Salts

Salts: definitions, formulae, solubility rules, precipitate prediction and selecting excess-solid, titration or precipitation methods.

SEC G3 Pure Chemistry 2027

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Learning objectives

describe the general rules of solubility for common salts to include nitrates, chlorides (including those of silver and lead), sulfates (including those of barium, calcium and lead), carbonates, hydroxides, salts of Group 1 cations and ammonium salts
suggest a method of preparing a given salt from suitable starting materials, given appropriate information.

A salt question may ask you to move between a name, an ionic formula, a solubility prediction and a practical preparation method. Keep those decisions connected.

1. Definition

A. Salt

A salt is an ionic compound formed when the H⁺ ion in an acid is replaced by a metal ion or an ammonium ion, NH₄ +.

2. Key Ideas

Salt names are made from:
- the cation (metal or ammonium), and
- the anion from the acid (chloride, sulfate, nitrate, ethanoate, carbonate, etc.).

Acid name → salt anion (memorise these):

Acid	Formula	Salt name ends with	Anion
hydrochloric acid	HCl	chloride	Cl⁻
nitric acid	HNO₃	nitrate	NO₃⁻
sulphuric acid	H₂SO₄	sulfate	SO₄²⁻
ethanoic acid	CH₃COOH	ethanoate	CH₃COO⁻

Solubility rules let you predict whether a salt remains dissolved or forms a precipitate; practise applying the exceptions rather than guessing.

3. Detailed Explanations

Salt = H⁺ in an acid replaced by a metal ion or NH₄ +.
Name = cation + anion (chloride/sulfate/nitrate/carbonate/ethanoate…).
Always soluble: all nitrates, all ammonium salts and all Group 1 salts.
Key exceptions: AgCl and PbCl₂ (insoluble chlorides); BaSO₄, PbSO₄, CaSO₄ (insoluble/sparingly soluble sulfates).

A. Where the “two parts” come from

In many salt-making reactions:

the cation comes from the metal/base, and
the anion comes from the acid.

Example: KOH(aq) + HCl(aq) → KCl(aq) + H₂O(l)

Here, K⁺ comes from KOH and Cl⁻ comes from HCl, so the salt is potassium chloride, KCl.

B. Common ways salts are formed (O-Level)

Reaction type	What you write	Example
acid + metal	salt + hydrogen	Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)
acid + metal oxide/hydroxide	salt + water	CuO(s) + 2HCl(aq) → CuCl₂(aq) + H₂O(l)
acid + alkali	salt + water	NaOH(aq) + HNO₃(aq) → NaNO₃(aq) + H₂O(l)
acid + carbonate	salt + water + carbon dioxide	CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)

Do not write “acid + base → salt + water” as an absolute rule.
Ammonia forms ammonium salts without water: NH₃(aq) + HCl(aq) → NH₄Cl(aq)

C. Solubility rules (the ones examiners expect)

Solubility rules tell you whether a salt is soluble in water (aq) or forms a precipitate (s).

Salt type	Usually soluble?	Common exceptions (memorise)
nitrates (NO₃⁻)	yes	none (at O-Level)
ammonium salts (NH₄ +)	yes	none
Group 1 salts (Li⁺, Na⁺, K⁺)	yes	none
chlorides (Cl⁻)	yes	AgCl, PbCl₂
sulfates (SO₄²⁻)	yes	BaSO₄, PbSO₄, CaSO₄ (sparingly soluble)
carbonates (CO₃²⁻)	no	Group 1 carbonates + ammonium carbonate are soluble
hydroxides (OH⁻)	no	Group 1 hydroxides are soluble; Ca(OH)₂ is sparingly soluble

Solubility rules appear constantly in qualitative analysis (precipitate formation): Qualitative Analysis (QA).

D. Choosing a preparation method (don’t lose yield)

The best method depends on whether the salt is soluble or insoluble.

Salt type	Preparation idea	Why
soluble salt	use excess insoluble solid + acid, then filter + crystallise	easy to remove excess solid
soluble salt (acid + alkali)	titration then crystallise	you cannot “filter out excess alkali”
insoluble salt	mix two solutions to form a precipitate, then filter + wash + dry	salt forms as a solid

Preparation steps and which reagents to choose: Preparation of Soluble Salts & Insoluble Salts.

E. Keep the three representations separate

For the precipitation of silver chloride:

Macroscopic: a white precipitate is seen.
Particle level: aqueous Ag⁺ and Cl⁻ ions form an insoluble ionic lattice; spectator ions remain in solution.
Symbolic: Ag + (aq) + Cl⁻(aq) → AgCl(s).

An observation describes what is seen. It should not be replaced by a particle explanation or an equation.

4. Common Mistakes

Thinking “salt” means only NaCl.
Using the wrong solubility exception (learn the chloride/sulfate/carbonate exceptions).
Saying “do titration” for every salt (wrong): titration is specifically for soluble salts made from acid + alkali.

5. Exam Tips

State the solubility rule, then name the precipitate (if any).
For preparation, justify the method by extraction: “use excess solid so I can filter it off.”

If asked to predict a precipitate when two solutions are mixed: write the insoluble salt and label it (s).
Always include state symbols if the question expects them.

6. Worked Examples

Problem

Name the salt formed when

H₂SO₄ reacts with

NaOH.

Study the worked solution

Identify the cation
Method
Take sodium ions from sodium hydroxide.
Reason
The base supplies the positive ion in the salt.
Working
Cation: Na⁺.
Identify the acid anion
Method
Take sulfate ions from sulfuric acid.
Reason
The acid determines the salt-family ending.
Working
Anion: SO₄²⁻; salt family: sulfate.
Balance charges and name
Method
Combine two sodium ions with one sulfate ion.
Reason
Total positive and negative charges must cancel.
Working
Sodium sulfate, Na₂SO₄.

Problem

A student mixes

AgNO₃(aq) and

NaCl(aq). Will a precipitate form? If so, name it.

Apply the exception, not only the general rule

Hints

Hint 1: chloride rule

Most chlorides are soluble, but silver chloride is an exception.

Hint 2: combine the ions

The possible insoluble pair is

Ag⁺ with

Cl⁻.

View solution step by step

Identify the possible products
Method
Pair Ag⁺ with Cl⁻.
Reason
Mixing the solutions brings those ions together.
Working
Candidate product: AgCl.
Apply the exception
Method
Classify silver chloride as insoluble.
Reason
It is an exception to the usual solubility of chlorides.
Working
Silver chloride, AgCl(s), precipitates.

Learner claim

A student says

CaCO₃ is soluble because calcium is a metal. Correct the claim using the carbonate solubility rule.

Check the allowed exceptions

View solution step by step

State the carbonate rule
Method
Begin with carbonates being insoluble.
Reason
Only Group 1 and ammonium carbonates are the stated soluble exceptions.
Working
Carbonates: insoluble except Group 1 and NH₄ + salts.
Test calcium against the exceptions
Method
Place calcium in Group 2.
Reason
It matches neither soluble exception.
Working
CaCO₃ is insoluble.

Examination question

Mixing

BaCl₂(aq) and

Na₂SO₄(aq) produces a white precipitate. Name it and give its formula with state. [2 marks]

Name and formulate the insoluble product

Write the answer

View solution step by step

Name the precipitate
1 mark
Method
Combine barium and sulfate names.
Reason
Those are the ions that form the insoluble product.
Working
Barium sulfate.
Give formula and state
1 mark
Method
Write a 1:1 neutral formula with (s).
Reason
Ba²⁺ and SO₄²⁻ balance, and a precipitate is solid.
Working
BaSO₄(s).

Self-mark with the mark scheme

Compare your response with each mark point. Select a point only when your response contains that evidence.

Self-mark name and state-labelled formula.

Mark points

1 mark: Names barium sulfate.
1 mark: Writes BaSO4(s).

Method-selection transfer

Pure sodium nitrate crystals are required from nitric acid and sodium hydroxide. Select the preparation method and justify it.

Use the separation constraint

Hints

Hint 1: can excess be filtered

Dissolved nitric acid and sodium hydroxide both pass through filter paper.

Hint 2: find exact volumes

Titration identifies the neutralising volumes before crystallisation.

View solution step by step

Identify the separation problem
Method
Recognise that both reactants are aqueous.
Reason
An excess of either would remain dissolved and contaminate the salt solution.
Working
Excess acid/alkali cannot be removed by filtration.
Select and complete the method
Method
Use titration, repeat without indicator, then concentrate and crystallise.
Reason
Exact neutralising volumes leave no excess reactant in the soluble salt solution.
Working
Titration → neutral solution → crystallisation of NaNO₃.

Next step

Try Mind Stretcher 1 independently.

7. Mind Stretchers

Mind stretcher 1: Fix the student’s claimExtension

Question: A student writes: “All chlorides are insoluble.” Correct this with one exception example.

Show Answer

Chlorides are usually soluble. Exceptions include silver chloride, AgCl(s), and lead(II) chloride, PbCl₂(s).

Mind stretcher 2: Extraction logicExtension

Question: Why is “acid + excess insoluble base” a good method for preparing some soluble salts?

Show Answer

Because the excess base is a solid and can be filtered off, leaving only the salt solution to be crystallised. This avoids excess acid contaminating the product.

8. Quiz

Test yourself on definitions, solubility rules, precipitate prediction, and selecting preparation methods.

Practice next

Lesson quizCheck this lesson quickly with targeted MCQs.Structured practiceSwitch to exam-style written responses after the quiz.

Salts

Continue where you stopped

Learning objectives

1. Definition

A. Salt

2. Key Ideas

3. Detailed Explanations

A. Where the “two parts” come from

B. Common ways salts are formed (O-Level)

C. Solubility rules (the ones examiners expect)

D. Choosing a preparation method (don’t lose yield)

E. Keep the three representations separate

4. Common Mistakes

5. Exam Tips

6. Worked Examples

Identify the salt from the reactants

Identify the cation

Identify the acid anion

Balance charges and name

Predict if a precipitate forms (chloride exception)

Identify the possible products

Apply the exception

Carbonate rule

State the carbonate rule

Test calcium against the exceptions

Sulfate exception

Name the precipitate

Give formula and state

Choose the preparation method (acid + alkali)

Identify the separation problem

Select and complete the method

7. Mind Stretchers

Mind stretcher 1: Fix the student’s claimExtension

Mind stretcher 2: Extraction logicExtension

8. Quiz