Balance equations, classify reaction types, test for gases, and understand how catalysts work — all in one place! ⚗️🔥
In any chemical reaction, mass is neither created nor destroyed. The total mass of reactants equals the total mass of products.
| Symbol | Meaning | Example |
|---|---|---|
| (s) | Solid | NaCl(s) |
| (l) | Liquid | H₂O(l) |
| (g) | Gas | CO₂(g) |
| (aq) | Aqueous (dissolved in water) | NaOH(aq) |
| Type | Pattern | Key Feature | Example |
|---|---|---|---|
| Combination (Synthesis) | A + B → AB | Two or more substances combine to make one product | 2Mg + O₂ → 2MgO |
| Decomposition | AB → A + B | One substance breaks down into two or more products | 2H₂O₂ → 2H₂O + O₂ |
| Displacement | A + BC → AC + B | More reactive element displaces less reactive from compound | Zn + CuSO₄ → ZnSO₄ + Cu |
| Double Decomposition | AB + CD → AD + CB | Two ionic compounds swap partners — often forms precipitate | AgNO₃ + NaCl → AgCl↓ + NaNO₃ |
| Neutralisation | Acid + Base → Salt + Water | Acid and base react to form salt and water | HCl + NaOH → NaCl + H₂O |
| Combustion | Fuel + O₂ → CO₂ + H₂O | Rapid reaction with oxygen, releases heat and light | CH₄ + 2O₂ → CO₂ + 2H₂O |
| Reaction | Balanced Equation | Type |
|---|---|---|
| Magnesium burning | 2Mg(s) + O₂(g) → 2MgO(s) | Combination |
| Hydrogen peroxide decomposition | 2H₂O₂(aq) → 2H₂O(l) + O₂(g) | Decomposition |
| Zinc + copper sulfate | Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s) | Displacement |
| Silver nitrate + sodium chloride | AgNO₃(aq) + NaCl(aq) → AgCl(s)↓ + NaNO₃(aq) | Double decomposition (ppt) |
| Neutralisation (HCl/NaOH) | HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l) | Neutralisation |
| Complete combustion of methane | CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g) | Combustion |
| Calcium carbonate decomposition | CaCO₃(s) → CaO(s) + CO₂(g) | Decomposition |
| Iron + steam | 3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g) | Displacement |
| Gas | Formula | Test | Positive Result |
|---|---|---|---|
| Hydrogen | H₂ | Apply a lit splint | Squeaky pop |
| Oxygen | O₂ | Apply a glowing splint | Splint relights |
| Carbon dioxide | CO₂ | Bubble through limewater (Ca(OH)₂) | Turns milky/cloudy |
| Ammonia | NH₃ | Damp red litmus paper | Turns blue (only alkaline gas) |
| Chlorine | Cl₂ | Damp blue litmus paper | Turns red then bleaches white |
| Sulfur dioxide | SO₂ | Acidified potassium dichromate | Orange → green |
| Hydrogen chloride | HCl | Hold near ammonia | White fumes (NH₄Cl) |
A catalyst is a substance that increases the rate of a chemical reaction without being used up (consumed) in the reaction. It works by providing an alternative pathway with a lower activation energy.
| Catalyst | Reaction it Speeds Up | Industry/Use |
|---|---|---|
| Iron (Fe) | N₂ + 3H₂ ⇌ 2NH₃ (Haber Process) | Fertiliser production |
| Manganese(IV) oxide (MnO₂) | 2H₂O₂ → 2H₂O + O₂ | Lab preparation of O₂ |
| Platinum (Pt) / Rhodium | Car exhaust catalytic converter | Reduces CO, NOₓ emissions |
| Enzymes (biological) | Fermentation: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ | Brewing, bread-making |
| Vanadium pentoxide (V₂O₅) | 2SO₂ + O₂ → 2SO₃ (Contact Process) | Sulfuric acid manufacture |
Some reactions can go in both directions and are called reversible reactions, shown by the symbol ⇌. When the rate of the forward reaction equals the rate of the reverse reaction, dynamic equilibrium is reached.
When two aqueous solutions are mixed and an insoluble solid forms, this is called a precipitate (shown by ↓). This is a type of double decomposition.
| Reactants Mixed | Precipitate Formed | Colour |
|---|---|---|
| AgNO₃(aq) + NaCl(aq) | AgCl(s) ↓ | White |
| BaCl₂(aq) + Na₂SO₄(aq) | BaSO₄(s) ↓ | White |
| Pb(NO₃)₂(aq) + KI(aq) | PbI₂(s) ↓ | Yellow |
| FeCl₃(aq) + NaOH(aq) | Fe(OH)₃(s) ↓ | Rust-brown |
| CuSO₄(aq) + NaOH(aq) | Cu(OH)₂(s) ↓ | Blue |
Click the + and − buttons to adjust coefficients and balance each equation. Get all atom counts equal to win! 🎯
Click each gas to see the test procedure, observation, and result. These are must-know tests for CSEC!
👆 Click a gas above to see its test
An overview of the 6 reaction types with their key properties visualised.
Click the card to flip!
Problem: Balance the equation for aluminium reacting with hydrochloric acid:
Al + HCl → AlCl₃ + H₂
Type each answer (or type show to reveal):
In the unbalanced equation Al + HCl → AlCl₃ + H₂, how many Cl atoms are on the RIGHT side (in AlCl₃)?
We need 3 Cl on the left to match 3 Cl in AlCl₃. What coefficient goes in front of HCl?
Now we have Al + 3HCl → AlCl₃ + H₂. Left: 3 H (from 3HCl). Right: 2 H (from H₂). To get 6 H on both sides, what coefficient goes in front of H₂?
Multiply everything by 2 to remove the fraction: 2Al + 6HCl → 2AlCl₃ + 3H₂. Verify: Al (2=2 ✓), H (6=6 ✓), Cl (6=6 ✓). What type of reaction is this?
Click a term, then its matching description. Green = correct!
1 (a) 4Fe + 3O₂ → 2Fe₂O₃ ✓ (check: Fe: 4=4, O: 6=6). Type: Combination (Synthesis) ✓
2 (b) Ca + 2H₂O → Ca(OH)₂ + H₂ ✓ (check: Ca:1=1, H:4=4, O:2=2). Type: Displacement ✓ (Ca displaces H from H₂O)
Award 1 mark per correct balanced equation (2 marks), 1 mark per state symbols if present (2 marks), 1 mark per reaction type (2 marks).
1 (a) Hydrogen: Hold a lit splint near the mouth of the tube. ✓ Positive result: a squeaky "pop" sound. ✓
2 (b) Carbon dioxide: Bubble the gas through limewater (calcium hydroxide solution). ✓ Positive result: the limewater turns milky/cloudy. ✓ (CaCO₃ precipitate forms: Ca(OH)₂ + CO₂ → CaCO₃↓ + H₂O)
3 (c) Ammonia: Hold damp red litmus paper in the gas. ✓ Positive result: the paper turns blue. ✓ (NH₃ is the only common alkaline gas)
1 (a) Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s) ✓
2 (b) Displacement reaction ✓
3 (c) The blue colour of the solution fades (as Cu²⁺ is used up). ✓ A reddish-brown/pink solid (copper) is deposited on the zinc. ✓
4 (d) Zinc is higher in the reactivity series than copper, so zinc can displace copper from its salt solution. ✓
1 (a) A catalyst is a substance that increases the rate of a reaction without being consumed (used up) in the reaction. ✓
2 (b) Iron (Fe) ✓
3 (c) N₂(g) + 3H₂(g) ⇌ 2NH₃(g) ✓ (accept without state symbols; reversible arrow needed for full mark)
4 (d) The catalyst provides an alternative reaction pathway with a lower activation energy ✓ so more particles have enough energy to react. ✓ The catalyst participates in the reaction but is regenerated at the end — it ends up unchanged. ✓
1 (a) Double decomposition (precipitation reaction) ✓
2 (b) Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) + 2KNO₃(aq) ✓ (balanced ✓, state symbols ✓)
3 (c) A yellow precipitate (lead iodide, PbI₂) forms immediately when the solutions are mixed. ✓
mass reactants = mass products
Balance by changing coefficients only
Combination · Decomposition
Displacement · Double Decomp.
Neutralisation · Combustion
H₂: squeaky pop 💥
O₂: relights splint 🔥
CO₂: milky limewater ☁️
NH₃: litmus → blue 🔵
AgCl ↓ = white
BaSO₄ ↓ = white
PbI₂ ↓ = yellow
Cu(OH)₂ ↓ = blue
Fe → Haber (NH₃)
MnO₂ → H₂O₂ decomp.
V₂O₅ → Contact (H₂SO₄)
(s) solid · (l) liquid
(g) gas · (aq) aqueous
↓ precipitate · ↑ gas given off