Ngā Tūāhua Matū · Chemical Reactions
Acids, bases, pH, combustion, and displacement reactions · Years 9–10
Ngā Whāinga Akoranga · Learning Intentions
- Identify the signs that a chemical reaction has taken place and distinguish reactants from products.
- Use the pH scale and chemical indicators to classify solutions as acidic, neutral, or basic.
- Write word equations for acid–base, acid–metal, and acid–carbonate reactions.
- Explain combustion reactions and represent them with word and balanced equations.
- Describe displacement reactions and relate them to the metal reactivity series.
Paearu Angitu · Success Criteria
- I can list at least four signs that a chemical reaction has occurred.
- I can read a pH value and classify it as acidic, neutral, or alkaline.
- I can write a correct word equation for neutralisation, acid + metal, and acid + carbonate reactions.
- I can explain what combustion is and write the word equation for burning a hydrocarbon.
- I can predict whether a displacement reaction will occur using the metal reactivity series.
Hononga Marautanga · Curriculum Connections
This activity addresses the following Science curriculum statements (Draft 2025, Phase 4 — Years 9, 10):
- Reactions: Reactants are the substances present at the start of a chemical reaction; products are the new substances formed.
- Reactions: Chemical reactions create new substances by rearranging particles and may be observed as bubbles, colour change, precipitate, or temperature change.
- Equations: Chemical reactions can be represented using word equations (e.g. sodium hydroxide + hydrochloric acid → sodium chloride + water).
- Equations: Chemical reactions can be represented using balanced chemical equations.
- Energy: Chemical reactions involve energy changes, which can be observed as thermal energy transfers to (exothermic) or from (endothermic) the surroundings.
- Acids and bases: Common acids and bases: hydrochloric acid (HCl), sulfuric acid (H₂SO₄), nitric acid (HNO₃), sodium hydroxide (NaOH), potassium hydroxide (KOH), ammonia (NH₃).
- pH: The pH scale can be used to measure acidity, identified by chemical indicators.
- pH: Classifying solutions using the pH scale.
- pH: Manipulating the pH of a solution by adding an acid or a base.
- Alkalis: Bases that are soluble in water are known as alkalis.
- Safety: Many acids and bases are corrosive materials.
- Neutralisation: Acids react with bases to produce water and salts.
- Acid + metal: Acids react with metals to produce hydrogen gas and salts.
- Acid + carbonate: Acids react with metal carbonates to produce carbon dioxide, water, and salts.
- Combustion: Combustion is a chemical reaction between a fuel and an oxidising agent.
- Combustion: Combustion is a process in which energy is transferred, primarily as heat and light.
- Combustion: Combustion of hydrocarbons with oxygen produces water and carbon dioxide.
- Combustion: Combustion of hydrogen with oxygen produces water.
- Displacement: A more reactive metal can replace a less reactive metal in a salt solution.
- Reaction rate: The rate of a chemical reaction can be increased with temperature, concentration, surface area, and catalysts.
- Historical: Søren Peter Lauritz Sørensen (1868–1939) introduced the pH scale.
- Historical: Jean Beguin (1550–1620) published the first recorded chemical equation.
Wāhanga 1 · What is a Chemical Reaction?
A chemical reaction occurs when substances (reactants) are rearranged to form new substances (products). Chemical reactions play key roles in both living systems and industry — from digestion and respiration to steel manufacturing and fuel combustion.
Substances present at the start of a reaction. Written on the left side of the arrow in a word equation.
e.g. hydrochloric acid + zinc
New substances formed by the reaction. Written on the right side of the arrow.
e.g. zinc chloride + hydrogen gas
Signs that a chemical reaction has taken place:
produced
change
change
(solid forms)
emitted
change
Energy changes:
Word equations — write the missing products:
| Reactants | Products | Exo or Endo? |
|---|---|---|
| sodium hydroxide + hydrochloric acid → | ||
| zinc + sulfuric acid → | ||
| methane + oxygen → |
Wāhanga 2 · Acids, Bases, and the pH Scale
The pH scale runs from 0 to 14 and measures how acidic or alkaline a solution is. It was devised by Danish chemist Søren Peter Lauritz Sørensen in 1909. Chemical indicators change colour depending on pH and are used to identify the acidity of a solution.
- Hydrochloric acid (HCl)
- Sulfuric acid (H₂SO₄)
- Nitric acid (HNO₃)
- Acetic acid (CH₃COOH) — vinegar
- Citric acid — lemon juice
- Sodium hydroxide (NaOH)
- Potassium hydroxide (KOH)
- Ammonia (NH₃)
- Sodium bicarbonate (baking soda)
- Calcium hydroxide (lime water)
Alkali = base soluble in water
- Litmus: red in acid, blue in base
- Universal indicator: full colour spectrum pH 1–14
- Phenolphthalein: colourless in acid, pink in base
- Red cabbage: natural indicator
Classify each substance and write its approximate pH:
| Substance | pH (approx) | Acid / Neutral / Alkaline | Litmus colour |
|---|---|---|---|
| Lemon juice | |||
| Pure water | |||
| Baking soda solution | |||
| Stomach acid (HCl) | |||
| Bleach (NaOH-based) |
Wāhanga 3 · Acid Reactions
Acids react with three main categories of substances, each producing a characteristic set of products.
When an acid reacts with a base, the two substances neutralise each other, producing a salt and water. This is an exothermic reaction.
HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)
Uses: antacid tablets (base) neutralise excess stomach acid; farmers add lime (calcium hydroxide) to acidic soil.
Acids react with many metals to produce a salt and hydrogen gas (H₂). The gas makes a squeaky pop when a lit splint is held near it — the hydrogen gas test.
Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g)
Not all metals react: gold and platinum are unreactive (this is why they are used in jewellery).
Acids react with metal carbonates to produce a salt, water, and carbon dioxide gas (CO₂). The CO₂ test: gas turns limewater milky.
CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)
Toheroa/pāua shells (calcium carbonate) fizz when placed in acid — this is why acid rain erodes marble and limestone buildings.
Complete the word equations and identify the type of acid reaction:
| Reactants | Products | Type |
|---|---|---|
| magnesium + hydrochloric acid → | ||
| potassium hydroxide + nitric acid → | ||
| sodium carbonate + hydrochloric acid → | ||
| copper(II) carbonate + sulfuric acid → |
Wāhanga 4 · Combustion Reactions
Combustion is a chemical reaction between a fuel and an oxidising agent (usually oxygen). It releases energy, primarily as heat and light — combustion reactions are always exothermic.
Hydrocarbons (compounds of hydrogen and carbon) burn with oxygen to produce carbon dioxide and water.
CH₄ + 2O₂ → CO₂ + 2H₂O
Common hydrocarbons: methane (CH₄, natural gas), octane (C₈H₁₈, petrol), ethanol (C₂H₅OH, biofuel)
Hydrogen burns cleanly — the only product is water. This is why hydrogen fuel cells are considered a clean energy source.
2H₂ + O₂ → 2H₂O
Oxygen is the oxidising agent — it accepts electrons from the fuel during combustion.
Combustion investigation — predictions and observations:
Your teacher will burn different fuels. Predict and observe what you can detect for each:
| Fuel | Predicted observations | Observed (heat / light / products) | Test confirms CO₂? (limewater) |
|---|---|---|---|
| Candle wax (hydrocarbon) | |||
| Ethanol (C₂H₅OH) | |||
| Magnesium ribbon |
Wāhanga 5 · Displacement Reactions and Reaction Rates
Displacement reactions occur when a more reactive metal pushes out (displaces) a less reactive metal from a salt solution. The reactivity series orders metals from most to least reactive.
Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)
Reaction Rate
The rate of a chemical reaction is how fast reactants are converted to products. It can be increased by:
More collisions per second
More particles available
More exposed particles
Lowers activation energy
Predict: will a displacement reaction occur? (✓ or ✗)
| Metal added to solution | Reaction? (✓/✗) | Products (if yes) |
|---|---|---|
| Copper added to silver nitrate solution | ||
| Silver added to copper sulfate solution | ||
| Zinc added to iron(II) sulfate solution |
Tūhono Aotearoa · New Zealand Connections
Māori used rāhui (temporary restrictions) to allow land and waterways to recover. Soil acidity affects plant growth — farmers and kaitiaki test soil pH and add lime (calcium hydroxide) to neutralise acidic soils, enabling growth of kūmara and other taonga species.
Toheroa (shellfish) shells are made of calcium carbonate. Acidic ocean water (caused by dissolved CO₂) reacts with these shells — an acid + carbonate reaction — eroding them. Ocean acidification threatens kaimoana (seafood) populations across Aotearoa.
Wai-o-Tapu and Rotorua's geothermal areas feature highly acidic waters (pH as low as 1–2) from dissolved sulfuric acid and hydrogen sulfide. Māori used these hot pools (wai māori pūia) for cooking and healing — applying knowledge of temperature effects on chemical reactions for centuries.
New Zealand is exploring green hydrogen as a clean fuel. Hydrogen combustion produces only water — no CO₂ — making it appealing for reducing emissions. The clean combustion equation (2H₂ + O₂ → 2H₂O) is simple, but producing hydrogen sustainably is the engineering challenge.
Whakaaro Hōhonu · Deeper Thinking
- A person takes an antacid tablet (contains calcium carbonate) to relieve heartburn (excess HCl in the stomach). Write a word equation for the reaction that occurs and explain why it relieves pain.
- Why does powdered marble react faster with hydrochloric acid than a marble chip of the same mass? Use particle theory to explain your answer.
- Burning fossil fuels (hydrocarbons) releases CO₂ into the atmosphere. Explain the connection between this combustion reaction and ocean acidification affecting kaimoana in Aotearoa.
- Extension: Catalytic converters in cars use platinum and palladium to speed up reactions that convert harmful exhaust gases (CO, NOₓ) into less harmful ones (CO₂, N₂, H₂O). Explain how a catalyst works and why the precious metals chosen are appropriate for this role.
Kuputaka · Key Vocabulary
Kaiako Planning Snapshot
Resources already provided. What to print: this handout (2–3 pages). All activities are self-contained.
Classroom use: Suitable as a formative assessment, paired investigation, or flipped classroom resource. Linked next step: Te Wānanga for a differentiated lesson plan, or save to My Kete.
NZ pedagogy basis: Aligned to Te Mātaiaho (Draft 2025). Tikanga and whanaungatanga principles inform the collaborative activity design. Suitable for all NZ kura and schools.
Mō ngā kaiako — For teachers and ākonga:
Inclusion: ESOL / ELL ākonga — visual supports, colour-coded diagrams, and bilingual headings. UDL / neurodiverse learners — chunked sections, multiple response formats. ADHD-friendly: short, scaffolded tasks.
Differentiation: Entry-level tasks provide word equations with guidance; on-level tasks are open; extension tasks require balanced equations and deeper analysis. Scaffold down with completed examples; stretch by removing supports.