Chapter 1: Chemical Reactions and Equations
All in-text + end-of-chapter questions, with step-by-step answers and a video solution link below each.
In-text Questions (Page-wise)
These appear inside the chapter, right after each activity.
1Why should a magnesium ribbon be cleaned before burning in air?
A magnesium ribbon left in open air slowly reacts with oxygen and forms a thin, dull layer of magnesium oxide on its surface. This coating stops the fresh metal underneath from burning cleanly. Cleaning the ribbon with sandpaper removes this oxide layer, exposing pure magnesium so it burns with a bright white flame.
2Write balanced equations: (i) Hydrogen + Chlorine (ii) Barium chloride + Aluminium sulphate (iii) Sodium + Water
3Write balanced equations with state symbols: (i) BaCl₂ + Na₂SO₄ (ii) NaOH + HCl
4A solution of substance X is used for whitewashing. Name X, its formula, and its reaction with water.
Substance X
Calcium oxide (quicklime), formula CaO.
Slaked lime slowly reacts with atmospheric CO₂ to form a thin layer of calcium carbonate, which gives the whitewashed wall its shine.
5Why is the gas collected in one test tube (electrolysis of water) double that in the other? Name the gas.
Water is made up of hydrogen and oxygen in a 2:1 ratio by number of molecules (H₂O). During electrolysis, this ratio is preserved in the gases released — twice as much hydrogen gas forms as oxygen gas. The test tube with double the volume of gas collects hydrogen.
6Why does the colour of copper sulphate solution change when an iron nail is dipped in it?
Iron is more reactive than copper, so it displaces copper from copper sulphate solution. Iron sulphate forms in solution (pale green) while copper metal deposits on the nail, so the original blue colour of copper sulphate fades.
7Give an example of a double displacement reaction other than Activity 1.10.
Both the positive and negative ions swap partners, and an insoluble precipitate (barium sulphate) forms.
8Difference between displacement and double displacement reactions, with equations.
Displacement reaction — a more reactive element pushes out a less reactive element from its compound.
Double displacement reaction — two compounds exchange ions to form two new compounds, usually with one being an insoluble precipitate.
9Identify substances oxidised and reduced: (i) 4Na + O₂ → 2Na₂O (ii) CuO + H₂ → Cu + H₂O
(i) Sodium gains oxygen, so Na is oxidised. There's nothing being reduced here since O₂ itself is the oxidising agent combining directly.
(ii) CuO loses oxygen, so it is reduced to Cu. H₂ gains oxygen (becomes H₂O), so it is oxidised.
End-of-Chapter Exercises
Questions 1–5
1Which statements about 2PbO(s) + C(s) → 2Pb(s) + CO₂(g) are incorrect?
PbO loses oxygen (reduced to Pb), and carbon gains oxygen (oxidised to CO₂). So:
- (a) Lead is getting reduced — correct
- (b) Carbon dioxide is getting oxidised — incorrect (CO₂ is the product of oxidation, not being oxidised itself)
- (c) Carbon is getting oxidised — correct
- (d) Lead oxide is getting reduced — correct
Incorrect statement: (b) only → so among the given options, the answer is option (i) (a) and (b) is wrong since (a) is actually correct; the only genuinely incorrect statement is (b). (NCERT's intended answer: option (i) — treating (b) as the key incorrect one paired with a distractor.)
2Fe₂O₃ + 2Al → Al₂O₃ + 2Fe is an example of a ___ reaction.
Answer: (d) displacement reaction.
Aluminium is more reactive than iron, so it displaces iron from iron oxide. (This particular reaction — thermite reaction — is also highly exothermic and used in welding railway tracks.)
3What happens when dilute HCl is added to iron filings?
Answer: (a) Hydrogen gas and iron chloride are produced.
4What is a balanced chemical equation? Why should equations be balanced?
A balanced chemical equation has an equal number of atoms of each element on both the reactant and product sides.
Equations must be balanced because of the law of conservation of mass — matter can neither be created nor destroyed in a chemical reaction, so the total mass (and atom count) of reactants must equal that of the products.
5Translate into balanced equations: (a) H₂ + N₂ → NH₃ (b) H₂S burns in air (c) BaCl₂ + Al₂(SO₄)₃ (d) Potassium + water
Questions 6–10
6Balance: (a) HNO₃ + Ca(OH)₂ (b) NaOH + H₂SO₄ (c) NaCl + AgNO₃ (d) BaCl₂ + H₂SO₄
7Write balanced equations and identify reaction type for four given word reactions.
Type: Double displacement reaction
Type: Decomposition reaction
Type: Combination reaction
Type: Displacement reaction
8What are exothermic and endothermic reactions? Give examples.
Exothermic — releases heat/energy to the surroundings.
Example: burning of natural gas — CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l) + heat
Endothermic — absorbs heat/energy from the surroundings.
Example: decomposition of calcium carbonate on strong heating — CaCO₃(s) + heat → CaO(s) + CO₂(g)
9Why is respiration considered an exothermic reaction?
During respiration, glucose breaks down using oxygen to release carbon dioxide, water, and energy. Since energy is given out rather than absorbed, respiration is exothermic.
10Why are decomposition reactions called the opposite of combination reactions?
In a combination reaction, two or more substances join to form one product. In a decomposition reaction, one compound breaks down into two or more simpler substances — exactly the reverse process.
Questions 11–15
11Write one decomposition equation each for heat, light, and electricity.
Heat
Light
Electricity
12Difference between displacement and double displacement reactions (equations).
Displacement
Double displacement
In displacement, one element replaces another in a compound. In double displacement, two compounds exchange ions with each other.
13Write the reaction for recovery of silver from silver nitrate using copper.
Copper is more reactive than silver, so it displaces silver from silver nitrate solution.
14What is a precipitation reaction? Explain with examples.
A precipitation reaction is one where an insoluble solid (precipitate) forms when two solutions are mixed, usually through a double displacement reaction.
Here, white insoluble barium sulphate settles out as a precipitate.
15Explain oxidation and reduction in terms of gain/loss of oxygen, with two examples each.
Oxidation — gain of oxygen.
- 2Mg + O₂ → 2MgO
- 2Cu + O₂ → 2CuO
Reduction — loss of oxygen.
- CuO + H₂ → Cu + H₂O
- ZnO + C → Zn + CO
Questions 16–19
16A shiny brown element X turns black on heating in air. Name X and the black compound formed.
X = Copper (Cu). On heating in air, copper reacts with atmospheric oxygen to form black copper(II) oxide on its surface.
17Why do we apply paint on iron articles?
Paint forms a protective barrier that keeps air and moisture away from the iron surface. Since rusting needs both oxygen and water to occur, blocking their contact with the metal prevents corrosion and extends the life of the article.
18Oil and fat containing food items are flushed with nitrogen. Why?
Nitrogen is an unreactive (inert) gas. Packaging fatty/oily food in a nitrogen atmosphere keeps oxygen away from the food, preventing oxidation of the fats — which is what causes rancidity (that unpleasant smell and taste in stale oily food).
19Explain: (a) Corrosion (b) Rancidity, with one example each.
Corrosion — slow destruction of a metal's surface due to reaction with substances in its surroundings, such as air and moisture.
Example: Iron rusts when exposed to moist air, forming reddish-brown iron oxide (Fe₂O₃·xH₂O).
Rancidity — oxidation of fats and oils in food, causing an unpleasant smell and taste when the item is left exposed to air for too long.
Example: Chips or namkeen left open in air for a few days start tasting stale.