation and anion, write each ion's formula with its charge, then balance the total positive and negative charges so they cancel. For example, sodium ions Na⁺ and sulfate ions SO₄²⁻ need two Na⁺ for each SO₄²⁻, giving Na₂SO₄. Aluminium Al³⁺ with chloride Cl⁻ needs three Cl⁻, giving AlCl₃. Where a polyatomic ion is needed more than once, bracket it: calcium hydroxide is Ca(OH)₂. Charges are not shown in the final formula; they are only a working step. This skill underpins writing balanced equations for neutralisation and predicting products of acid reactions.
Your focus
- State the products of reactions between acids and alkalis or bases, and between acids and metal carbonates.
- Classify substances as alkalis, bases or metal carbonates and predict the salt formed in each reaction.
- Write word equations and balanced symbol equations, including state symbols, for neutralisation reactions.
Show all 18 objectives
- Identify the anion contributed by a given acid and the metal or ammonium ion contributed by a given base or alkali.
- Name the salt produced from a specified acid and base or alkali, including salts of variable-charge metals.
- Write correct formulae for salts by balancing the charges on the cation and anion.
- Identify the negative ion supplied by hydrochloric, nitric and sulfuric acids.
- Construct the correct salt name from a named acid and a named base, alkali or carbonate.
- Apply the naming rule accurately to salts containing variable-charge positive ions.
- Identify the positive ion in a given base, alkali or carbonate.
- Use the positive ion to name the salt formed when it reacts with a named acid.
- Deduce the formula of a salt by balancing the charges of its positive and negative ions.
- Classify given reactants as acids, metal oxides, metal hydroxides, carbonates or ammonia.
- Predict the salt, water and any carbon dioxide formed from a stated acid-base or acid-carbonate reaction, noting ammonia forms only a salt.
- Construct word and balanced symbol equations for the predicted products using correct formulae and state symbols.
- State the formulae and charges of common cations and anions accurately.
- Combine a named cation and anion to produce a correctly balanced salt formula.
- Apply brackets correctly when a polyatomic ion appears more than once in a formula.
Neutralisation of acids and salt production exam tips
Marking Points
- Acid + alkali or base → salt + water; this is neutralisation and no gas is produced in these reactions.
- Alkalis are soluble metal hydroxides, for example NaOH(aq) and KOH(aq); bases include insoluble metal hydroxides and metal oxides such as CuO(s) and Fe(OH)₃(s).
- Acid + metal carbonate → salt + water + carbon dioxide; the gas is identified by bubbling through limewater, which turns milky or cloudy.
- Word equations and balanced symbol equations must use correct formulae, including state symbols where required, for example HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l).
- The salt name is built from the metal or ammonium ion from the base or carbonate and the acid anion: chloride from hydrochloric acid, nitrate from nitric acid, sulfate from sulfuric acid.
- Neutralisation can be followed by pH change towards pH 7, and in practical work an indicator or pH meter can show when the acid has been neutralised.
- The acid used determines the anion in the salt: hydrochloric acid → chloride, nitric acid → nitrate, sulfuric acid → sulfate.
- The base or alkali used determines the metal or ammonium ion in the salt: sodium hydroxide → sodium salts, potassium hydroxide → potassium salts, copper(II) oxide → copper(II) salts, ammonia → ammonium salts.
- Salt names combine the metal or ammonium ion with the acid anion, for example sodium nitrate from sodium hydroxide and nitric acid.
- Formulae must have balanced charges: for example, Mg²⁺ with Cl⁻ gives MgCl₂, and Al³⁺ with SO₄²⁻ gives Al₂(SO₄)₃.
- Roman numerals in names such as copper(II) oxide show the charge on the metal ion and must be reflected in the salt formula.
- Word equations and balanced symbol equations should show the salt and water as products for acid + metal oxide/hydroxide reactions, but only the salt for ammonia + acid reactions.
- State that hydrochloric acid produces chloride salts, for example sodium chloride from sodium hydroxide.
- State that nitric acid produces nitrate salts, for example potassium nitrate from potassium hydroxide.
- State that sulfuric acid produces sulfate salts, for example copper(II) sulfate from copper(II) oxide.
- Name a salt by combining the positive ion from the base, alkali or carbonate with the negative ion supplied by the acid.
- Write the salt name correctly, including any Roman numeral where the positive ion has variable charge, such as iron(II) sulfate or iron(III) chloride.
- Identify the positive ion present in a named base, alkali or carbonate, such as Na⁺ in sodium hydroxide or Cu²⁺ in copper(II) oxide.
- Use that positive ion to write the first part of the salt name, for example sodium in sodium sulfate or copper(II) in copper(II) nitrate.
- Recognise that the positive ion may be a metal ion or the ammonium ion, NH₄⁺, when an ammonium compound is used.
- Deduce the salt formula by balancing the charge on the positive ion from the base, alkali or carbonate with the charge on the negative ion from the acid.
- Explain that the positive ion is unchanged during neutralisation and appears in the salt product.
- Classify each reactant as an acid or a base, alkali or carbonate before predicting products.
- Apply the pattern acid + metal hydroxide or metal oxide → salt + water.
- Apply the pattern acid + metal carbonate → salt + water + carbon dioxide.
- Apply the pattern acid + ammonia → ammonium salt.
- Derive the salt name from the metal or ammonium ion and the acid, for example sulfate from sulfuric acid and sodium from sodium hydroxide.
- Write the products as a word equation and, where asked, as a balanced symbol equation with correct formulae and state symbols.
- Identify the cation and anion present in the named salt, using the metal or ammonium ion as the cation and the acid-derived ion as the anion.
- State the charge on each common ion correctly, for example Na⁺, Mg²⁺, Al³⁺, Cl⁻, O²⁻, OH⁻, NO₃⁻, SO₄²⁻, CO₃²⁻.
- Apply the rule that total positive charge must equal total negative charge in the final neutral formula.
- Use the lowest whole-number ratio of ions, so magnesium chloride is MgCl₂ rather than Mg₂Cl₄.
- Place polyatomic ions in brackets when more than one is required, as in Ca(OH)₂ and Al₂(SO₄)₃.
- Write the final formula without charge symbols, since the compound itself is electrically neutral.
Examiner Tips
- 💡Underline the acid and the base or carbonate in the question, then name the salt from the metal or ammonium ion and the acid anion before writing the equation.
- 💡When a gas is produced, state the test and the positive result, for example carbon dioxide turns limewater milky.
- 💡Balance symbol equations by checking each element in turn, and include state symbols if the question asks for them.
- 💡For required practical work, describe adding the solid base or carbonate gradually to the acid until it is in excess, then filtering to remove excess solid.
- 💡Identify the acid first to fix the anion, then identify the base or alkali to fix the metal or ammonium ion, and combine them to name the salt.
- 💡Check the charge on each ion and use the lowest whole-number ratio when writing a formula, for example Ca²⁺ and NO₃⁻ give Ca(NO₃)₂.
- 💡If a question gives a salt name, work backwards to identify the acid and base that could produce it.
- 💡Include state symbols only when they are requested or when they help show that a solid base has reacted with an aqueous acid.
- 💡Underline the acid name in the question, then write the negative ion it supplies before naming the salt.
- 💡Check every salt name contains two parts: the positive ion from the base, alkali or carbonate and the negative ion from the acid.
- 💡Practise a three-column table of acid, negative ion and salt ending so the pattern becomes automatic.
- 💡Write the positive ion symbol and charge above the base, alkali or carbonate before attempting the salt name or formula.
- 💡For variable-charge metals, read the Roman numeral in the name and use it as the ion charge, for example iron(III) means Fe³⁺.
- 💡Cross-check the final formula so the total positive charge equals the total negative charge.
- 💡Write the general pattern for the reaction type in the margin before naming the specific products.
- 💡Check the metal or ammonium ion in the base, because this fixes the first part of the salt name.
- 💡For carbonate reactions, include the gas test observation, such as bubbling or limewater turning milky, if the question asks for evidence.
- 💡Write the two ion formulae with their charges side by side before combining them; this makes the balancing step visible and easy to check.
- 💡Cross-check by multiplying the subscript by the ion charge for each ion and confirming the two totals are equal in size.
- 💡If a question gives an unfamiliar salt name, break it into the familiar cation and anion parts rather than trying to recall the whole formula.
- 💡Show the bracketed form for hydroxide, nitrate, sulfate and carbonate salts whenever the subscript is greater than one.
Common Mistakes
- Writing that acid + alkali produces carbon dioxide: correct this by stating that carbon dioxide is produced only when a metal carbonate reacts with an acid.
- Confusing soluble and insoluble hydroxides: correct this by defining alkalis as soluble metal hydroxides and noting that insoluble metal hydroxides are bases but not alkalis.
- Omitting water from the products of acid + metal carbonate: correct this by writing the full word equation acid + metal carbonate → salt + water + carbon dioxide.
- Using incorrect salt formulae such as CaCl instead of CaCl₂: correct this by balancing charges, since Ca²⁺ needs two Cl⁻ ions.
- Naming the salt from the base only and ignoring the acid: correct this by always combining the metal or ammonium ion with the anion from the acid.
- Writing sulfate as SO₄ without the 2⁻ charge or using S₂O₄: correct this by using the sulfate ion SO₄²⁻.
- Including water as a product when ammonia reacts with an acid: correct this by remembering that ammonia + acid → ammonium salt only.
- Balancing formulae incorrectly, such as NaCl₂: correct this by balancing charges so that Na⁺ and Cl⁻ give NaCl.
- Naming the salt from the base alone, for example calling the product of hydrochloric acid and sodium hydroxide 'sodium oxide'; correct this by naming the positive ion first and the acid-derived negative ion second.
- Confusing nitric acid with nitrous acid and writing 'nitrite' salts; correct this by linking nitric acid to nitrate salts only.
- Omitting the Roman numeral for variable-charge ions, for example writing 'iron chloride' for FeCl₂; correct this by writing iron(II) chloride.
- Taking the positive ion from the acid instead of the base, alkali or carbonate; correct this by identifying the metal or ammonium ion in the base, alkali or carbonate first.
- Ignoring the charge on a variable-charge ion and writing an incorrect formula, for example CuCl for copper(II) chloride; correct this by using Cu²⁺ and two Cl⁻ to give CuCl₂.
- Treating the hydroxide ion, OH⁻, as the positive ion in an alkali; correct this by recognising that OH⁻ is negative and the positive ion is the metal or ammonium ion present.
- Predicting carbon dioxide for every neutralisation. Correction: carbon dioxide is produced only when the base is a metal carbonate or hydrogencarbonate.
- Mixing up acid endings, for example writing 'sulfuric' salt names such as sodium sulfuric. Correction: use the salt ending, so sodium sulfate.
- Assuming ammonia reacting with an acid produces water. Correction: ammonia reacts with an acid to produce only an ammonium salt, with no water formed.
- Writing the charges into the final formula, such as Na⁺Cl⁻ instead of NaCl. Correction: charges are a balancing step only; the finished salt formula shows no charge symbols.
- Swapping the ion ratio, for example writing MgCl instead of MgCl₂. Correction: check that 1 × 2⁺ balances 2 × 1⁻, so two chloride ions are needed.
- Forgetting brackets around a repeated polyatomic ion, writing CaOH₂ instead of Ca(OH)₂. Correction: bracket the whole ion before adding the subscript when more than one unit is needed.
- Using the acid's hydrogen count as the metal count without checking charge, for example assuming H₂SO₄ always gives a 2⁺ metal sulfate. Correction: balance charges from the ions themselves, not from the acid's formula.