10 min readConcept sheet1-mark MCQ and assertion-reason; often a 3 or 5-mark question with equations

Combination and decomposition reactions; exothermic and endothermic

In a combination reaction, two or more substances form one product. In a decomposition reaction, one substance breaks into simpler ones, using heat, electricity or light.

CombinationA + B → AB, e.g. CaO + H₂O → Ca(OH)₂

DecompositionAB → A + B, e.g. CaCO₃ → CaO + CO₂

ExothermicHeat is given out (burning, respiration, quicklime with water)

EndothermicHeat is taken in (most decomposition reactions)

Activity: water on quicklime (a combination reaction)

1
Put a small amount of quicklime (calcium oxide) in a beaker.
2
Slowly add water, then touch the outside of the beaker.
3
Observe: a hissing sound, and the beaker becomes very hot.
4★
CaO(s) + H₂O(l) → Ca(OH)₂(aq) + Heat
Quicklime + water gives slaked lime (calcium hydroxide)
5
On a whitewashed wall, slaked lime reacts with carbon dioxide in air: Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)
In 2 to 3 days a thin, shiny layer of calcium carbonate forms

★ key step

More combination reactions, and three kinds of decomposition

Kind	Energy	Example	What you see
Combination	Heat given out	C(s) + O₂(g) → CO₂(g)	Burning of coal
Combination	Heat given out	2H₂(g) + O₂(g) → 2H₂O(l)	Formation of water
Thermal decomposition	Heat taken in	CaCO₃(s) → CaO(s) + CO₂(g)	Limestone gives quicklime, which is used to make cement
Thermal decomposition	Heat taken in	2Pb(NO₃)₂(s) → 2PbO(s) + 4NO₂(g) + O₂(g)	Brown fumes of nitrogen dioxide
Electrolytic decomposition	Electricity	2H₂O(l) → 2H₂(g) + O₂(g)	Hydrogen at the cathode has double the volume of oxygen at the anode
Photolytic decomposition (by light)	Sunlight	2AgCl(s) → 2Ag(s) + Cl₂(g)	White silver chloride turns grey; also 2AgBr(s) → 2Ag(s) + Br₂(g). Used in black and white photography

Activity: heating ferrous sulphate crystals

1
Take about 2 g of pale green ferrous sulphate crystals in a dry boiling tube.
2
Heat over a burner, holding the tube with a holder, its mouth pointing away from people.
3
Observe: water drops near the mouth of the tube, a change of colour, and the smell of burning sulphur.
The crystals (FeSO₄·7H₂O) first lose their water of crystallisation
4★
2FeSO₄(s) → Fe₂O₃(s) + SO₂(g) + SO₃(g)
On heating. Ferric oxide, Fe₂O₃, is the reddish-brown solid left behind

★ key step

How to draw the electrolysis of water set-up

Draw a plastic mug with two carbon electrodes fixed through rubber stoppers in its base.
Fill the mug with water and write "water with a few drops of dilute sulphuric acid".
Draw two test tubes full of water, turned upside down over the two electrodes.
Join the electrodes to a 6 V battery through a switch.
Show gas at the top of each tube: twice as much hydrogen over the cathode (negative) as oxygen over the anode (positive).
Label: mug, water, carbon electrodes, rubber stoppers, test tubes, hydrogen, oxygen, cathode, anode, battery.

Exothermic vs endothermic

Exothermic: heat given outQuicklime with water. Burning of natural gas: CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g). Respiration: C₆H₁₂O₆(aq) + 6O₂(aq) → 6CO₂(aq) + 6H₂O(l) + energy. Vegetable waste turning into compost.

Endothermic: heat taken inMost decomposition reactions need heat, light or electricity to break the reactant. Example: heating limestone, CaCO₃ → CaO + CO₂.

Respiration is exothermic: glucose from rice, roti and fruit is broken down in our cells to give energy.

In real life

Before Diwali or Pongal, many homes are whitewashed with slaked lime, which slowly turns into shiny calcium carbonate on the walls. Cement is made using quicklime, which comes from heating limestone.

Exam angle

2 marks: “Rice and roti give us energy after they are digested. Name the reaction in our cells that releases this energy, write its equation, and say why it is exothermic.” Respiration: C₆H₁₂O₆(aq) + 6O₂(aq) → 6CO₂(aq) + 6H₂O(l) + energy. Glucose combines with oxygen in our cells and energy is given out, so it is exothermic.

2 marks: “Slaked lime is applied on walls. After 2 to 3 days the walls shine. Why? Write the equation.” Calcium hydroxide reacts with carbon dioxide in air to form a thin layer of calcium carbonate, which gives the shine. Ca(OH)₂ + CO₂ → CaCO₃ + H₂O.

3 marks: “A student heats lead nitrate powder in a boiling tube. (i) What does she see? (ii) Write the balanced equation. (iii) Is the reaction exothermic or endothermic? Why?” (i) Brown fumes of nitrogen dioxide. (ii) 2Pb(NO₃)₂(s) → 2PbO(s) + 4NO₂(g) + O₂(g). (iii) Endothermic: heat must be supplied to break down the lead nitrate.

Remember it

Combine = come together; decompose = break apart. Breaking needs energy (heat, light or electricity), so decomposition is usually endothermic.