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Acid-Base & Acid-Carbonate Reactions

Two common antacid ingredients neutralise stomach acid by different reactions. Calcium carbonate (CaCO₃) reacts with acid to give a salt, water and carbon dioxide, while magnesium hydroxide (Mg(OH)₂) gives only a salt and water. That extra CO₂ is why a carbonate antacid can make you belch. In this lesson you compare antacids by the chemistry of their reactions, not by brand or health claims. (This is chemistry, not medical advice.)

Today's hook, Two common antacid ingredients neutralise stomach acid by different reactions. Calcium carbonate (CaCO₃) reacts with acid to give a salt, water and carbon dioxide, while magnesium hydroxide (Mg(OH)₂) gives only a salt and water. That extra CO₂ is why a carbonate antacid can make you belch. In this lesson you compare antacids by the chemistry of their reactions, not by brand or health claims. (This is chemistry, not medical advice.)
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Warm up and recall

Warm up first

Three quick questions from earlier lessons. Pulling old material back to mind before you learn something new makes the new material stick better, so this is not busywork.

Worksheets

Practise this lesson

Four printable worksheets that build from the foundations up to exam-style questions, start at whatever level suits you.

Build Foundations & guided practice Apply Application practice Master Mastery challenge Exam Exam-style questions
01
Recall, your gut answer first
+5 XP warm-up

Picture an antacid tablet that contains calcium carbonate (CaCO₃) being added to excess stomach acid (dilute hydrochloric acid). A reaction begins straight away. What reaction is it, and what products form? This lesson models that chemistry, it is not medical advice.

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What you'll master, and the words for it

03
What you'll master
Know

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
Understand

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
Can do

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
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Key terms
Acid
A substance that produces H⁺ ions in aqueous solution (Arrhenius definition). In NSW Year 11 Chemistry, use this definition.
Base
A substance that produces OH⁻ ions in aqueous solution (Arrhenius definition), or a metal oxide/carbonate that reacts with acid. Alkalis are soluble bases.
Neutralisation
Reaction between an acid and a hydroxide or oxide base: acid + base → salt + water. (Ammonia and carbonates are bases too but follow related, different patterns.)
Alkali
A soluble base. Most are hydroxides that release OH⁻ directly in water (NaOH, KOH, Ca(OH)₂); ammonia (NH₃) is also an alkali because it produces OH⁻ by reacting with water.
Acid + carbonate reaction
Acid + carbonate → salt + water + CO₂(g); CO₂ gas production (bubbles) confirms the reaction.
Salt
An ionic compound formed from the cation of a base and the anion of an acid; e.g., NaCl, CaSO₄.
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Acid-Base Neutralisation Reactions

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Acid-Base Neutralisation Reactions
core concept

When you add dilute hydrochloric acid to magnesium hydroxide powder, the solid dissolves and the mixture warms slightly; an indicator added to the acid changes colour as the acid is consumed (never smell the acid or the mixture). An acid and a hydroxide base have reacted, leaving a dissolved salt and water. This is neutralisation of a hydroxide base: Acid + metal hydroxide → Salt + Water. The H⁺ from the acid combines with the OH⁻ from the base to form water; the remaining ions form the salt. (Other bases react differently: ammonia forms an ammonium salt with no separate water, and carbonates also release CO₂.)

Acid Base Salt Balanced Equation
HClNaOHNaCl HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)
H₂SO₄Ca(OH)₂CaSO₄ H₂SO₄(aq) + Ca(OH)₂(aq) → CaSO₄(s) + 2H₂O(l)
HNO₃KOHKNO₃ HNO₃(aq) + KOH(aq) → KNO₃(aq) + H₂O(l)
HClMgOMgCl₂ 2HCl(aq) + MgO(s) → MgCl₂(aq) + H₂O(l)
Balancing tip: When the base has two OH⁻ groups (e.g. Ca(OH)₂, Mg(OH)₂), you need two moles of a monoprotic acid (HCl, HNO₃) to neutralise one mole of the base. The H₂SO₄ + Ca(OH)₂ row is not a simple all-aqueous case: both Ca(OH)₂ and the product CaSO₄ are only sparingly soluble, which is why CaSO₄ is written (s). Use the NaOH/KOH rows as the clean fully-soluble neutralisation pattern.
Common error: Students write H₂O₂ instead of H₂O as the product. Neutralisation always produces H₂O, one O from OH⁻ combined with one H⁺. H₂O₂ is hydrogen peroxide, a completely different compound.
Universal net ionic equation: H⁺(aq) + OH⁻(aq) → H₂O(l) is the net ionic equation for every strong acid-strong base neutralisation, regardless of which specific acid and base are used. This is why all strong acid-strong base reactions release the same energy per mole of water formed.
Neutralisation reaction as an ordered exothermic sequence from ions to water plus energy.

Neutralisation (hydroxide base): Acid + base → Salt + Water. The net ionic equation is 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. (H⁺(aq) is the course shorthand for the hydrated proton, written more fully as H₃O⁺.)

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
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Acid-Carbonate & Acid-Hydrogen Carbonate Reactions

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Acid-Carbonate & Acid-Hydrogen Carbonate Reactions
core concept

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 with excess acid present, three products form: salt + water + CO₂ gas. (With only limited acid, hydrogen carbonate can be an intermediate.) The fizzing you observe is one of the most recognisable indicators of chemical change in chemistry.

SWC mnemonic: S alt, W ater, C arbon dioxide. These three products are fixed for every acid-carbonate reaction. Forgetting one costs marks.

Examples:

2HCl(aq) + CaCO₃(s) → CaCl₂(aq) + H₂O(l) + CO₂(g)
Left: 2H, 2Cl, 1Ca, 1C, 3O. Right: 1Ca, 2Cl, 2H, 1O+2O=3O, 1C. ✓
HCl(aq) + NaHCO₃(aq) → NaCl(aq) + H₂O(l) + CO₂(g)
Left: 2H, 1Cl, 1Na, 1C, 3O. Right: 1Na, 1Cl, 2H, 1O+2O=3O, 1C. ✓
CO vs CO₂ confusion: Students sometimes write CO as the gas product of acid-carbonate reactions. This is wrong, CO is a product of incomplete combustion. The carbon in carbonate (CO₃²⁻) is already at oxidation state +4, so CO₂ is the only possible gaseous carbon product.

Acid + carbonate → salt + water + CO₂(g). Acid + hydrogen carbonate → salt + water + CO₂(g). With excess acid, both 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?

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Antacids, Clinical Connection

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Antacids, Clinical Connection
core concept

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 (one releases CO₂, the other does not).

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

Active ingredient: CaCO₃
Reaction type: Acid-carbonate
CO₂ produced?: Yes, belching
Balanced equation with HCl: 2HCl + CaCO₃ → CaCl₂ + H₂O + CO₂

Mylanta

Active ingredient: Mg(OH)₂
Reaction type: Neutralisation
CO₂ produced?: No
Balanced equation with HCl: 2HCl + Mg(OH)₂ → MgCl₂ + 2H₂O

Baking soda

Active ingredient: NaHCO₃
Reaction type: Acid-H carbonate
CO₂ produced?: Yes, belching
Balanced equation with HCl: HCl + NaHCO₃ → NaCl + H₂O + CO₂
💊 Real-World Anchor, Antacids: This context appears in Short Answer Q3. Be ready to: write a balanced equation for a specific antacid with HCl, state the reaction type, name the salt produced, and explain whether CO₂ is generated and why that matters (for example, CO₂ causes belching).
Common error, Mg(OH)₂ classified as carbonate: Mylanta contains Mg(OH)₂, which is a hydroxide base, it undergoes neutralisation and produces NO CO₂. The OH⁻ groups, not a CO₃²⁻ ion, are what react with H⁺. Mg(OH)₂ is not a carbonate.
Choose acid + base (including antacid combinations) · drag to add base · watch pH curve and equivalence point Interactive

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, so, unlike a carbonate antacid, it does not cause belching from CO₂.

Cross-lesson links: In L03 you predicted precipitation reactions using solubility rules, acid-base reactions use a similarly predictable pattern: acid + base always gives salt + water. In L06, you will apply these reaction types to explain how Aboriginal and Torres Strait Islander peoples' cycad detoxification methods relate to chemical principles developed throughout L01–L05.
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Worked examples

Worked examples · reveal as you go

Worked Example 1, Predicting Products and Balancing Neutralisation +5 XP on full reveal

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.

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Acid (H₂SO₄) + base (KOH) → neutralisation reaction.
(a) Identify reaction type
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Cation from base: K⁺ (from KOH). Anion from acid: SO₄²⁻ (from H₂SO₄). Salt = potassium sulfate, K₂SO₄. Other product: water.
(b) Identify the salt
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H₂SO₄(aq) + KOH(aq) → K₂SO₄(aq) + H₂O(l)
(c) Write unbalanced equation
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H₂SO₄(aq) + 2KOH(aq) → K₂SO₄(aq) + 2H₂O(l)
Check: Left, 4H, 2K, 1S, 6O. Right, 4H, 2K, 1S, 4+2=6O. ✓
Balance, need 2 KOH to supply 2 K⁺; then 2 H₂O
Step 1 of 4
Worked Example 2, Acid-Carbonate Reaction +5 XP on full reveal

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.

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Products: NaCl(aq), H₂O(l), CO₂(g), Salt, Water, CO₂ (SWC).
Identify salt: Na⁺ from Na₂CO₃ + Cl⁻ from HCl → NaCl
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2HCl(aq) + Na₂CO₃(s) → 2NaCl(aq) + H₂O(l) + CO₂(g)
Check: Left, 2H, 2Cl, 2Na, 1C, 3O. Right, 2Na, 2Cl, 2H, 1O+2O=3O, 1C. ✓
Write and balance
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Vigorous fizzing/bubbling as CO₂ gas is evolved; solid Na₂CO₃ disappears as it is consumed by the reaction.
(c) Observable indicators
Step 1 of 3

Put the method in order

Predict, then reveal+8 XP
1 · Predict
2 · Reveal
3 · Compare

A student takes two antacid tablets (each containing 500 mg CaCO₃; molar mass 100 g/mol). (a) Calculate the maximum amount, in mol, of HCl these tablets can neutralise. (b) The student says "I've taken enough antacid to neutralise all my stomach acid, so my stomach should now be at pH 7." Evaluate this claim using your calculation and the reaction chemistry.

Confidence: 50%
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What you'll master, and the words for it

02
Formula reference · this lesson
core formula
📐

Key Patterns, This Lesson

$\text{Acid} + \text{Base} \rightarrow \text{Salt} + \text{H}_2\text{O}$  (neutralisation)
$\text{H}^+(aq) + \text{OH}^-(aq) \rightarrow \text{H}_2\text{O}(l)$, net ionic equation for any strong acid + strong base
$\text{Acid} + \text{Carbonate} \rightarrow \text{Salt} + \text{H}_2\text{O} + \text{CO}_2\uparrow$
$\text{CO}_2\uparrow$ gas always produced, visible fizzing; also: $\text{Acid} + \text{HCO}_3^- \rightarrow \text{Salt} + \text{H}_2\text{O} + \text{CO}_2\uparrow$
Salt identification: cation from base/carbonate  |  anion from acid  |  HCl → chloride, H₂SO₄ → sulfate, HNO₃ → nitrate
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Common misconception

Common errors · the 3 traps that cost marks

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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.

2

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 an antacid reacts with (consumes) some of the excess acid; how much is neutralised depends on the moles of antacid and acid (stoichiometry). "Neutralisation" names the acid-base reaction, it does not by itself fix the final pH, which depends on the amounts of each. The stomach normally stays acidic (about pH 1.5–3.5) so the enzyme pepsin can work. Calculating an exact final pH is a Year 12 topic.

3

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.

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Drill, then revisit

Quick-fire practice · 5 reps +2 XP per reveal

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Student writes: HNO₃(aq) + Ca(OH)₂(aq) → CaNO₃(aq) + H₂O(l)

2

Student writes: HCl(aq) + CaCO₃(s) → CaCl₂(aq) + H₂O(l) + CO(g)

3

Student writes: H₂SO₄(aq) + 2NaOH(aq) → 2NaOH(aq) + H₂SO₄(aq) + H₂O(l)

4

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.

5

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.

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Revisit your thinking

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.

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Interactive Tool, pH Scale Interactive Open fullscreen ↗
Use the pH Scale tool. A solution with pH = 2 is…
✓

Multiple choice

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Multiple choice
+5 XP per correct · +25 XP all-correct

Pick your answer, then rate your confidence. That tells the system what to drill next.

Spot the error+5 XP

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 ✓
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Short answer
UnderstandBand 34 MARKS

Q1. 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.

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ApplyBand 44 MARKS

Q2. 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)

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EvaluateBand 55 MARKS

Q3. 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)

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📖 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

Acid + base → neutralisation (salt + water, no CO₂). Salt: Ca²⁺ + NO₃⁻ → Ca(NO₃)₂; no carbonate → no CO₂.

CaCl₂ and H₂O are colourless, no blue colour is produced. Gas, disappearing solid, and warmth are all expected.

Mg(OH)₂ undergoes neutralisation (no CO₃²⁻ ion → no CO₂). All carbonate/hydrogen carbonate options produce CO₂.

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).

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].