SPEC 4.4.2.3 • RP1 • MODULE 4 • PAPER 1
Soluble Salts — RP1
The most-tested practical on Paper 1. Seven steps. Four examiner traps that catch the same students every year. Master the method, the reasoning, and the mark-scheme phrasing — and this is reliable easy marks.
Why This Lesson Matters
RP1 has appeared on every Paper 1 in recent years. The same examiner errors repeat — not because the chemistry is hard, but because students don't use the precise mark-scheme language.
7
Steps in the Method
40%
Scored Q04.1 Jun 2022 (observation)
RP1
First Required Practical
Spec 4.4.2.3 verbatim
"Soluble salts can be made from acids by reacting them with solid insoluble substances, such as metals, metal oxides, hydroxides or carbonates. The solid is added to the acid until no more reacts and the excess solid is filtered off to produce a solution of the salt. Salt solutions can be crystallised to produce solid salts."
Two Levels Of Scope
Spec 4.4.2.3 (general): any of metals, metal oxides, hydroxides, or carbonates can be used to make a soluble salt from acid. The spec skill is to "describe how to make pure, dry samples of named soluble salts from information provided".
RP1 (specific): uses an insoluble oxide or carbonate with a Bunsen burner to heat dilute acid and a water bath or electric heater to evaporate the solution. Common test combinations: copper / zinc oxide or carbonate + acid.
Key Definitions
Soluble salt
A salt that dissolves in water to form an aqueous solution.
Insoluble base
A base that does not dissolve in water. Includes insoluble metal oxides (CuO, ZnO, MgO), hydroxides (Cu(OH)₂, Mg(OH)₂), and carbonates (CuCO₃, ZnCO₃).
Crystallisation
The formation of pure solid crystals from a saturated solution, typically by cooling after gentle evaporation of the solvent (water).
Saturated solution
A solution containing the maximum amount of solute that can dissolve at a given temperature. Cooling a hot saturated solution causes crystals to form.
Filtrate / residue
Filtrate = the liquid that passes through the filter paper. Residue = the solid that stays on the filter paper.
Why Use An Insoluble Base?
The whole method depends on filterability of the excess. This is the elegant trick of RP1.
Spec 4.4.2.3 — All 4 Categories Of Insoluble Reactant
Metal (must react with acid; e.g. Mg, Zn, Fe — gives off H₂)
Insoluble metal oxide (e.g. CuO, ZnO, MgO)
Insoluble metal hydroxide (e.g. Cu(OH)₂, Mg(OH)₂)
Insoluble metal carbonate (e.g. CuCO₃, ZnCO₃ — gives off CO₂)
RP1 specifically uses the oxide or carbonate options. Hydroxides also work for general spec questions.
✓ Insoluble Reactant Works
Add the solid in excess to the acid.
All the acid reacts (no acid contamination)
Excess solid stays as a solid — doesn't dissolve
Excess solid filtered off easily
Filtrate = pure salt solution
✗ Soluble Base (Alkali) Fails
If you tried to add NaOH (or any alkali) in excess:
Excess alkali would dissolve in the solution
Cannot be filtered off — it's already in solution
Salt solution would be contaminated with alkali
This is why titrations are needed for alkalis (Lesson 4.06)
The Logic In One Sentence
Insoluble reactant in excess → excess can be filtered off → pure salt solution. This is exactly the answer AQA wants for "why use excess solid rather than excess acid?" questions — the filterability is the point.
The 7-Step Method
Memorise the sequence and the reason for each step — examiners test both.
1
Warm dilute acid in a beaker (using a Bunsen burner) — gentle heat speeds up the reaction. Don't boil.
2
Add insoluble solid (oxide, hydroxide, or carbonate) in excess, stirring until no more reacts. Excess ensures all the acid reacts.
3
Filter the mixture (filter paper in funnel). Removes excess unreacted solid → filtrate is pure salt solution.
4
Heat the filtrate gently in an evaporating basin using a water bath or electric heater (NOT direct Bunsen heating).
5
Heat to crystallisation point — solution becomes saturated; crystals form at the edge / on a cold rod.
6
Leave to cool slowly — slow cooling produces larger, purer crystals.
7
Pour off liquid; pat crystals dry with filter paper.
The Bunsen Burner Subtlety
This is the single most-misunderstood point on the entire RP1 method. The spec uses Bunsen twice differently — students conflate them and lose marks.
✓ Step 1 — Bunsen IS Used
Warming the dilute acid at the start.
This is gentle warming to speed up the reaction. The acid won't suffer because it has lots of water — short heating just warms it.
✗ Step 4 — Bunsen NOT Used
Evaporating the salt solution later.
Direct Bunsen flame would heat too fast → boil off all the water → heat the dry crystals → destroy them (decomposition / discolouration). Use a water bath OR electric heater here.
Spec Verbatim (RP1)
"Required practical 1: preparation of a pure, dry sample of a soluble salt from an insoluble oxide or carbonate using a Bunsen burner to heat dilute acid and a water bath or electric heater to evaporate the solution ."
Mental Test
If the question is about warming the acid → Bunsen. If the question is about warming the filtrate / evaporating → water bath or electric heater. Read carefully — which step is being asked about?
Trap 1 — Observation That Reaction Is Complete
P1 Jun 2022 Q04.1: The student adds zinc oxide to hydrochloric acid until the zinc oxide is in excess. Give one observation that the student could make to show that the zinc oxide is in excess.1 MARK
"Only about 40% of students scored this mark. A visible observation was required, such as solid remaining at the bottom of the beaker, or solid no longer disappearing. Merely rewording the idea that zinc oxide is in excess did not gain credit." — AQA Examiner Report, P1 Jun 2022 Q04.1
✗ Reword (No Mark)
"Zinc oxide is in excess."
Just restates what the question says — no observation made.
✗ Reword (No Mark)
"The reaction has stopped."
A deduction, not an observation. What can you SEE?
✓ MS Verbatim
"(Zinc oxide) solid remaining" or "solid no longer disappears"
✓ MS Verbatim (carbonate version)
"No more effervescence / bubbles / fizzing" (Q01.2 Jun 2024 — when carbonate is used)
Naming Discipline
Q04.1 ER: "Many students referred incorrectly to zinc rather than zinc oxide." Be precise — say "zinc oxide" remaining (not "zinc"). The reactant added was zinc oxide; that's what's still visible in excess.
Trap 2 — Why Excess Of The SOLID, Not The Acid?
P1 Jun 2022 Q04.2: Why is excess zinc oxide used rather than excess hydrochloric acid?1 MARK
"Few students gained this mark. Many students said the reason was to use up all the acid, without considering why that was necessary — answers in terms of ease of removal of the excess reactant were required." — AQA Examiner Report, P1 Jun 2022 Q04.2
✗ Wrong Direction
"To use up all the acid."
MS literally says "ignore to ensure all the acid is used up" for this question. It doesn't answer why the OXIDE specifically is the one in excess.
✓ MS Verbatim
"(Excess) zinc oxide can be filtered off."
The point: excess solid is removable by filtration. Excess acid would stay dissolved and contaminate the salt.
The Distinction Two Different Questions
Two related-but-different questions on this topic:
"Why is excess used?" → "to ensure all the acid reacts" (avoids contamination)
"Why is excess of the SOLID rather than the acid?" → "the excess solid can be filtered off" (the acid couldn't be removed easily)
Read the question carefully — the wording tells you which answer is wanted.
Trap 3 — The Heating Method (Step 4)
P1 Jun 2024 Q01.4: Name the equipment that can be used to warm the filtrate gently in Step 6 (evaporation).1 MARK
"Mark scheme: 'electric heater' OR 'water bath'. Extra information: 'ignore Bunsen burner'." — AQA Mark Scheme, P1 Jun 2024 Q01.4
✗ Doesn't Score
"Bunsen burner"
Direct flame heats too fast → evaporates to dryness → destroys crystals.
✓ MS Verbatim
"Electric heater" OR "water bath"
Gentle, indirect heating that won't reach the destruction temperature.
Why Indirect Heating?
A water bath cannot exceed 100 °C (because water boils at 100 °C). An electric heater can be set to a controlled temperature. Both prevent the salt solution from being heated past dryness, which would burn or decompose the crystals.
Bunsen Confusion Source
Bunsen IS used in Step 1 (warming acid). Many students remember "Bunsen" from that step and apply it incorrectly to Step 4. Step 1 = Bunsen OK; Step 4 = water bath / electric heater.
Trap 4 — Crystallisation, Not Dryness
P1 Jun 2022 Q04.4: Describe how the student should obtain crystals of zinc chloride from a solution of zinc chloride.2 MARKS
"Many students restricted themselves to one mark; either they did not heat the solution, they did not give a reason for heating, or they heated to dryness. A few students referred to evaporating the zinc chloride, rather than the water, from the solution." — AQA Examiner Report, P1 Jun 2022 Q04.4
The 2-Mark Answer
Heat the solution until crystallisation point is reached (allow: until crystals start to form / to reduce the volume / to evaporate some of the water)
Leave the solution to cool / crystallise
Heating to dryness scores at most 1 mark (only if no other mark is awarded). It's the wrong technique — heat damages crystals after the water is gone.
✗ Wrong Substance Evaporates
"The acid evaporates" / "The salt evaporates"
Wrong. The water evaporates. The acid has already reacted (it's now part of the salt). The salt stays dissolved until saturation.
✓ Correct Substance
"Some of the water evaporates, leaving a saturated solution."
Full 6-Mark Method — P1 Jun 2023 Q03.1
The actual question: "Plan a method to make pure, dry crystals of zinc chloride from zinc carbonate and a dilute acid."6 MARKS
"This question discriminated very well, with nearly two fifths of students giving a Level 3 answer (5–6 marks), another third reaching Level 2... Some students only described the crystallisation process, leaving out essential steps. A few would not have made crystals because they evaporated to dryness." — AQA Examiner Report, P1 Jun 2023 Q03.1
Level 3 (5–6 marks) — Indicative Content
Use zinc carbonate and hydrochloric acid(naming the right acid is critical — chloride salt → HCl)
Add zinc carbonate to the acid in a beaker, stir
Continue adding until zinc carbonate is in excess — shown by excess solid remaining and no more effervescence
Filter the mixture to remove excess zinc carbonate
Heat filtrate gently to crystallisation point (using water bath or electric heater)
Leave the solution to crystallise; pat crystals dry with filter paper
Pick The Right Acid
For zinc chloride, the acid must be hydrochloric acid (HCl). For zinc sulfate, sulfuric acid. For zinc nitrate, nitric acid. The salt name's anion determines the acid. (Carries forward from Lesson 4.03.)
The Five Common Errors — Summary Table
Verbatim mark-scheme rules from the past four years of Paper 1.
ConceptWhat loses the markWhat scores it
Observation reaction is complete"solid no longer reacts" (rewords Q)" solid remaining / no longer disappears "; OR " no more effervescence " (carbonate)
Why excess of solid (not acid)?"to use up the acid" (wrong question)" excess solid can be filtered off "
Heating method (Step 4)"Bunsen burner"" water bath " OR " electric heater "
Crystallisation"evaporate to dryness"" heat to crystallisation point " + " leave to cool "
What evaporates"acid evaporates" / "salt evaporates"" water evaporates "
Naming Reactants For A Given Salt
P1 Jun 2022 Q04.3: Name one other compound the student could add to hydrochloric acid to produce zinc chloride. ("Other" — i.e. not the zinc oxide already used.)1 MARK
MS Allowed Answers
Zinc hydroxide (Zn(OH)₂) — insoluble base
Zinc carbonate (ZnCO₃) — insoluble carbonate
Both react with HCl to give ZnCl₂. Both are insoluble — fit RP1's method.
ER Common Errors (Q04.3)
"Some suggested zinc or chlorine; whilst zinc would work, it is not a compound. Others suggested using salts such as zinc sulfate."
Why these don't score:
"Zinc" — works but not a compound (question asked for compound)
"Chlorine" — wouldn't give zinc chloride (it's a non-metal anyway)
"Zinc sulfate" — already a salt; wouldn't give zinc chloride from it
Mark-Scheme Phrases — RP1 Vocabulary
The exact phrases AQA accepts. Memorise these — paraphrasing loses marks.
Step / ConceptMark-scoring phrase
Step 1 (warming acid)"warm dilute acid in a beaker (with a Bunsen)"
Step 2 (excess)"add insoluble base/carbonate in excess " or "until no more reacts"
Observation reaction complete"solid remains / no longer disappears " / " no more effervescence "
Why excess? (general)"to ensure all the acid reacts"
Why solid in excess? (specific)" excess solid can be filtered off "
Step 3 (filter)"filter to remove excess (unreacted) solid"
What's collected after filter" filtrate " = salt solution; " residue " = excess solid
Step 4 (heating filtrate)" water bath " / " electric heater " — NOT Bunsen
Step 5 (when to stop heating)"heat to crystallisation point " / "until crystals start to form"
Step 6 (crystallisation)" leave to cool (and crystallise)"
What evaporates"the water evaporates" (NOT the acid; NOT the salt)
Step 7 (drying)" pat dry with filter paper"
Practice 1 — Foundation
A student wants to make pure, dry copper sulfate crystals. They have access to dilute sulfuric acid and solid copper oxide.
(a) Write a balanced equation for the reaction between copper oxide and sulfuric acid.2 MARKS
(b) Give one observation that would show all the acid has reacted (i.e. the copper oxide is in excess).1 MARK
(c) Name the equipment that should be used to evaporate the salt solution gently after filtering.1 MARK
a / 1
Correct formulae: CuO, H₂SO₄, CuSO₄, H₂O.
a / 2
Balanced: CuO + H₂SO₄ → CuSO₄ + H₂O (1:1 ratio).
b / 1
Copper oxide solid remains (allow: solid no longer disappears / unreacted solid visible at the bottom of the beaker). Don't write "copper oxide is in excess" — that rewords the question. MS ignores effervescence here (no CO₂ from oxide).
c / 1
Water bath OR electric heater. Do NOT write "Bunsen burner".
⚠️
Common errors: writing "Bunsen" for the evaporation step; writing the observation as a re-statement of the question; not balancing the equation.
Practice 2 — Full Method
Plan a method to make pure, dry crystals of magnesium chloride from magnesium carbonate and a suitable dilute acid. Include the reagents, the steps, and how you would know each step was complete.6 MARKS
1
Use magnesium carbonate and hydrochloric acid (chloride salt → HCl).
2
Warm the dilute hydrochloric acid in a beaker (Bunsen burner OK for this step).
3
Add magnesium carbonate in excess, stirring. Continue until no more effervescence / solid remains undissolved at the bottom.
4
Filter the mixture to remove excess unreacted magnesium carbonate. The filtrate is the magnesium chloride solution.
5
Heat the filtrate gently (using a water bath or electric heater) until crystallisation point is reached — crystals start to form / volume reduces.
6
Leave to cool slowly — crystals form. Filter or pour off the remaining liquid; pat crystals dry with filter paper.
⚠️
Equation: MgCO₃ + 2HCl → MgCl₂ + H₂O + CO₂. The CO₂ bubbles are why effervescence stopping signals completion. Do NOT heat to dryness; do NOT use Bunsen for the evaporation step.
Summary
The 7-Step Method
1. Warm dilute acid (Bunsen OK for THIS step)
2. Add insoluble base/carbonate in excess; stir until no more reacts
3. Filter to remove excess unreacted solid → filtrate is salt solution
4. Heat filtrate gently — water bath or electric heater (NOT Bunsen)
5. Heat to crystallisation point (saturation; crystals form at edge)
6. Leave to cool slowly — crystals form
7. Pour off liquid; pat crystals dry with filter paper
Why Use An Insoluble Base?
Excess can be added without dissolving
Excess can then be filtered off (the elegant trick of RP1)
This is the answer to "why is excess solid used rather than excess acid?"
The Five Common Errors
Observation: "solid no longer reacts" rewords the question; need "solid remains" or "no more effervescence"
Why excess solid: "use up the acid" misses the point; need "excess can be filtered off"
Heating: "Bunsen" is wrong for Step 4; need "water bath / electric heater"
Crystallisation: never "evaporate to dryness"; need "heat to crystallisation point" + "leave to cool"
What evaporates: water — not the acid, not the salt
Exam Tip
If the question asks any RP1 step, your first move is to identify which step. Step 1 (warming acid) and Step 4 (evaporating salt solution) use different equipment. Step 2 (adding excess) and the question "why excess solid?" have different correct answers. Read precisely; answer precisely.
Next: Why This Lesson Matters