NSWChemistrySyllabus dot point

Inquiry Question 1: What is an acid and a base?

Predict and write balanced molecular, ionic and net ionic equations for reactions of acids with active metals, metal carbonates and hydrogencarbonates, and bases (including metal oxides and hydroxides)

Q: 2019 HSC HSC: A small piece of zinc is dropped into dilute sulfuric acid. Describe the observations and write the balanced ionic equation. Explain how the identity of the gas produced could be confirmed.

**Observations.** Vigorous bubbling at the zinc surface. The zinc gradually decreases in size and dissolves. The solution may warm noticeably (the reaction is exothermic). **Equation.** Sulfuric acid is fully dissociated, so the net ionic equation is: $$Zn_{(s)} + 2H^+_{(aq)} \rightarrow Zn^{2+}_{(aq)} + H_{2(g)}$$ The $SO_4^{2-}$ ion is a spectator and is omitted. **Confirmation of gas.** Collect a sample over water in a test tube. Bring a lit splint to the mouth of the tube. A "pop" sound confirms hydrogen gas. (The hydrogen burns explosively in the small volume of air.) Markers reward (1) two specific observations, (2) a correctly balanced net ionic equation, (3) naming the pop test with the expected result.

A focused answer to the HSC Chemistry Module 6 dot point on acid reactions. The four reaction types, balanced molecular, full ionic and net ionic equations, the activity series, gas tests, and worked HSC past exam questions.

Generated by Claude OpusReviewed by Better Tuition Academy8 min answerUpdated 2026-05-18

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What this dot point is asking

NESA wants you to predict the products of acid reactions with active metals, metal carbonates and hydrogencarbonates, and bases (metal oxides and hydroxides), write balanced molecular, full ionic and net ionic equations, identify spectator ions, and describe the observable evidence (bubbling, dissolution, temperature change, gas tests). The chemistry builds on the Arrhenius properties of acids and feeds directly into enthalpy of neutralisation and titration analysis.

The answer

Reaction type 1: acid with an active metal

General form: acid + metal -> salt + hydrogen.

2HCl_{(aq)} + Mg_{(s)} \rightarrow MgCl_{2(aq)} + H_{2(g)}

Net ionic:

2H^+_{(aq)} + Mg_{(s)} \rightarrow Mg^{2+}_{(aq)} + H_{2(g)}

This is a redox reaction. The metal is oxidised; $H^+$ is reduced.

Activity series. Only metals more reactive than hydrogen displace it from dilute acids. K, Na, Ca, Mg, Al, Zn, Fe, Pb (slowly) react; Cu, Ag, Au do not. Lead reacts slowly because of an insoluble lead salt coating.

Observations. Bubbling (hydrogen gas), the metal disappears, the solution may warm.

Gas test. Hydrogen gives a "pop" with a lit splint.

Reaction type 2: acid with a metal carbonate

General form: acid + carbonate -> salt + water + carbon dioxide.

2HCl_{(aq)} + Na_2CO_{3(aq)} \rightarrow 2NaCl_{(aq)} + H_2O_{(l)} + CO_{2(g)}

Net ionic (for the soluble carbonate above):

2H^+_{(aq)} + CO_3^{2-}_{(aq)} \rightarrow H_2O_{(l)} + CO_{2(g)}

For an insoluble carbonate ( $CaCO_3$ , $MgCO_3$ ), keep the carbonate as a solid in the ionic equation:

2H^+_{(aq)} + CaCO_{3(s)} \rightarrow Ca^{2+}_{(aq)} + H_2O_{(l)} + CO_{2(g)}

Observations. Bubbling (carbon dioxide), the carbonate dissolves.

Gas test. Carbon dioxide turns limewater ( $Ca(OH)_2_{(aq)}$ ) milky/cloudy. The chemistry: $CO_2 + Ca(OH)_2 \rightarrow CaCO_3 \downarrow + H_2O$ .

Reaction type 3: acid with a metal hydrogencarbonate

General form: acid + hydrogencarbonate -> salt + water + carbon dioxide.

HCl_{(aq)} + NaHCO_{3(aq)} \rightarrow NaCl_{(aq)} + H_2O_{(l)} + CO_{2(g)}

Net ionic:

H^+_{(aq)} + HCO_3^-_{(aq)} \rightarrow H_2O_{(l)} + CO_{2(g)}

This is the chemistry of common antacids (sodium bicarbonate neutralising stomach acid).

Reaction type 4: acid with a base (neutralisation)

General form: acid + base -> salt + water.

With a soluble hydroxide:

HCl_{(aq)} + NaOH_{(aq)} \rightarrow NaCl_{(aq)} + H_2O_{(l)}

Net ionic:

H^+_{(aq)} + OH^-_{(aq)} \rightarrow H_2O_{(l)}

This single net ionic equation describes every strong acid + strong base reaction.

With a metal oxide (a basic oxide):

2HCl_{(aq)} + CuO_{(s)} \rightarrow CuCl_{2(aq)} + H_2O_{(l)}

Net ionic:

2H^+_{(aq)} + CuO_{(s)} \rightarrow Cu^{2+}_{(aq)} + H_2O_{(l)}

With ammonia:

HCl_{(aq)} + NH_{3(aq)} \rightarrow NH_4Cl_{(aq)}

Net ionic:

H^+_{(aq)} + NH_{3(aq)} \rightarrow NH_4^+_{(aq)}

Observations. Heat is released. With a coloured oxide ( $CuO$ black), the solid dissolves and the solution takes on the colour of the metal cation ( $Cu^{2+}$ blue).

Writing ionic and net ionic equations

Write a balanced molecular equation with state symbols.
Split every aqueous strong electrolyte into its ions. Strong acids ( $HCl$ , $HNO_3$ , $H_2SO_4$ , $HClO_4$ ), strong bases ( $NaOH$ , $KOH$ , $Ca(OH)_2$ , $Ba(OH)_2$ ), and soluble salts split. Solids, liquids, gases, and weak electrolytes do not split.
Cancel ions that appear unchanged (same species, same coefficient) on both sides. These are the spectators.
Check that the net ionic equation balances for atoms and for charge.

Worked example

Predict the products and write the balanced molecular, full ionic and net ionic equations for the reaction of dilute sulfuric acid with solid magnesium oxide.

Step 1: Reaction type. Acid + metal oxide -> salt + water.

Step 2: Molecular equation.

H_2SO_{4(aq)} + MgO_{(s)} \rightarrow MgSO_{4(aq)} + H_2O_{(l)}

Step 3: Full ionic equation. Split aqueous strong electrolytes:

2H^+_{(aq)} + SO_4^{2-}_{(aq)} + MgO_{(s)} \rightarrow Mg^{2+}_{(aq)} + SO_4^{2-}_{(aq)} + H_2O_{(l)}

Step 4: Net ionic equation. Cancel $SO_4^{2-}$ :

2H^+_{(aq)} + MgO_{(s)} \rightarrow Mg^{2+}_{(aq)} + H_2O_{(l)}

Step 5: Check. Atoms: 2 H, 1 Mg, 1 O on each side. Charge: $+2$ on each side. Balanced.

Common traps

Splitting insoluble solids. $CaCO_3$ , $MgO$ , $CuO$ are all solids in the relevant equations. Do not split them into ions, even when they appear in an ionic equation.

Forgetting the diprotic acid stoichiometry. $H_2SO_4$ provides two protons, so reactions with monoprotic bases or hydrogencarbonates need a 2:1 ratio.

Writing $H^+$ instead of $H_3O^+$ in ionic equations. Both are accepted at HSC. Be consistent within an answer.

Missing the gas test. Markers expect students to know the limewater test for $CO_2$ and the pop test for $H_2$ .

Saying copper reacts with dilute HCl. Copper sits below hydrogen in the activity series. It does not react with dilute non-oxidising acids. (It does react with concentrated $HNO_3$ via a different, oxidative mechanism, but this is beyond HSC.)

Ignoring state symbols. Markers deduct for missing $(aq)$ , $(s)$ , $(l)$ or $(g)$ on ionic equations.

In one sentence

Acids react with active metals to give salt and hydrogen, with carbonates and hydrogencarbonates to give salt, water and $CO_2$ , and with bases (metal oxides, hydroxides, ammonia) to give a salt and water; to write the net ionic equation, split aqueous strong electrolytes, cancel spectators, and check that atoms and charge balance.

Past exam questions, worked

Real questions from past NESA papers on this dot point, with our answer explainer.

2021 HSC4 marksDilute hydrochloric acid is added to solid calcium carbonate. Write the balanced molecular equation, the full ionic equation, and the net ionic equation for the reaction. Identify the spectator ion(s).

Show worked answer →

A 4 mark answer needs all three equations and the spectator identification.

Molecular equation.

2HCl_{(aq)} + CaCO_{3(s)} \rightarrow CaCl_{2(aq)} + H_2O_{(l)} + CO_{2(g)}

Full ionic equation. Split soluble strong electrolytes into ions. $CaCO_3$ is a solid (insoluble) so it is not split. $H_2O$ and $CO_2$ are molecular so they are not split.

2H^+_{(aq)} + 2Cl^-_{(aq)} + CaCO_{3(s)} \rightarrow Ca^{2+}_{(aq)} + 2Cl^-_{(aq)} + H_2O_{(l)} + CO_{2(g)}

Net ionic equation. Cancel ions that appear unchanged on both sides ( $2Cl^-$ ).

2H^+_{(aq)} + CaCO_{3(s)} \rightarrow Ca^{2+}_{(aq)} + H_2O_{(l)} + CO_{2(g)}

Spectator ion. $Cl^-_{(aq)}$ . It appears on both sides unchanged and takes no part in the reaction.

Markers reward (1) a correctly balanced molecular equation with state symbols, (2) keeping the solid carbonate intact in the ionic equation, (3) a correctly cancelled net ionic equation, (4) explicit identification of the spectator.

2019 HSC3 marksA small piece of zinc is dropped into dilute sulfuric acid. Describe the observations and write the balanced ionic equation. Explain how the identity of the gas produced could be confirmed.

Show worked answer →

Observations. Vigorous bubbling at the zinc surface. The zinc gradually decreases in size and dissolves. The solution may warm noticeably (the reaction is exothermic).

Equation. Sulfuric acid is fully dissociated, so the net ionic equation is:

Zn_{(s)} + 2H^+_{(aq)} \rightarrow Zn^{2+}_{(aq)} + H_{2(g)}

The $SO_4^{2-}$ ion is a spectator and is omitted.

Confirmation of gas. Collect a sample over water in a test tube. Bring a lit splint to the mouth of the tube. A "pop" sound confirms hydrogen gas. (The hydrogen burns explosively in the small volume of air.)

Markers reward (1) two specific observations, (2) a correctly balanced net ionic equation, (3) naming the pop test with the expected result.

What this dot point is asking

The answer

Reaction type 1: acid with an active metal

Reaction type 2: acid with a metal carbonate

Reaction type 3: acid with a metal hydrogencarbonate

Reaction type 4: acid with a base (neutralisation)

Writing ionic and net ionic equations

Worked example

Common traps

In one sentence

Past exam questions, worked

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