Chemical reactions, equations & balancing
Milk turning sour, an iron nail rusting and LPG burning are all changes where new substances form.
A chemical reaction changes the starting substances (reactants) into new substances (products) with different properties. Signs: change of state, change of colour, gas bubbles, change of temperature, a solid (precipitate) appearing.
Equation / formula : Word equation: magnesium + oxygen → magnesium oxide. Skeletal: Mg + O₂ → MgO. Balanced: 2Mg + O₂ → 2MgO (atoms of each element are equal on both sides, because mass is conserved). States: (s) solid, (l) liquid, (g) gas, (aq) in water.
In the figure : Left: oxygen atoms are unequal (2 and 1). Middle: change only the coefficients. Right: both sides now have 2 Mg and 2 O.
Rule 1 : Balancing a skeletal equation — Count atoms of each element on both sides, then change only the numbers in front (coefficients). Never change the small subscripts.
- Easy : H₂ + Cl₂ → 2HCl
- Medium : Fe + H₂O → Fe₃O₄ + H₂ becomes 3Fe + 4H₂O → Fe₃O₄ + 4H₂.
- For exams : 3BaCl₂ + Al₂(SO₄)₃ → 3BaSO₄ + 2AlCl₃
Rule 2 : Reading a chemical equation — A good equation also shows states and conditions (heat, light, catalyst) written above or below the arrow.
- Easy : CaCO₃(s) → CaO(s) + CO₂(g) happens on heating.
- Medium : NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l): all dissolved in water except the water formed.
- For exams : Zinc + dilute sulphuric acid: Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g).
A balanced equation obeys the law of conservation of mass: total mass of reactants = total mass of products.
Writing H₂O₂ instead of 2H₂O to "balance" oxygen. This changes the substance itself. Only coefficients may change.
Start with the biggest formula, leave H and O for the end. Memory clue : "Count, Coefficient, Check" (CCC).
Spot the difference
| Point | Skeletal equation | Balanced equation |
|---|---|---|
| Atoms | may be unequal | equal on both sides |
| Example | Mg + O₂ → MgO | 2Mg + O₂ → 2MgO |
Practice questions
- 1Name two signs of a chemical reaction.
- 2Balance: N₂ + H₂ → NH₃.
- 3What does (aq) mean in an equation?
- 4Balance: Al + O₂ → Al₂O₃.
- 5Why must we not change subscripts while balancing?
- 6Balance: Na + H₂O → NaOH + H₂.
- 7Balance Pb(NO₃)₂ → PbO + NO₂ + O₂.
- 8Write the balanced equation for burning of methane (CH₄) in oxygen and count atoms on both sides.
Show answers (try them yourself first)
- 1Gas bubbles, colour change, new solid / temperature change, change of state
- 2N₂ + 3H₂ → 2NH₃
- 3Dissolved in water (aqueous).
- 44Al + 3O₂ → 2Al₂O₃
- 5It would change the substance itself.
- 62Na + 2H₂O → 2NaOH + H₂
- 72Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂
- 8CH₄ + 2O₂ → CO₂ + 2H₂O ; C 1=1, H 4=4, O 4=4.
Chemical reactions, equations & balancing
In the app you can move this figure with your finger.
Open in the appCombination and decomposition reactions
Mixing quicklime with water gets the mixture hot (combination). Heating baking soda makes cakes rise (decomposition).
In a combination reaction two or more substances join to form a single product. In a decomposition reaction one substance breaks into two or more simpler substances, using heat, light or electricity.
Equation / formula : Combination: A + B → AB. Decomposition: AB → A + B.
In the figure : Decomposition is named after what breaks the compound: heat, light or electricity.
Rule 1 : Combination reactions — Elements or compounds join. Most are exothermic (give out heat).
- Easy : C + O₂ → CO₂
- Medium : CaO + H₂O → Ca(OH)₂ + heat (slaked lime for whitewash).
- For exams : N₂ + 3H₂ → 2NH₃
Rule 2 : Decomposition reactions — One compound splits. Most need energy, so most are endothermic.
- Easy : Electrolysis of water: 2H₂O → 2H₂ + O₂.
- Medium : Heating limestone: CaCO₃ → CaO + CO₂.
- For exams : 2FeSO₄ → Fe₂O₃ + SO₂ + SO₃ (green crystals turn brown, smell of burning sulphur); 2AgCl → 2Ag + Cl₂ in sunlight (white turns grey).
Exothermic: heat is released (burning, respiration). Endothermic: heat is absorbed (most decompositions).
Thinking only elements can combine. Compounds also combine: CaO + H₂O → Ca(OH)₂.
One product = combination. One reactant = decomposition. Memory clue : "Combine = Come together; Decompose = De-part".
Spot the difference
| Point | Combination | Decomposition |
|---|---|---|
| Reactants | two or more | one |
| Products | one | two or more |
| Heat | usually released | usually absorbed |
Practice questions
- 1Is C + O₂ → CO₂ a combination or decomposition reaction?
- 2Name the three energy sources used in decomposition.
- 3What is the other product when limestone is heated?
- 4Why is the reaction of quicklime with water called exothermic?
- 5Write the balanced equation for electrolysis of water.
- 6Silver chloride turns grey in sunlight. Which type of decomposition is this?
- 7Green FeSO₄ crystals are heated. Write observations and the equation.
- 8Give one reason why decomposition reactions are called the opposite of combination reactions.
Show answers (try them yourself first)
- 1Combination
- 2Heat, light, electricity
- 3CO₂ (carbon dioxide)
- 4Heat is given out.
- 52H₂O → 2H₂ + O₂
- 6Photolytic (by light)
- 7Turns brown; smell of burning sulphur. 2FeSO₄ → Fe₂O₃ + SO₂ + SO₃
- 8Combination joins substances; decomposition splits them.
Combination and decomposition reactions
In the app you can move this figure with your finger.
Open in the appDisplacement and double displacement reactions
A shiny iron nail dipped in blue copper sulphate gets a brown coat and the blue colour fades.
In a displacement reaction a more reactive element pushes out a less reactive element from its compound. In a double displacement reaction two compounds exchange their ions; often an insoluble solid (precipitate) forms.
Equation / formula : A + BC → AC + B | AB + CD → AD + CB
In the figure : Top left: one element is replaced. Top right: two compounds exchange partners. Bottom: real examples; ↓ means a precipitate.
Rule 1 : Displacement: reactivity decides — A metal displaces another only if it is more reactive.
- Easy : Fe + CuSO₄ → FeSO₄ + Cu
- Medium : Zn + CuSO₄ → ZnSO₄ + Cu ; Pb + CuCl₂ → PbCl₂ + Cu
- For exams : Cu + 2AgNO₃ → Cu(NO₃)₂ + 2Ag (solution turns blue, silver deposits).
Rule 2 : Double displacement and precipitation — Positive ions and negative ions swap. If one new compound does not dissolve, it appears as a precipitate.
- Easy : Na₂SO₄ + BaCl₂ → BaSO₄↓ (white) + 2NaCl
- Medium : Neutralisation is double displacement: NaOH + HCl → NaCl + H₂O.
- For exams : Pb(NO₃)₂ + 2KI → PbI₂↓ (yellow) + 2KNO₃ ; AgNO₃ + NaCl → AgCl↓ (white) + NaNO₃
A precipitate is an insoluble solid formed in a solution. Its colour helps identify the ions.
Writing Fe₂(SO₄)₃ for the product of iron with copper sulphate. In this reaction iron gives FeSO₄ (pale green).
Single swap = displacement. Two couples swap = double displacement. Memory clue : "A new dance partner": one person cuts in (displacement) or two couples swap (double).
Spot the difference
| Point | Displacement | Double displacement |
|---|---|---|
| What happens | one element replaced | ions exchanged |
| Typical sign | colour change, metal coating | precipitate |
Practice questions
- 1Which is more reactive: iron or copper?
- 2What is a precipitate?
- 3Name the white precipitate in Na₂SO₄ + BaCl₂.
- 4Why does the blue colour of CuSO₄ fade when zinc is added?
- 5Write: copper + silver nitrate → ?
- 6Is NaOH + HCl a displacement or double displacement reaction?
- 7Will silver displace copper from CuSO₄? Give a reason.
- 8Write the balanced equation for lead nitrate with potassium iodide and give the colour of the precipitate.
Show answers (try them yourself first)
- 1Iron
- 2An insoluble solid formed in a solution.
- 3BaSO₄
- 4Cu²⁺ is replaced by Zn²⁺ (colourless).
- 5Cu + 2AgNO₃ → Cu(NO₃)₂ + 2Ag
- 6Double displacement
- 7No; silver is less reactive than copper.
- 8Pb(NO₃)₂ + 2KI → PbI₂ + 2KNO₃ ; yellow
Oxidation and reduction (redox)
A copper vessel turns black when heated in air. Passing hydrogen over the black powder brings back the brown copper.
Oxidation is gain of oxygen or loss of hydrogen. Reduction is loss of oxygen or gain of hydrogen. Both happen together in a redox reaction. The substance that gives oxygen (or takes hydrogen) is the oxidising agent; the other is the reducing agent.
Equation / formula : 2Cu + O₂ → 2CuO (Cu oxidised) CuO + H₂ → Cu + H₂O (CuO reduced, H₂ oxidised)
In the figure : Top: the two definitions. Bottom: copper is oxidised to black CuO, then CuO is reduced back to brown copper.
Rule 1 : Spot what is oxidised and reduced — Look for oxygen or hydrogen moving between substances.
- Easy : 2Mg + O₂ → 2MgO : Mg is oxidised.
- Medium : CuO + H₂ → Cu + H₂O : CuO reduced; H₂ oxidised.
- For exams : ZnO + C → Zn + CO : ZnO reduced, C oxidised. MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂ : HCl oxidised (loses H), MnO₂ reduced (loses O).
Rule 2 : Oxidising and reducing agents — The oxidising agent gets reduced; the reducing agent gets oxidised.
- Easy : In CuO + H₂ → Cu + H₂O, CuO is the oxidising agent.
- Medium : In Fe₂O₃ + 2Al → Al₂O₃ + 2Fe, aluminium is the reducing agent.
- For exams : H₂S + Cl₂ → 2HCl + S : H₂S is oxidised (loses H), Cl₂ is reduced and is the oxidising agent.
Oxidation and reduction never happen alone; where one substance is oxidised, another is reduced.
Saying "the oxidising agent is oxidised". It is the reducing agent that gets oxidised.
Ask two questions: who gained oxygen? who lost hydrogen? Memory clue : "OG-HL" : Oxidation = Oxygen Gain / Hydrogen Loss; "RL-HG" : Reduction = oxygen Loss / Hydrogen Gain.
Spot the difference
| Point | Oxidation | Reduction |
|---|---|---|
| Oxygen | gained | lost |
| Hydrogen | lost | gained |
Practice questions
- 1Define oxidation in terms of oxygen.
- 2In 2Cu + O₂ → 2CuO, which substance is oxidised?
- 3What is a redox reaction?
- 4In CuO + H₂ → Cu + H₂O name the oxidising and reducing agents.
- 5Why does the black CuO turn brown in a stream of hydrogen?
- 6In ZnO + C → Zn + CO, which is reduced?
- 7In H₂S + Cl₂ → 2HCl + S find the substance oxidised and the oxidising agent.
- 8Explain why the thermite reaction Fe₂O₃ + 2Al → Al₂O₃ + 2Fe is a redox reaction.
Show answers (try them yourself first)
- 1Gain of oxygen (or loss of hydrogen).
- 2Copper
- 3Oxidation and reduction occur together.
- 4Oxidising: CuO. Reducing: H₂.
- 5CuO loses oxygen and becomes copper.
- 6ZnO
- 7H₂S is oxidised; Cl₂ is the oxidising agent.
- 8Fe₂O₃ loses oxygen (reduced); Al gains oxygen (oxidised).
Corrosion and rancidity
A rusty gate, a green copper pot and stale oily snacks all show slow oxidation in everyday life.
Corrosion is the slow eating away of a metal by air, moisture or chemicals. Iron rusts (hydrated iron(III) oxide, Fe₂O₃·xH₂O), copper turns green, silver turns black. Rancidity is the oxidation of fats and oils in food, giving a bad smell and taste.
Equation / formula : Rusting needs oxygen and water. Prevent it by painting, oiling, galvanising (zinc coat), plating or alloying.
In the figure : The first three show damage; the last one lists remedies.
Rule 1 : Rusting and its prevention — Remove either air or water, or cover the iron with a safe layer.
- Easy : An iron nail in dry air with calcium chloride does not rust (no moisture).
- Medium : A nail in boiled water under a layer of oil does not rust (no air).
- For exams : Galvanised iron stays safe even when scratched, because zinc is more reactive and corrodes first (sacrificial protection).
Rule 2 : Rancidity and its prevention — Fats and oils react slowly with oxygen. Keep oxygen, light and heat away.
- Easy : Chips packets are flushed with nitrogen gas.
- Medium : Antioxidants are added to oils and butter.
- For exams : Oil is stored in airtight containers in a cool dark place to slow oxidation.
Rust is reddish-brown, flaky and weakens iron; corrosion costs a lot of money every year.
Thinking rust forms with water alone, or air alone. Both oxygen and moisture are needed.
No air or no water = no rust. Cover it, coat it, or sacrifice zinc. Memory clue : "AWE": Air + Water = rust, so remove one of them!
Spot the difference
| Point | Corrosion | Rancidity |
|---|---|---|
| Affects | metals | fats and oils in food |
| Example | rusting of iron | stale chips |
Practice questions
- 1Name the two things needed for rusting.
- 2What is the colour of rust?
- 3Why are chips packets filled with nitrogen?
- 4Give two methods to prevent rusting.
- 5What is rancidity?
- 6Why does a silver spoon turn black?
- 7Explain galvanisation with its advantage.
- 8A nail is kept in boiled water covered with oil, and another in tap water. Which rusts, and why?
Show answers (try them yourself first)
- 1Oxygen (air) and water (moisture)
- 2Reddish brown
- 3To stop oxidation of the oil.
- 4Painting, oiling, galvanising, alloying (any two)
- 5Oxidation of fats and oils giving bad smell and taste.
- 6It forms black silver sulphide.
- 7Zinc coating on iron; protects even when scratched.
- 8The tap-water nail rusts; the other has no air.