Where Mindful Learning Begins
Where Mindful Learning Begins
Class 10 Science NCERT Solutions Chapter 1 (CBSE 2026-27) — every in-text question and all 20 back-of-chapter Exercises on this one page, solved with the reasoning behind each answer.
Chemical Reactions and Equations covers everything from writing and balancing chemical equations, to the four main types of reactions — combination, decomposition, displacement and double displacement — through exothermic and endothermic reactions, oxidation-reduction (redox reactions), and the everyday effects of oxidation such as corrosion and rancidity. These NCERT solutions work through every Activity-based in-text question and the full Exercise set in detail, making this one of the most exam-relevant chapters in the chemistry portion of the Class 10 Science syllabus.
Chemical Reactions and Equations is the opening chapter of Class 10 Science chemistry, and it lays the foundation for everything that follows in Acids, Bases and Salts and Metals and Non-Metals. It moves from writing a simple word-equation, to balancing a skeletal equation step by step, to classifying reactions into four broad types, and finally to oxidation-reduction and its everyday consequences — corrosion and rancidity. Every question woven through the chapter is solved here, section by section, in the order the textbook presents them: the in-text questions after Section 1.1, the in-text questions after Section 1.2.2, the embedded "Recall Activity" and "Do You Know" checks, the in-text questions after Section 1.2.5, and the full 20-question Exercise set at the end.
Word-equations, skeletal equations, the hit-and-trial balancing method, and state symbols — (s), (l), (g), (aq).
Combination, decomposition (thermal, photolytic, electrolytic), displacement, and double displacement/precipitation reactions.
Oxidation and reduction in terms of gain/loss of oxygen, plus everyday effects — rusting of iron and rancid food.
Magnesium metal reacts slowly with atmospheric oxygen to form a thin layer of magnesium oxide (MgO) on its surface. This oxide layer is unreactive and prevents the metal underneath from burning properly. Cleaning the ribbon with sandpaper removes this oxide coating, exposing fresh metal so that it burns readily and completely when heated.
(i) Hydrogen + Chlorine → Hydrogen chloride
(ii) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride
(iii) Sodium + Water → Sodium hydroxide + Hydrogen
(i) Solutions of barium chloride and sodium sulphate react to give insoluble barium sulphate and a solution of sodium chloride.
(ii) Sodium hydroxide solution reacts with hydrochloric acid solution to produce sodium chloride solution and water.
(i) 'X' is Calcium oxide (quick lime), formula CaO. (A solution of slaked lime, Ca(OH)2, made from it, is what is actually used for whitewashing.)
(ii) Reaction of 'X' with water:
In the electrolysis of water, water decomposes into hydrogen and oxygen in the mole ratio 2:1:
Since two molecules of hydrogen gas are produced for every one molecule of oxygen gas, the volume of hydrogen collected at the cathode is double the volume of oxygen collected at the anode. The gas present in double the amount is hydrogen.
(i) Colour of precipitate: bright yellow. Compound precipitated: Lead iodide (PbI2).
(ii) Balanced chemical equation:
(iii) Yes — this is also a double displacement reaction (and specifically a precipitation reaction), since the ions of the two reactant compounds are exchanged to form a new insoluble compound (PbI2) and a soluble one (KNO3).
Magnesium gains oxygen to form magnesium oxide (2Mg + O2 → 2MgO), so magnesium is being oxidised.
Yes. In respiration, glucose combines with oxygen in body cells and releases energy:
Since energy is given out, it is an exothermic reaction.
The bottom of the test tube feels cold — the mixture absorbs heat from the surroundings. This is an endothermic reaction.
Iron is more reactive than copper, so it displaces copper from copper sulphate solution, forming iron sulphate and depositing copper metal on the nail:
As blue copper sulphate is converted into pale green iron sulphate, the blue colour of the solution fades, and the iron nail gets a brownish (copper) coating.
(A white precipitate of silver chloride is formed, while sodium and nitrate ions are exchanged.)
(i) 4Na(s) + O2(g) → 2Na2O(s) — Na is oxidised (gains oxygen); O2 is reduced.
(ii) CuO(s) + H2(g) → Cu(s) + H2O(l) — CuO is reduced (loses oxygen); H2 is oxidised (gains oxygen).
Answer: (i) (a) and (b)
Statement (a) is imprecise/incorrect because it is lead oxide (not elemental lead) that undergoes reduction — statement (d) correctly says this. Statement (b) is incorrect because carbon dioxide is the product formed, not something that is itself getting oxidised — it is carbon that gets oxidised to carbon dioxide (statement c is correct).
Answer: (d) displacement reaction
Aluminium, being more reactive than iron, displaces iron from iron(III) oxide. (This particular reaction is also known as the thermite reaction and is a redox reaction.)
Answer: (a) Hydrogen gas and iron chloride are produced
A balanced chemical equation is one in which the number of atoms of each element is the same on both the reactant side (LHS) and the product side (RHS).
Equations must be balanced because of the Law of Conservation of Mass — mass can neither be created nor destroyed in a chemical reaction. Since atoms are neither created nor destroyed during a reaction, their number must remain equal before and after the reaction, i.e., the total mass of reactants must equal the total mass of products.
(a) Hydrogen gas combines with nitrogen to form ammonia.
(b) Hydrogen sulphide gas 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 metal reacts with water to give potassium hydroxide and hydrogen gas.
(a) HNO3 + Ca(OH)2 → Ca(NO3)2 + H2O
(b) NaOH + H2SO4 → Na2SO4 + H2O
(c) NaCl + AgNO3 → AgCl + NaNO3
(d) BaCl2 + H2SO4 → BaSO4 + HCl
(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
(a) Potassium bromide(aq) + Barium iodide(aq) → Potassium iodide(aq) + Barium bromide(s)
Type: Double displacement (precipitation) reaction
(b) Zinc carbonate(s) → Zinc oxide(s) + Carbon dioxide(g)
Type: Decomposition reaction (thermal decomposition)
(c) Hydrogen(g) + Chlorine(g) → Hydrogen chloride(g)
Type: Combination reaction
(d) Magnesium(s) + Hydrochloric acid(aq) → Magnesium chloride(aq) + Hydrogen(g)
Type: Displacement reaction
Exothermic reactions are reactions in which heat/energy is released along with the formation of products.
Example: CH4(g) + 2O2(g) → CO2(g) + 2H2O(g) + heat (burning of natural gas); respiration.
Endothermic reactions are reactions in which energy/heat is absorbed from the surroundings.
Example: CaCO3(s) →(heat) CaO(s) + CO2(g) (thermal decomposition of limestone); electrolysis of water.
During respiration, food (carbohydrates broken down to glucose) combines with oxygen inside the cells of the body, releasing energy that keeps us alive, along with carbon dioxide and water as by-products:
Since energy is released (given out) rather than absorbed, respiration is an exothermic process.
In a combination reaction, two or more simpler substances (reactants) combine to form a single, more complex product: A + B → AB.
In a decomposition reaction, a single compound breaks down into two or more simpler substances: AB → A + B.
Since one is the reverse process of the other, decomposition reactions are considered the opposite of combination reactions.
Combination: CaO(s) + H2O(l) → Ca(OH)2(aq)
Decomposition: CaCO3(s) →(heat) CaO(s) + CO2(g)
Heat (thermal decomposition):
Light (photolytic decomposition):
Electricity (electrolytic decomposition):
In a displacement reaction, a single, more reactive element displaces (removes) a less reactive element from its compound: A + BC → AC + B.
In a double displacement reaction, two compounds in solution exchange ions (radicals) with each other, usually forming an insoluble precipitate: AB + CD → AD + CB.
Copper, being more reactive than silver, displaces silver from silver nitrate solution.
A precipitation reaction is a reaction in which an insoluble solid (called a precipitate) is formed when two solutions of soluble salts react together in water, due to an exchange of ions.
In both examples, a white insoluble precipitate (barium sulphate / silver chloride) settles out of the solution.
(a) Oxidation — a substance is oxidised when it gains oxygen during a reaction.
(b) Reduction — a substance is reduced when it loses oxygen during a reaction.
Element 'X' is Copper (Cu). On heating in air it forms the black-coloured compound Copper(II) oxide (CuO).
Paint forms a protective coating that keeps moisture and oxygen (air) from coming into direct contact with the iron surface. Since rusting/corrosion of iron needs both moisture and air, painting prevents the iron from rusting and protects it from corrosion.
Nitrogen is an unreactive (inert) gas. Flushing food packets with nitrogen displaces the oxygen present, which prevents the oils and fats in the food from being oxidised. This stops the food from becoming rancid (developing a bad smell and taste) and helps it stay fresh for longer.
(a) Corrosion — the process by which a metal is slowly attacked/eaten away by substances around it such as moisture, acids, or gases in the air, forming a coating on its surface.
Example: Iron articles develop a reddish-brown coating of rust (hydrated iron(III) oxide) when exposed to moist air.
(b) Rancidity — the oxidation of fats and oils present in food when exposed to air over time, which changes their smell and taste.
Example: Ghee or chips left exposed to air for a long time develop an unpleasant smell and taste due to rancidity.
Next up is Chapter 2 — Acids, Bases and Salts. Or explore the full chapter list, browse the Class 10 Science hub, or book a free demo class for personalised coaching.
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