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NCERT Solutions · Class 10 Science · Chapter 1

Chapter 1: Chemical Reactions and Equations (Chemistry)

Answers to all in-text and exercise questions of Chapter 1, Chemical Reactions and Equations (NCERT Class 10 Science, 2026-27 reprint): balancing equations with state symbols, combination, decomposition, displacement, double displacement and redox reactions, exothermic and endothermic reactions, corrosion and rancidity. All 25 questions are answered, with the key answer highlighted.

Free NCERT solutions by Notes Bazar · www.notesbazar.in/ncert-solutions/class-10-science/chapter-1-chemical-reactions-and-equations

A balanced equation has the same number of atoms of each element on both sides (law of conservation of mass). The main types of reaction are combination (A + B → AB), decomposition (AB → A + B, by heat, light or electricity), displacement (a more reactive element pushes out a less reactive one), double displacement (ions are exchanged, often forming a precipitate) and redox. Oxidation is the gain of oxygen or loss of hydrogen; reduction is the loss of oxygen or gain of hydrogen. State symbols: (s) solid, (l) liquid, (g) gas, (aq) aqueous solution.

In-text questions (Section 1.1)

Why should a magnesium ribbon be cleaned before burning in air?

Solution

Magnesium reacts slowly with air on standing and gets a coating of magnesium oxide (and some carbonate). This dull layer stops the metal underneath from burning properly. Rubbing with sandpaper removes the layer, so the clean magnesium can react with oxygen.

To remove the layer of magnesium oxide on its surface, which would stop the ribbon from burning readily.

Write balanced equations: (i) Hydrogen + Chlorine → Hydrogen chloride (ii) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride (iii) Sodium + Water → Sodium hydroxide + Hydrogen

Solution

$H X_{2} + Cl X_{2} 2 HCl$
$3 BaCl X_{2} + Al X_{2} (SO X_{4}) X_{3} 3 BaSO X_{4} + 2 AlCl X_{3}$
$2 Na + 2 H X_{2} O 2 NaOH + H X_{2}$

(i) H₂ + Cl₂ → 2HCl (ii) 3BaCl₂ + Al₂(SO₄)₃ → 3BaSO₄ + 2AlCl₃ (iii) 2Na + 2H₂O → 2NaOH + H₂

Write balanced equations with state symbols: (i) Barium chloride and sodium sulphate solutions react to give insoluble barium sulphate and sodium chloride solution. (ii) Sodium hydroxide solution reacts with hydrochloric acid solution to give sodium chloride solution and water.

Solution

$BaCl X_{2} (aq) + Na X_{2} SO X_{4} (aq) BaSO X_{4} (s) + 2 NaCl (aq)$
$NaOH (aq) + HCl (aq) NaCl (aq) + H X_{2} O (l)$

See the two equations above.

In-text questions (Section 1.2)

A solution of a substance X is used for whitewashing. (i) Name X and write its formula. (ii) Write its reaction with water.

Solution

X is calcium oxide (quicklime), CaO.
It reacts vigorously with water to form slaked lime, giving out a lot of heat: $CaO (s) + H X_{2} O (l) Ca (OH) X_{2} (aq) + heat$

The slaked lime solution is used for whitewashing. On the wall it slowly reacts with carbon dioxide in the air to form a thin, shiny layer of calcium carbonate: $Ca (OH) X_{2} + CO X_{2} CaCO X_{3} + H X_{2} O$ .

(i) Calcium oxide (quicklime), CaO (ii) CaO + H₂O → Ca(OH)₂ + heat

Why is the amount of gas collected in one test tube in Activity 1.7 double that in the other? Name this gas.

Solution

In the electrolysis of water, $2 H X_{2} O (l) electricity 2 H X_{2} (g) + O X_{2} (g)$ .

Each 2 molecules of water give 2 molecules of hydrogen but only 1 of oxygen, so the volume of hydrogen is twice that of oxygen.

Water contains hydrogen and oxygen in the ratio 2 : 1 by volume, so twice as much hydrogen is released. The gas in double amount is hydrogen.

Exercises

Which statements about 2PbO(s) + C(s) → 2Pb(s) + CO₂(g) are incorrect? (a) Lead is getting reduced. (b) Carbon dioxide is getting oxidised. (c) Carbon is getting oxidised. (d) Lead oxide is getting reduced. Options: (i) (a) and (b) (ii) (a) and (c) (iii) (a), (b) and (c) (iv) all

Solution

Lead oxide loses oxygen, so it is reduced; carbon gains oxygen, so it is oxidised. Statement (b) is wrong because CO₂ is a product, not something being oxidised. Statement (a) is also counted as incorrect, because the substance being reduced is lead oxide, not lead.

(i) (a) and (b)

Fe₂O₃ + 2Al → Al₂O₃ + 2Fe is an example of (a) combination (b) double displacement (c) decomposition (d) displacement reaction.

Solution

Aluminium, being more reactive, displaces iron from iron oxide. (This reaction is also a redox reaction, and is used as the thermit reaction to join railway tracks.)

(d) displacement reaction

What happens when dilute hydrochloric acid is added to iron filings? (a) Hydrogen gas and iron chloride are produced. (b) Chlorine gas and iron hydroxide are produced. (c) No reaction takes place. (d) Iron salt and water are produced.

Solution

$Fe (s) + 2 HCl (aq) FeCl X_{2} (aq) + H X_{2} (g)$

(a) Hydrogen gas and iron chloride are produced.

What is a balanced chemical equation? Why should chemical equations be balanced?

Solution

A balanced chemical equation has an equal number of atoms of each element on the reactant and product sides.

Equations must be balanced because of the law of conservation of mass: mass can neither be created nor destroyed in a chemical reaction. Atoms are only rearranged, so the same atoms must appear on both sides.

An equation with equal numbers of atoms of each element on both sides; balancing is needed to obey the law of conservation of mass.

Translate into balanced equations: (a) Hydrogen gas combines with nitrogen to form ammonia. (b) Hydrogen sulphide burns in air to give water and sulphur dioxide. (c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate. (d) Potassium reacts with water to give potassium hydroxide and hydrogen.

Solution

$3 H X_{2} (g) + N X_{2} (g) 2 NH X_{3} (g)$
$2 H X_{2} S (g) + 3 O X_{2} (g) 2 H X_{2} O (l) + 2 SO X_{2} (g)$
$3 BaCl X_{2} (aq) + Al X_{2} (SO X_{4}) X_{3} (aq) 2 AlCl X_{3} (aq) + 3 BaSO X_{4} (s)$
$2 K (s) + 2 H X_{2} O (l) 2 KOH (aq) + H X_{2} (g)$

See the four equations above.

Balance: (a) HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + H₂O (b) NaOH + H₂SO₄ → Na₂SO₄ + H₂O (c) NaCl + AgNO₃ → AgCl + NaNO₃ (d) BaCl₂ + H₂SO₄ → BaSO₄ + HCl

Solution

$2 HNO X_{3} + Ca (OH) X_{2} Ca (NO X_{3}) X_{2} + 2 H X_{2} O$
$2 NaOH + H X_{2} SO X_{4} Na X_{2} SO X_{4} + 2 H X_{2} O$
$NaCl + AgNO X_{3} AgCl + NaNO X_{3}$ (already balanced)
$BaCl X_{2} + H X_{2} SO X_{4} BaSO X_{4} + 2 HCl$

See the balanced equations above.

Write balanced equations: (a) Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water (b) Zinc + Silver nitrate → Zinc nitrate + Silver (c) Aluminium + Copper chloride → Aluminium chloride + Copper (d) Barium chloride + Potassium sulphate → Barium sulphate + Potassium chloride

Solution

$Ca (OH) X_{2} + CO X_{2} CaCO X_{3} + H X_{2} O$
$Zn + 2 AgNO X_{3} Zn (NO X_{3}) X_{2} + 2 Ag$
$2 Al + 3 CuCl X_{2} 2 AlCl X_{3} + 3 Cu$
$BaCl X_{2} + K X_{2} SO X_{4} BaSO X_{4} + 2 KCl$

See the balanced equations above.

Write balanced equations and identify the type of reaction: (a) KBr(aq) + BaI₂(aq) → KI(aq) + BaBr₂(s) (b) ZnCO₃(s) → ZnO(s) + CO₂(g) (c) H₂(g) + Cl₂(g) → HCl(g) (d) Mg(s) + HCl(aq) → MgCl₂(aq) + H₂(g)

Solution

$2 KBr (aq) + BaI X_{2} (aq) 2 KI (aq) + BaBr X_{2} (s)$ : double displacement (precipitation) reaction
$ZnCO X_{3} (s) heat ZnO (s) + CO X_{2} (g)$ : decomposition (thermal decomposition)
$H X_{2} (g) + Cl X_{2} (g) 2 HCl (g)$ : combination reaction
$Mg (s) + 2 HCl (aq) MgCl X_{2} (aq) + H X_{2} (g)$ : displacement reaction

(a) double displacement (b) decomposition (c) combination (d) displacement

What are exothermic and endothermic reactions? Give examples.

Solution

Exothermic reactions give out heat. Examples: burning of natural gas, $CH X_{4} + 2 O X_{2} CO X_{2} + 2 H X_{2} O$ + heat; quicklime with water; respiration.
Endothermic reactions absorb heat. Examples: decomposition of calcium carbonate, $CaCO X_{3} heat CaO + CO X_{2}$ ; photosynthesis (absorbs light energy); barium hydroxide with ammonium chloride (the test tube feels cold).

Exothermic: heat is released (e.g. burning of methane). Endothermic: heat is absorbed (e.g. decomposition of CaCO₃).

Why is respiration considered an exothermic reaction?

Solution

During respiration, glucose from digested food combines with oxygen in our cells and releases energy:

$C X_{6} H X_{12} O X_{6} (aq) + 6 O X_{2} (aq) 6 CO X_{2} (aq) + 6 H X_{2} O (l)$ + energy

Since energy is released, respiration is exothermic. This energy keeps us warm and powers all life processes.

Because glucose is oxidised in the cells and energy is given out.

Why are decomposition reactions called the opposite of combination reactions? Write equations.

Solution

In a combination reaction, two or more substances join to form one product. In a decomposition reaction, one substance breaks down into two or more simpler products. One builds up, the other breaks down.

Combination: $CaO + H X_{2} O Ca (OH) X_{2}$ ; $2 H X_{2} + O X_{2} 2 H X_{2} O$
Decomposition: $CaCO X_{3} heat CaO + CO X_{2}$ ; $2 H X_{2} O electricity 2 H X_{2} + O X_{2}$

Combination joins reactants into a single product; decomposition splits a single reactant into simpler products.

Write one equation each for decomposition reactions where energy is supplied as heat, light or electricity.

Solution

Heat: $2 FeSO X_{4} (s) heat Fe X_{2} O X_{3} (s) + SO X_{2} (g) + SO X_{3} (g)$
Light: $2 AgCl (s) sunlight 2 Ag (s) + Cl X_{2} (g)$ (used in black-and-white photography)
Electricity: $2 H X_{2} O (l) electricity 2 H X_{2} (g) + O X_{2} (g)$

Thermal: ferrous sulphate; photolytic: silver chloride; electrolytic: water.

What is the difference between displacement and double displacement reactions? Write equations.

Solution

In a displacement reaction, a more reactive element displaces a less reactive element from its compound: $Fe (s) + CuSO X_{4} (aq) FeSO X_{4} (aq) + Cu (s)$
In a double displacement reaction, two compounds exchange ions: $Na X_{2} SO X_{4} (aq) + BaCl X_{2} (aq) BaSO X_{4} (s) + 2 NaCl (aq)$

One element replaces another in displacement; two compounds swap ions in double displacement.

In refining silver, silver is recovered from silver nitrate solution by displacement with copper. Write the reaction.

Solution

Copper is more reactive than silver, so it displaces silver:

$Cu (s) + 2 AgNO X_{3} (aq) Cu (NO X_{3}) X_{2} (aq) + 2 Ag (s)$

Cu + 2AgNO₃ → Cu(NO₃)₂ + 2Ag

What is a precipitation reaction? Explain with examples.

Solution

A reaction that produces an insoluble solid (a precipitate) is a precipitation reaction. These are usually double displacement reactions.

$Na X_{2} SO X_{4} (aq) + BaCl X_{2} (aq) BaSO X_{4} (s) + 2 NaCl (aq)$ : a white precipitate of barium sulphate forms.
$Pb (NO X_{3}) X_{2} (aq) + 2 KI (aq) PbI X_{2} (s) + 2 KNO X_{3} (aq)$ : a yellow precipitate of lead iodide forms.

A reaction that forms an insoluble precipitate, such as BaSO₄ (white) or PbI₂ (yellow).

Explain in terms of gain or loss of oxygen, with two examples each: (a) Oxidation (b) Reduction

Solution

Oxidation is the gain of oxygen by a substance. $2 Cu + O X_{2} heat 2 CuO$ (copper is oxidised); $C + O X_{2} CO X_{2}$ (carbon is oxidised).
Reduction is the loss of oxygen. $CuO + H X_{2} heat Cu + H X_{2} O$ (copper oxide is reduced); $ZnO + C Zn + CO$ (zinc oxide is reduced).

In each of these reactions one substance is oxidised while the other is reduced, so they are redox reactions.

Oxidation: gain of oxygen. Reduction: loss of oxygen. Examples as above.

A shiny brown element X becomes black on heating in air. Name X and the black compound.

Solution

X is copper. On heating, its surface combines with oxygen to form black copper(II) oxide:

$2 Cu + O X_{2} heat 2 CuO$

X is copper (Cu); the black compound is copper oxide (CuO).

Why do we apply paint on iron articles?

Solution

Iron rusts when it is exposed to moist air (oxygen and water). Paint forms a coat that keeps air and moisture away from the iron, which prevents rusting (corrosion).

To prevent rusting, by keeping air and moisture away from the iron.

Why are foods containing oil and fat flushed with nitrogen?

Solution

Oils and fats get oxidised by air and become rancid, which changes their smell and taste. Nitrogen is an unreactive gas. Filling the packet with nitrogen removes oxygen, so the food is not oxidised and stays fresh longer (e.g. chips packets).

To prevent oxidation of the oils and fats, which would make the food rancid.

Explain with one example each: (a) Corrosion (b) Rancidity

Solution

Corrosion: a metal is slowly attacked by substances around it such as moisture, air and acids. Example: rusting of iron, which forms reddish-brown hydrated iron oxide (Fe₂O₃·xH₂O). (Silver turning black and copper turning green are other examples.)
Rancidity: fats and oils in food get oxidised, so the food smells and tastes bad. Example: chips or butter left open for a long time. It is prevented by adding antioxidants, packing in nitrogen, or keeping the food in airtight containers.

Corrosion: e.g. rusting of iron. Rancidity: e.g. stale fried snacks.

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