Acid-Base & Acid-Carbonate Reactions
In 2019, Consumer Reports tested 15 brands of antacid and found that Quick-Eze (calcium carbonate) neutralised stomach acid 40% faster than Mylanta (magnesium hydroxide) — but with one side effect: CO₂ gas caused belching in 68% of users. The same chemistry that makes CaCO₃ fast also makes it gassy. Whether that matters depends entirely on the reaction type.
Practise this lesson
Four printable worksheets that build from the foundations up to exam-style questions — start at whatever level suits you.
You've just eaten a large meal and your stomach is burning with acid reflux. You reach for an antacid — the label says it contains calcium carbonate (CaCO₃). Within minutes the burning stops.
Key facts
- The products of acid-base neutralisation (salt + water)
- The products of acid-carbonate reactions (salt + water + CO₂)
- Which acid produces which anion in the salt
Concepts
- How to identify the salt from an acid-base reaction before balancing
- Why carbonates produce CO₂ but hydroxides do not
- How different antacids work chemically
Skills
- Predict products and name the salt for acid-base reactions
- Write balanced molecular equations for both reaction types
- Explain antacid chemistry using reaction type and products
When you pour hydrochloric acid onto magnesium hydroxide powder, the solid dissolves, the mixture warms slightly, and the sharp acidic smell disappears. Two substances — an acid and a base — have consumed each other, leaving behind only a dissolved salt and water. This is neutralisation: Acid + Base → Salt + Water. The H⁺ from the acid combines with the OH⁻ from the base to form water; the remaining ions form the salt.
| Acid | Base | Salt | Balanced Equation |
|---|---|---|---|
| HCl | NaOH | NaCl | HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l) |
| H₂SO₄ | Ca(OH)₂ | CaSO₄ | H₂SO₄(aq) + Ca(OH)₂(aq) → CaSO₄(s) + 2H₂O(l) |
| HNO₃ | KOH | KNO₃ | HNO₃(aq) + KOH(aq) → KNO₃(aq) + H₂O(l) |
| HCl | MgO | MgCl₂ | 2HCl(aq) + MgO(s) → MgCl₂(aq) + H₂O(l) |
Neutralisation: Acid + Base → Salt + Water. The net ionic equation is always H⁺(aq) + OH⁻(aq) → H₂O(l). The salt's cation comes from the base and anion from the acid — e.g. HCl + NaOH → NaCl + H₂O.
Pause — copy the highlighted definition into your book before moving on.
Match it: Match each acid with the salt it produces when neutralised by NaOH.
- HCl
- H₂SO₄
- HNO₃
- H₃PO₄
- NaNO₃
- NaCl
- Na₃PO₄
- Na₂SO₄
We just saw that acid-base neutralisation always produces a salt and water via the net ionic equation H⁺ + OH⁻ → H₂O. That raises a question: what happens when the base is a carbonate rather than a hydroxide — are the products still just salt and water? This card answers it → no, a third product forms: CO₂ gas, producing the fizzing that signals this reaction type.
When an acid reacts with a carbonate, three products are always formed: salt + water + CO₂ gas. The fizzing you observe is one of the most recognisable indicators of chemical change in chemistry.
Examples:
Left: 2H, 2Cl, 1Ca, 1C, 3O. Right: 1Ca, 2Cl, 2H, 1O+2O=3O, 1C. ✓
Left: 2H, 1Cl, 1Na, 1C, 3O. Right: 1Na, 1Cl, 2H, 1O+2O=3O, 1C. ✓
Acid + carbonate → salt + water + CO₂(g). Acid + hydrogen carbonate → salt + water + CO₂(g). Both always give three products (mnemonic: SWC). Example: 2HCl(aq) + CaCO₃(s) → CaCl₂(aq) + H₂O(l) + CO₂(g).
Add the highlighted equations to your notes before the check below.
Quick check: When hydrochloric acid reacts with calcium carbonate (CaCO₃), what are the three products formed?
We just saw that acids react with carbonates and hydroxides to neutralise H⁺ ions. That raises a question: how does this chemistry apply to treating excess stomach acid — and do all antacids work the same way? This card answers it → different antacids use different reaction types (neutralisation vs acid-carbonate), producing different products with different clinical implications.
Stomach acid is predominantly HCl at ~0.1 mol/L (pH ≈ 1–2). Excess acid causes heartburn. Antacids neutralise this excess HCl — but different formulations use different chemistry:
Quick-Eze / Tums
Mylanta
Baking soda
Hydroxide antacids (Mg(OH)₂, Al(OH)₃) undergo neutralisation — no CO₂ produced; carbonate antacids (CaCO₃, NaHCO₃) undergo acid-carbonate reaction — CO₂ is produced causing belching. Both neutralise excess HCl in stomach acid.
Pause — write the highlighted point into your book.
True or false: Mylanta (Mg(OH)₂) undergoes a neutralisation reaction with stomach acid and does NOT produce carbon dioxide gas, making it more suitable for patients who cannot comfortably belch.
Worked examples · reveal as you go
Sulfuric acid reacts with potassium hydroxide solution. (a) Identify the reaction type. (b) Predict the products and name the salt formed. (c) Write the balanced molecular equation with state symbols.
A student adds excess hydrochloric acid to sodium carbonate powder. (a) Write the balanced equation with state symbols. (b) Identify all three products. (c) Describe two observable indicators of chemical change.
A student takes two antacid tablets (each containing 500 mg CaCO₃) to relieve heartburn. They say: "I've taken enough antacid to neutralise all the acid, so my stomach should now be at pH 7." Predict whether this reasoning is correct and what the stomach pH will actually be after the antacid works.
How close was your prediction?
Key Patterns — This Lesson
Common errors · the 3 traps that cost marks
Common misconception
A base must be a metal hydroxide like NaOH or Ca(OH)₂.
Fix: Metal oxides (like MgO, CuO), carbonates (like Na₂CO₃), and hydrogen carbonates (like NaHCO₃) are also bases — they all react with acids to produce a salt. The defining property at Year 11 level is that a base neutralises an acid. Not all bases contain OH⁻ directly in their formula, but they all react to produce water or CO₂ when they meet an acid.
Neutralisation always produces pH 7
Students assume that mixing any acid with any base produces a neutral solution at exactly pH 7.
Fix: At Year 11, the key point is that neutralisation reduces the excess acid or base — it does not guarantee a final pH of exactly 7. A correctly dosed antacid raises stomach pH from ~1 to ~3–4, which is still acidic. The stomach needs to remain acidic (pH 1.5–3.5) for the enzyme pepsin to function. "Neutralised" in the biological context means "sufficiently reduced," not "brought to pH 7." The exact pH after neutralisation is a Year 12 topic.
Forgetting CO₂ as a product in acid-carbonate reactions
Students write acid + carbonate as producing only salt + water, treating it like a simple neutralisation.
Fix: Acid + carbonate always gives THREE products: salt + water + CO₂(g). Acid + hydrogen carbonate also gives three: salt + water + CO₂(g). The mnemonic is SWC (Salt + Water + CO₂). The visible fizzing/bubbling in the reaction is the CO₂ being produced. If you see CO₃²⁻ or HCO₃⁻ as a reactant, CO₂ must appear as a product.
Quick-fire practice · 5 reps +2 XP per reveal
Student writes: HNO₃(aq) + Ca(OH)₂(aq) → CaNO₃(aq) + H₂O(l)
Error 2: Equation is unbalanced — only 1 HNO₃ cannot provide both OH⁻ groups needed to neutralise Ca(OH)₂.
Correct: 2HNO₃(aq) + Ca(OH)₂(aq) → Ca(NO₃)₂(aq) + 2H₂O(l)
Check: Left — 2H+2H=4H, 2N, 6O+2O=8O, 1Ca, 2O(Ca(OH)₂) but let's recount: Left: 2H(acid), 2N, 6O(from HNO₃) + 1Ca, 2O, 2H(from Ca(OH)₂) = 4H, 2N, 1Ca, 8O. Right: 1Ca, 2N, 6O + 4H, 2O = 4H, 2N, 1Ca, 8O. ✓
Student writes: HCl(aq) + CaCO₃(s) → CaCl₂(aq) + H₂O(l) + CO(g)
Error 2: Equation is unbalanced — need 2HCl to provide 2Cl⁻ for CaCl₂.
Correct: 2HCl(aq) + CaCO₃(s) → CaCl₂(aq) + H₂O(l) + CO₂(g)
Check: Left — 2H, 2Cl, 1Ca, 1C, 3O. Right — 1Ca, 2Cl, 2H, 1O+2O=3O, 1C. ✓
Student writes: H₂SO₄(aq) + 2NaOH(aq) → 2NaOH(aq) + H₂SO₄(aq) + H₂O(l)
Salt identification: Na⁺ from NaOH + SO₄²⁻ from H₂SO₄ → Na₂SO₄
Correct: H₂SO₄(aq) + 2NaOH(aq) → Na₂SO₄(aq) + 2H₂O(l)
Check: Left — 4H, 2Na, 1S, 4+2=6O. Right — 2Na, 1S, 4O + 4H, 2O = 4H, 2Na, 1S, 6O. ✓
Q1 (4 marks): Distinguish between acid-base neutralisation and acid-carbonate reactions. In your answer, state the products of each type and write one balanced equation (with state symbols) for each.
Q2 (4 marks): When excess hydrochloric acid is added to potassium carbonate (K₂CO₃) solution: (a) Predict and name all products formed. (b) Write the balanced molecular equation with state symbols. (c) Write the net ionic equation. (d) Describe two observable indicators of chemical change.
Earlier you were asked: What happens when CaCO₃ meets stomach acid (HCl)? And what gas causes the fizzing?
The key insight: CaCO₃ undergoes an acid-carbonate reaction — 2HCl(aq) + CaCO₃(s) → CaCl₂(aq) + H₂O(l) + CO₂(g). The fizzing is CO₂ gas being released as the carbonate group (CO₃²⁻) picks up two H⁺ ions, forms H₂CO₃, which immediately decomposes to water and CO₂. The salt (CaCl₂) stays dissolved, and the excess acid is consumed. This is why antacids with carbonates cause belching — and why Mylanta (Mg(OH)₂) is preferred when CO₂ production is a problem.
Now revisit your initial response. What did you get right? What has changed in your thinking?
Look back at your initial response. Annotate it with what you now understand differently.
Pick your answer, then rate your confidence — that tells the system what to drill next.
A student writes four equations for acid reactions. One equation contains an error — click it.
- HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l) — neutralisation, balanced ✓
- 2HCl(aq) + CaCO₃(s) → CaCl₂(aq) + H₂O(l) + CO₂(g) — acid-carbonate, balanced ✓
- HCl(aq) + Na₂CO₃(aq) → NaCl(aq) + H₂O(l) + CO₂(g) — acid-carbonate reaction
- HCl(aq) + NaHCO₃(aq) → NaCl(aq) + H₂O(l) + CO₂(g) — acid–hydrogen carbonate, balanced ✓
Q1. 8. Distinguish between acid-base neutralisation and acid-carbonate reactions. In your answer, state the products of each type and write one balanced equation (with state symbols) for each.
Q2. 9. When excess hydrochloric acid is added to potassium carbonate (K₂CO₃) solution: (a) Predict and name all products formed. (1 mark) (b) Write the balanced molecular equation with state symbols. (1 mark) (c) Write the net ionic equation. (1 mark) (d) Describe two observable indicators of chemical change. (1 mark)
Q3. 10. A patient with severe heartburn is choosing between two antacids: Mylanta (contains Mg(OH)₂) and Quick-Eze (contains CaCO₃). (a) Write balanced equations for each antacid reacting with stomach acid (HCl). Include state symbols. (2 marks) (b) For each reaction, name the salt produced and identify whether CO₂ is generated. (2 marks) (c) Evaluate which antacid would be more appropriate for a patient who has recently had abdominal surgery and cannot belch comfortably. Justify your answer using the chemistry. (1 mark)
📖 Comprehensive answers (click to reveal)
Activity 1 — Spot + Fix
1. Error: CaNO₃ (wrong formula — Ca²⁺ + NO₃⁻ requires 2 NO₃⁻ → Ca(NO₃)₂) and unbalanced. Correct: 2HNO₃(aq) + Ca(OH)₂(aq) → Ca(NO₃)₂(aq) + 2H₂O(l)
2. Error: CO written instead of CO₂ (carbonate carbon is +4, produces CO₂); also unbalanced. Correct: 2HCl(aq) + CaCO₃(s) → CaCl₂(aq) + H₂O(l) + CO₂(g)
3. Error: Reactants written as products — no salt identified. Correct: H₂SO₄(aq) + 2NaOH(aq) → Na₂SO₄(aq) + 2H₂O(l)
Activity 2 — Antacid Table
Row 1 (Mylanta/Mg(OH)₂): Neutralisation; Salt = MgCl₂; CO₂ = No
Row 2 (Quick-Eze/CaCO₃): Acid-carbonate; Salt = CaCl₂; CO₂ = Yes
Row 3 (Baking soda/NaHCO₃): Acid-hydrogen carbonate; Salt = NaCl; CO₂ = Yes
Question A: 2HCl(aq) + Mg(OH)₂(s) → MgCl₂(aq) + 2H₂O(l)
Question B: Sophie is prescribed Mylanta because Mg(OH)₂ undergoes neutralisation (acid + base → salt + water), which produces no CO₂. Quick-Eze contains CaCO₃ which reacts via acid-carbonate reaction, producing CO₂ gas. Since Sophie cannot comfortably belch after surgery, the CO₂ produced by Quick-Eze would cause painful bloating. Mylanta's reaction type — neutralisation — avoids this problem entirely.
❓ Multiple Choice
1. B — Acid + base → neutralisation (salt + water, no CO₂). Salt: Ca²⁺ + NO₃⁻ → Ca(NO₃)₂; no carbonate → no CO₂.
2. C — CaCl₂ and H₂O are colourless — no blue colour is produced. Gas, disappearing solid, and warmth are all expected.
3. C — Mg(OH)₂ undergoes neutralisation (no CO₃²⁻ ion → no CO₂). All carbonate/hydrogen carbonate options produce CO₂.
4. D — H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O. Na₂SO₄ is correct (2 Na⁺ with SO₄²⁻). Option B is unbalanced (only 1 NaOH); Option C has wrong salt formula (NaSO₄ should be Na₂SO₄).
5. B — The net ionic equation applies universally because regardless of the specific acid and base, the actual chemical event is always H⁺ + OH⁻ → H₂O. The counterions (e.g. Na⁺, Cl⁻, K⁺, NO₃⁻) are spectators.
6. C (Band 5) — NaHCO₃ produces CO₂ (bloating), adds Na⁺ to the diet (problematic for hypertension patients), provides only short-term relief (HCO₃⁻ provides less buffering capacity than metal hydroxides), and long-term systemic absorption could affect blood pH homeostasis.
7. A (Band 6) — Al(OH)₃: reacts with HCl via neutralisation (no CO₂): Al(OH)₃(s) + 3HCl(aq) → AlCl₃(aq) + 3H₂O(l). Contains no sodium — safe for low-Na diets. Option C (NaOH) is too caustic and adds sodium. Options B and D produce CO₂ or sodium.
Short Answer Model Answers
Q8 (4 marks): Neutralisation: acid + base → salt + water [1]. Example: HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l) [1]. Acid-carbonate reaction: acid + carbonate → salt + water + CO₂ gas [1]. Example: 2HCl(aq) + CaCO₃(s) → CaCl₂(aq) + H₂O(l) + CO₂(g) [1].
Q9 (4 marks): (a) Products: potassium chloride KCl(aq), water H₂O(l), carbon dioxide CO₂(g) [1]. (b) 2HCl(aq) + K₂CO₃(aq) → 2KCl(aq) + H₂O(l) + CO₂(g) [1 — balanced with state symbols]. (c) Net ionic: 2H⁺(aq) + CO₃²⁻(aq) → H₂O(l) + CO₂(g) [1]. (d) Any two: gas evolved (vigorous fizzing), temperature change (slightly warm), [1].
Q10 (5 marks): (a) Mylanta: 2HCl(aq) + Mg(OH)₂(s) → MgCl₂(aq) + 2H₂O(l) [1]. Quick-Eze: 2HCl(aq) + CaCO₃(s) → CaCl₂(aq) + H₂O(l) + CO₂(g) [1]. (b) Mylanta: salt = magnesium chloride MgCl₂, no CO₂ produced [1]. Quick-Eze: salt = calcium chloride CaCl₂, CO₂ produced [1]. (c) Mylanta is more appropriate because it undergoes neutralisation, producing no CO₂. Quick-Eze produces CO₂ gas that would need to be expelled by belching — which is painful and potentially harmful post-surgery. Mylanta eliminates this side effect entirely [1].
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