CBSE Class 10 Science
Chapter-wise Previous Year Questions & Solutions
Built from the original CBSE question papers — 2022 (Term 2), 2023, 2023 Compartment, 2024, 2024 Supplementary, 2025 Supplementary and 2026
These are the CBSE Class 10 Science previous year questions from the 2022 (Term 2), 2023, 2023 Compartment, 2024, 2024 Supplementary, 2025 Supplementary and 2026 board papers, sorted chapter by chapter. Every question carries its marks, its type and the years it appeared in, and a complete step-by-step solution follows it — including ray diagrams, circuit diagrams, balanced chemical equations and worked numericals.
Chapter 1 · ChemistryChemical Reactions and Equations
When 2 mL of sodium hydroxide solution is added to a few pieces of granulated zinc in a test tube and then warmed, the reaction that occurs can be written in the form of a balanced chemical equation as:
- (A) NaOH + Zn → NaZnO2 + H2O
- (B) 2NaOH + Zn → Na2ZnO2 + H2
- (C) 2NaOH + Zn → NaZnO2 + H2
- (D) 2NaOH + Zn → Na2ZnO2 + H2O
(B)
2NaOH + Zn → Na2ZnO2 + H2↑Zinc is amphoteric, so it reacts with a strong alkali to give sodium zincate and hydrogen gas. Check the balance: Na = 2 on both sides, O = 2 on both sides, H = 2 on both sides. Options (A) and (C) leave sodium unbalanced; (D) produces water instead of H2, which contradicts the observed effervescence.
To balance the following chemical equation, the values of the coefficients x and y respectively are:
x Pb(NO3)2 →Heat 2 PbO + y NO2 + O2- (A) 2, 4
- (B) 2, 2
- (C) 2, 3
- (D) 4, 2
A close variant in 2024 asked for x, y, z in: x Zn(NO3)2 → y ZnO + z NO2 + O2 — answer 2, 2, 4.
(A) x = 2, y = 4
2 Pb(NO3)2 →Heat 2 PbO + 4 NO2 + O2Balance nitrogen first: 2 × 2 = 4 N atoms on the left, so y = 4. Oxygen check — LHS: 2 × 6 = 12; RHS: 2 + (4×2) + 2 = 12. ✓
To balance the following chemical equation the values of x and y should respectively be:
2NaOH + x Al2O3 → y NaAlO2 + H2O- (A) 1, 4
- (B) 1, 2
- (C) 2, 4
- (D) 2, 3
(B) x = 1, y = 2
2NaOH + Al2O3 → 2NaAlO2 + H2ONa = 2 ✓, Al = 2 ✓, H = 2 ✓, O: LHS 2 + 3 = 5, RHS 4 + 1 = 5 ✓
Select from the following a decomposition reaction in which the source of energy for decomposition is light:
- (A) 2FeSO4 → Fe2O3 + SO2 + SO3
- (B) 2H2O → 2H2 + O2
- (C) 2AgBr → 2Ag + Br2
- (D) CaCO3 → CaO + CO2
(C)
2AgBr →Sunlight 2Ag + Br2This is photolytic decomposition — pale-yellow silver bromide turns grey as silver metal is set free. (A) and (D) are thermal decompositions; (B) is electrolytic decomposition.
Select from the following decomposition reactions in which the source of energy for decomposition is heat:
- (i) CaCO3 → CaO + CO2
- (ii) 2H2O → 2H2 + O2
- (iii) 2AgBr → 2Ag + Br2
- (iv) 2FeSO4 → Fe2O3 + SO2 + SO3
- (A) (i) and (ii)
- (B) (ii) and (iii)
- (C) (iii) and (iv)
- (D) (i) and (iv)
(D) (i) and (iv)
(i) Limestone on strong heating gives quick lime — thermal. (iv) Green ferrous sulphate crystals on heating give reddish-brown Fe2O3 — thermal. (ii) needs electricity, (iii) needs sunlight.
Which of the following is not a thermal decomposition reaction?
- (A) 2 FeSO4 → Fe2O3 + SO2 + SO3
- (B) CaCO3 → CaO + CO2
- (C) 2 AgCl → 2 Ag + Cl2
- (D) Pb(NO3)2 → 2 PbO + 4 NO2 + O2
(C)
Silver chloride decomposes in sunlight, not by heat — it is a photolytic (photochemical) decomposition used in black-and-white photography.
Select from the following a process in which a combination reaction is involved:
- (A) Black and white photography
- (B) Burning of coal (carbon)
- (C) Refining of copper
- (D) Digestion of food
(B)
C(s) + O2(g) → CO2(g) + HeatTwo reactants combine to give a single product — the definition of a combination reaction. Photography is decomposition, digestion is a series of decomposition (hydrolysis) reactions.
Careful: in the 2024 version of this question the options included Burning of methane as well. Burning of methane (CH4 + 2O2 → CO2 + 2H2O) gives two products, so it is combustion but not a combination reaction. Only burning of carbon qualifies.
In the following case(s) the combination reaction occurs in:
- I. CuO + H2
- II. ZnO + C
- III. Na + O2
- IV. CH4 + O2
- (A) Only III
- (B) Only IV
- (C) II and III
- (D) I, III and IV
(A) Only III
4Na + O2 → 2Na2OOnly III gives a single product. I and II are displacement/reduction reactions (CuO + H2 → Cu + H2O; ZnO + C → Zn + CO), and IV gives two products.
The above reaction is a:
- (A) Decomposition reaction
- (B) Displacement reaction
- (C) Double displacement reaction
- (D) Combination reaction
(B)
Zinc, being more reactive than hydrogen, displaces hydrogen from acetic acid. A single element replaces another element from a compound → displacement.
Consider the following chemical equations I and II:
I. Mg + 2HCl → MgCl2 + H2 II. NaOH + HCl → NaCl + H2OThe correct statement about these equations is:
- (A) 'I' is a displacement reaction and 'II' is a decomposition reaction.
- (B) 'I' is a displacement reaction and 'II' is a double displacement reaction.
- (C) Both 'I' and 'II' are displacement reactions.
- (D) Both 'I' and 'II' are double displacement reactions.
(B)
In I, magnesium displaces hydrogen → displacement. In II, Na+ and H+ exchange their partners (Cl− and OH−) → double displacement (here specifically a neutralisation reaction).
Which one of the following reactions is different from the remaining three?
- (A) NaCl + AgNO3 → AgCl + NaNO3
- (B) CaO + H2O → Ca(OH)2
- (C) KNO3 + H2SO4 → KHSO4 + HNO3
- (D) ZnCl2 + H2S → ZnS + 2HCl
(B)
(A), (C) and (D) are all double displacement reactions (exchange of ions). (B) is a combination reaction — two reactants give one product.
When aqueous solutions of barium chloride and sodium sulphate react together, an insoluble substance along with aqueous sodium chloride is formed. This reaction is an example of a:
- (A) combination reaction
- (B) decomposition reaction
- (C) displacement reaction
- (D) double displacement reaction
(D)
BaCl2(aq) + Na2SO4(aq) → BaSO4(s)↓ + 2NaCl(aq)Ions are exchanged and an insoluble white precipitate of barium sulphate separates out, so it is also called a precipitation reaction.
When aqueous solutions of potassium iodide and lead nitrate are mixed, an insoluble substance separates out. The chemical equation for the reaction involved is:
- (A) KI + PbNO3 → PbI + KNO3
- (B) 2KI + Pb(NO3)2 → PbI2 + 2KNO3
- (C) KI + Pb(NO3)2 → PbI + KNO3
- (D) KI + PbNO3 → PbI2 + KNO3
(B)
2KI(aq) + Pb(NO3)2(aq) → PbI2(s)↓ + 2KNO3(aq)Lead is divalent, so the correct formulae are Pb(NO3)2 and PbI2. The yellow precipitate is lead(II) iodide.
Consider the following cases:
- (a) CaSO4 + Al →
- (b) CuSO4 + Ca →
- (c) FeSO4 + Cu →
- (d) ZnSO4 + Mg →
The cases in which new products will form are:
- (A) (a) and (b)
- (B) (b) and (c)
- (C) (c) and (d)
- (D) (b) and (d)
(D) (b) and (d)
A displacement occurs only when the free metal is more reactive than the metal in the salt.
- (b) Ca is above Cu → reaction occurs: Ca + CuSO4 → CaSO4 + Cu
- (d) Mg is above Zn → reaction occurs: Mg + ZnSO4 → MgSO4 + Zn
- (a) Al is below Ca, (c) Cu is below Fe → no reaction
A metal ribbon 'X' burns in oxygen with a dazzling white flame forming a white ash 'Y'. The correct description of X, Y and the type of reaction is:
- (A) X = Ca ; Y = CaO ; Type of reaction = Decomposition
- (B) X = Mg ; Y = MgO ; Type of reaction = Combination
- (C) X = Al ; Y = Al2O3 ; Type of reaction = Thermal decomposition
- (D) X = Zn ; Y = ZnO ; Type of reaction = Endothermic
(B)
2Mg(s) + O2(g) →Burn 2MgO(s)The dazzling white light is the signature of burning magnesium ribbon; the white ash is magnesium oxide. It is a combination reaction and also exothermic.
Which of the following is a redox reaction, but not a combination reaction?
- (A) C + O2 → CO2
- (B) 2H2 + O2 → 2H2O
- (C) 2Mg + O2 → 2MgO
- (D) Fe2O3 + 3CO → 2Fe + 3CO2
(D)
In (D), Fe2O3 loses oxygen (reduced) while CO gains oxygen (oxidised) — a redox reaction. But it gives two products, so it is not a combination reaction. (A), (B) and (C) are redox and combination reactions.
The reaction given above is a redox reaction because in this case:
- (A) MnO2 is oxidised and HCl is reduced.
- (B) HCl is oxidised.
- (C) MnO2 is reduced.
- (D) MnO2 is reduced and HCl is oxidised.
(D)
MnO2 loses oxygen → it is reduced to MnCl2 (so MnO2 is the oxidising agent). HCl loses hydrogen → it is oxidised to Cl2 (so HCl is the reducing agent). Both happen simultaneously, which is exactly what makes it a redox reaction.
Identify the correct statement about the following reaction:
2H2S + SO2 → 2H2O + 3S- (A) H2S is the oxidising agent and SO2 is the reducing agent.
- (B) H2S is reduced to sulphur.
- (C) SO2 is the oxidising agent and H2S is the reducing agent.
- (D) SO2 is oxidised to sulphur.
(C)
H2S loses hydrogen → oxidised → it is the reducing agent. SO2 loses oxygen → reduced → it is the oxidising agent.
A chemical reaction in which exchange of ions occurs between the reactants, is known as:
- (A) Endothermic Reaction
- (B) Exothermic Reaction
- (C) Double Displacement Reaction
- (D) Displacement Reaction
(C) Double displacement reaction. The general form is AB + CD → AD + CB.
Solid calcium oxide reacts vigorously with water to form calcium hydroxide accompanied by the liberation of heat. From the information given above it may be concluded that this reaction:
- (A) is endothermic and pH of the solution formed is more than 7.
- (B) is exothermic and pH of the solution formed is 7.
- (C) is endothermic and pH of the solution formed is 7.
- (D) is exothermic and pH of the solution formed is more than 7.
(D)
CaO(s) + H2O(l) → Ca(OH)2(aq) + HeatHeat is released → exothermic. The product, calcium hydroxide, is a base, so pH > 7.
Which of the following is an example of an endothermic process/reaction?
- (A) Formation of slaked lime / Burning of coal
- (B) Decomposition of vegetable matter into compost
- (C) Dissolution of ammonium chloride in water
- (D) Digestion of food in our body / Process of respiration
A 2024 variant of the same question replaced option (C) with “Decomposition of calcium carbonate to form quick lime and carbon dioxide” — that then becomes the answer.
(C) Dissolution of ammonium chloride in water.
NH4Cl absorbs heat from the surroundings as it dissolves, so the test tube feels cold. Formation of slaked lime, burning of coal, composting and respiration are all exothermic.
In the 2024 variant, thermal decomposition of limestone absorbs heat continuously:
CaCO3 →Heat CaO + CO2 (endothermic)Select exothermic processes from the following:
- (i) Dilution of acid
- (ii) Burning of natural gas
- (iii) Evaporation of water
- (iv) Electrolysis of water
- (A) (i) and (ii)
- (B) (ii) and (iii)
- (C) (iii) and (iv)
- (D) (i) and (iv)
(A) (i) and (ii)
Dilution of a concentrated acid releases a large amount of heat, and combustion of methane is strongly exothermic. Evaporation absorbs latent heat and electrolysis absorbs electrical energy — both endothermic.
When lead nitrate powder is heated strongly in a boiling tube, three substances are produced. These are:
- (A) PbO, NO and O2
- (B) PbO2, NO and NO2
- (C) PbO, NO2 and O2
- (D) PbO2, NO2 and O2
The 2026 set asked the same idea as: “On heating lead nitrate crystals, one would get — (A) yellow fumes (B) rotten egg smell (C) burning smell of sulphur (D) brown fumes” → answer (D) brown fumes.
(C)
2Pb(NO3)2(s) →Heat 2PbO(s) + 4NO2(g) + O2(g)Yellow lead monoxide residue, brown fumes of nitrogen dioxide, and colourless oxygen gas.
The emission of brown fumes in the given experimental set-up is due to:
- (A) thermal decomposition of lead nitrate which produces brown fumes of nitrogen dioxide.
- (B) thermal decomposition of lead nitrate which produces brown fumes of lead oxide.
- (C) oxidation of lead nitrate forming lead oxide and nitrogen dioxide.
- (D) oxidation of lead nitrate forming lead oxide and oxygen.
(A)
Heat breaks a single compound into three simpler ones, so it is thermal decomposition, and the brown colour belongs to NO2 gas (lead oxide is a yellow solid, not a fume).
On heating ferrous sulphate crystals, one would get:
- (A) Sweet smell
- (B) Rotten egg smell
- (C) Characteristic smell of burning sulphur
- (D) No smell
(C)
2FeSO4·7H2O →Heat 2FeSO4 + 14H2O 2FeSO4 →Heat Fe2O3 + SO2 + SO3SO2 has the choking, characteristic smell of burning sulphur.
Which of the following compounds is formed on heating limestone with the release of CO2 gas?
- (A) Ca(OH)2
- (B) CaCO3
- (C) CaO
- (D) CaCl2
(C) Calcium oxide (quick lime).
CaCO3(s) →Heat CaO(s) + CO2(g)Assertion (A): Reaction of quicklime with water is an exothermic reaction.
Reason (R): Quicklime reacts vigorously with water releasing a large amount of heat.
(A) Both A and R are true and R is the correct explanation of A.
CaO(s) + H2O(l) → Ca(OH)2(aq) + HeatAssertion (A): MnO2 + 4HCl → MnCl2 + 2H2O + Cl2 is a redox reaction.
Reason (R): In this reaction, HCl is oxidised to Cl2 and MnO2 is reduced to MnCl2.
(A) Both A and R are true and R correctly explains A.
Oxidation and reduction occur together in the same equation, so it is a redox reaction, and R states exactly which species undergoes which change.
Assertion (A): It is advised that while diluting an acid one should add water to acid and not acid to water, keeping the solution continuously stirred.
Reason (R): The process of dissolving an acid into water is highly exothermic.
(D) A is false, but R is true.
The correct rule is the opposite: acid must be added slowly to water with constant stirring. Since dilution releases a lot of heat, adding water to concentrated acid can make the mixture splash out and cause burns.
Assertion (A): Decomposition of vegetable matter into compost is an exothermic reaction.
Reason (R): Decomposition reactions need energy to break down the reactants.
(B) Both A and R are true, but R is not the correct explanation of A.
Composting really does release heat (that is why a compost heap feels warm), and it is true that most decomposition reactions need an input of energy. But the reason does not explain why composting is exothermic — composting is a slow biological oxidation that gives out more energy than it takes in.
Assertion (A): Burning of natural gas is an endothermic reaction.
Reason (R): Methane gas reacts with oxygen to produce CO2 and H2O.
(D) A is false, but R is true.
CH4(g) + 2O2(g) → CO2(g) + 2H2O(g) + HeatCombustion always releases heat, so burning of natural gas is exothermic, not endothermic. The products named in R are correct.
Name the type of chemical reaction in which calcium oxide reacts with water. Justify your answer by giving the balanced chemical equation for the reaction.
It is a combination reaction (and also an exothermic reaction).
CaO(s) + H2O(l) → Ca(OH)2(aq) + HeatJustification: two reactants — calcium oxide and water — combine to form a single product, calcium hydroxide (slaked lime). A large amount of heat is evolved, which is why the container becomes hot.
Translate the following statement into a balanced chemical equation:
“When barium chloride reacts with aluminium sulphate, aluminium chloride and barium sulphate are formed.”
State the type of this reaction, giving a reason to justify your answer.
Type: Double displacement reaction (specifically a precipitation reaction).
Reason: Ba2+ and Al3+ exchange their partner ions (Cl− and SO42−), and an insoluble white precipitate of barium sulphate separates out of the solution.
When a few drops of barium chloride solution are added to an aqueous solution of sodium sulphate, a white precipitate is obtained.
- (a) Write the balanced chemical equation for the reaction involved.
- (b) What is the other name of this precipitation reaction? Why is it called so?
(a)
BaCl2(aq) + Na2SO4(aq) → BaSO4(s)↓ + 2NaCl(aq)(b) It is also called a double displacement reaction, because the two reactants exchange their ions — Ba2+ pairs with SO42− and Na+ pairs with Cl− — producing two new compounds.
When magnesium ribbon is burnt in air, an ash of white colour is produced. Write the chemical equation for the reaction, giving the chemical name of the ash produced. State the type of chemical reaction, giving justification for your answer.
White ash: magnesium oxide (MgO).
Type: Combination reaction — also an oxidation and an exothermic reaction.
Justification: Two substances, magnesium and oxygen, combine to form a single product. Magnesium gains oxygen, so it is oxidised.
Translate the following statements into balanced chemical equations:
- (a) Aluminium reacts with copper chloride to form aluminium chloride and copper.
- (b) Zinc reacts with sodium hydroxide to give sodium zincate and hydrogen gas.
(a)
2Al(s) + 3CuCl2(aq) → 2AlCl3(aq) + 3Cu(s)(b)
Zn(s) + 2NaOH(aq) → Na2ZnO2(aq) + H2(g)↑Write balanced chemical equations for the reactions taking place when:
- (i) Aluminium metal reacts with steam.
- (ii) Magnesium reacts with dilute HCl.
(i)
2Al(s) + 3H2O(g) → Al2O3(s) + 3H2(g)(ii)
Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)↑State whether the given chemical reaction is a redox reaction or not. Justify your answer.
MnO2 + 4HCl → MnCl2 + 2H2O + Cl2Yes, it is a redox reaction.
Justification:
- MnO2 loses oxygen → it is reduced to MnCl2. MnO2 therefore acts as the oxidising agent.
- HCl loses hydrogen (equivalently, Cl− loses electrons) → it is oxidised to Cl2. HCl therefore acts as the reducing agent.
Since oxidation and reduction take place simultaneously in the same reaction, it is a redox reaction.
“No precipitation reaction can occur without exchange of ions between the two reactants.” Justify this statement, giving a balanced chemical equation for the reaction.
Giving one example of each, differentiate between a displacement reaction and a double displacement reaction.
Option (A). A precipitate forms only when the ions already present in the two solutions regroup into a new combination that happens to be insoluble. That regrouping is an exchange of ions, so a precipitation reaction is necessarily a double displacement reaction.
Na2SO4(aq) + BaCl2(aq) → BaSO4(s)↓ + 2NaCl(aq)Here Ba2+ leaves Cl− and joins SO42−; without this exchange the insoluble BaSO4 could never appear.
Option (B).
| Displacement reaction | Double displacement reaction |
|---|---|
| A more reactive element displaces a less reactive element from its compound. | Two compounds exchange their ions to form two new compounds. |
| Form: A + BC → AC + B | Form: AB + CD → AD + CB |
| Only one element is set free. | No element is set free; usually a precipitate or gas or water forms. |
| Fe + CuSO4 → FeSO4 + Cu | Na2SO4 + BaCl2 → BaSO4↓ + 2NaCl |
Define a decomposition reaction. Write an equation to show the thermal decomposition of ferrous sulphate crystals.
What is meant by a balanced chemical equation? Why is it necessary for the equation to be balanced?
Option (A). A decomposition reaction is a reaction in which a single compound breaks down into two or more simpler substances on supplying energy in the form of heat, light or electricity.
2FeSO4·7H2O(s) →Heat 2FeSO4(s) + 14H2O(g) 2FeSO4(s) →Heat Fe2O3(s) + SO2(g) + SO3(g)The green crystals first lose their water of crystallisation and finally leave a reddish-brown residue of ferric oxide.
Option (B). A balanced chemical equation is one in which the number of atoms of each element is the same on both sides of the equation.
Why it is necessary: because of the law of conservation of mass — mass can neither be created nor destroyed in a chemical reaction. The total mass of reactants must equal the total mass of products, which is possible only if every atom is accounted for on both sides. A balanced equation also tells us the exact mole ratio in which substances react.
- (a) Write a chemical equation for the reaction in which a change in colour is observed when a metal is kept immersed in a salt solution of another metal.
- (b) When hydrogen gas is passed over heated copper(II) oxide, copper and steam are formed. Write the balanced chemical equation with physical states for this reaction. State what kind of chemical reaction this is.
(a) An iron nail dipped in blue copper sulphate solution turns brownish, and the solution fades to pale green:
Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s) (blue) → (pale green) (reddish-brown deposit)(b)
CuO(s) + H2(g) →Heat Cu(s) + H2O(g)This is a redox (oxidation–reduction) reaction: CuO loses oxygen and is reduced to copper, while H2 gains oxygen and is oxidised to water. It may also be described as a displacement reaction.
- (i) Define a decomposition reaction. Write the chemical equation for the reaction that occurs when lead nitrate is heated strongly in a boiling tube.
- (ii) In the electrolytic decomposition of water, two gases are liberated at the electrodes. Give the mass ratio of the gas liberated at the cathode and at the anode.
(i) A decomposition reaction is one in which a single compound breaks down into two or more simpler substances using heat, light or electricity.
2Pb(NO3)2(s) →Heat 2PbO(s) + 4NO2(g) + O2(g)(ii) Hydrogen is collected at the cathode, oxygen at the anode. The volume ratio is 2 : 1, so the mass ratio is:
mass H2 : mass O2 = (2 × 2) : (1 × 32) = 4 : 32 = 1 : 8It is observed that calcium on reaction with water floats on its surface. Explain why it happens. Also write a balanced chemical equation for the reaction that occurs. What happens when the aqueous solution of the product of this reaction reacts with carbon dioxide gas? Write a balanced chemical equation for the reaction.
Why calcium floats: hydrogen gas is produced during the reaction and the bubbles of this gas stick to the surface of the calcium metal. The bubbles make the effective density of the piece less than that of water, so the metal floats.
Ca(s) + 2H2O(l) → Ca(OH)2(aq) + H2(g)↑With carbon dioxide: the calcium hydroxide solution (lime water) turns milky because insoluble calcium carbonate is formed.
Ca(OH)2(aq) + CO2(g) → CaCO3(s)↓ + H2O(l)On passing excess CO2, the milkiness disappears as soluble calcium hydrogencarbonate is formed: CaCO3 + H2O + CO2 → Ca(HCO3)2.
A small amount of copper oxide was taken in a beaker and dilute hydrochloric acid was added with continuous stirring of the solution. Name the compound formed and state the colour of its solution. Write the balanced chemical equation for the reaction that occurs. Based on the reaction, state the nature (acidic/basic) of copper oxide.
Compound formed: copper(II) chloride, CuCl2.
Colour of solution: the black copper oxide dissolves and the solution turns blue-green.
Nature of CuO: basic — it is a metal oxide that neutralises an acid to give a salt and water.
Answer the following questions in the context of the electrolysis of water:
- (a) Why is this reaction/process called a decomposition reaction?
- (b) Giving a reason, state whether this reaction is exothermic or endothermic.
- (c) Name the gases collected at the anode and cathode.
- (d) What is the mass ratio of the gases collected at the anode and cathode?
(a) A single compound, water, breaks down into two simpler substances, hydrogen and oxygen — that is precisely a decomposition reaction (here, electrolytic decomposition).
(b) Endothermic, because electrical energy has to be supplied continuously to break the O–H bonds; the moment the current is switched off, the reaction stops.
(c) Anode → oxygen; Cathode → hydrogen.
(d) Volume ratio O2 : H2 = 1 : 2, so
mass O2 : mass H2 = 32 : 4 = 8 : 1- (i) While electrolysing water, before passing the current some drops of an acid are added. Why? Name the gases liberated at the cathode and anode. Write the relationship between the volume of gas collected at the anode and the volume of gas collected at the cathode.
- (ii) What is observed when silver chloride is exposed to sunlight? Give the type of reaction involved.
(i) Pure water is a very poor conductor of electricity. A few drops of dilute sulphuric acid supply free ions and make the water conducting, so electrolysis can proceed.
- Cathode → hydrogen gas
- Anode → oxygen gas
i.e. the volume of gas collected at the anode is 12 of that collected at the cathode.
(ii) White silver chloride slowly turns grey in sunlight, because it decomposes into silver metal and chlorine gas.
2AgCl(s) →Sunlight 2Ag(s) + Cl2(g)Type: photolytic (photochemical) decomposition reaction.
Silver chloride kept in a china dish turns grey in sunlight.
- (a) Write the colour of silver chloride when it was kept in the china dish.
- (b) Name the type of chemical reaction taking place and write the chemical equation for the reaction.
- (c) State one use of the reaction. Name one more chemical which can be used for the same purpose.
(a) White.
(b) Photolytic decomposition (decomposition by light).
2AgCl(s) →Sunlight 2Ag(s) + Cl2(g)(c) Use: in black-and-white photography. Another chemical used for the same purpose is silver bromide (AgBr).
Write down the balanced chemical equations for the following reactions and identify the type of reaction in each case.
- (a) Nitrogen gas is treated with hydrogen gas to form ammonia gas.
- (b) Lead nitrate is heated strongly to form lead monoxide, nitrogen dioxide and oxygen.
- (c) A copper wire is dipped in silver nitrate solution and a shining deposit of silver is produced.
(a) Combination reaction
N2(g) + 3H2(g) → 2NH3(g)(b) Thermal decomposition reaction
2Pb(NO3)2(s) →Heat 2PbO(s) + 4NO2(g) + O2(g)(c) Displacement reaction
Cu(s) + 2AgNO3(aq) → Cu(NO3)2(aq) + 2Ag(s)The colourless silver nitrate solution turns blue and shining silver is deposited on the copper wire.
- (a) Identify the reducing agent in the following reactions:
- (i) 4NH3 + 5O2 → 4NO + 6H2O
- (ii) H2O + F2 → HF + HOF
- (iii) Fe2O3 + 3CO → 2Fe + 3CO2
- (iv) 2H2 + O2 → 2H2O
- (b) Define a redox reaction in terms of gain or loss of oxygen.
(a) The reducing agent is the substance that is itself oxidised (gains oxygen / loses hydrogen).
- (i) NH3 — it gains oxygen to form NO.
- (ii) H2O — it loses hydrogen to F2.
- (iii) CO — it gains oxygen to form CO2.
- (iv) H2 — it gains oxygen to form H2O.
(b) A redox reaction is a reaction in which one reactant gains oxygen (is oxidised) while, in the same reaction, another reactant loses oxygen (is reduced). Oxidation and reduction always occur together.
Study the experimental set-up shown in the diagram and write the chemical equation for the chemical reaction involved. Name and define the type of reaction. List two other metals which can be used in place of iron to show the same type of reaction with copper sulphate solution.
Type of reaction: Displacement reaction.
Definition: A reaction in which a more reactive element displaces a less reactive element from its compound.
Two other metals: zinc (Zn) and magnesium (Mg) — both lie above copper in the reactivity series. (Aluminium also works.)
Observe the given diagram and answer the following questions:
- (a) Identify the type of reaction taking place and mention why the beaker becomes hot. Write a balanced chemical equation for the reaction taking place in the beaker.
- (b) Name the two types of reactions in which the above reaction can be placed, giving justification for each.
(a) It is an exothermic combination reaction. The beaker becomes hot because the reaction between quick lime and water releases a large amount of heat energy to the surroundings.
(b) The reaction belongs to two categories:
- Combination reaction — two reactants (CaO and H2O) unite to form a single product, Ca(OH)2.
- Exothermic reaction — heat is given out along with the product, so the temperature of the mixture and the beaker rises.
The product, slaked lime, is used for whitewashing walls: Ca(OH)2 + CO2 → CaCO3 + H2O gives the shiny white finish.
A reddish-brown metal used in electrical wires, when powdered and heated strongly, turns black. When hydrogen gas is passed over this black substance, it regains its original colour. Based on this information answer the following:
- (a) Name the metal and the black substance formed.
- (b) Write balanced chemical equations for the two reactions involved.
(a) The metal is copper (Cu); the black substance is copper(II) oxide, CuO.
(b)
2Cu(s) + O2(g) →Heat 2CuO(s) (oxidation / combination) CuO(s) + H2(g) →Heat Cu(s) + H2O(g) (reduction)State the change in colour observed in each of the following cases, mentioning the reason:
- (a) Silver chloride is exposed to sunlight.
- (b) A piece of zinc is dipped in ferrous sulphate solution.
- (c) Copper powder is strongly heated in air.
(a) White → grey. Sunlight decomposes AgCl and grey silver metal is set free.
2AgCl →Sunlight 2Ag + Cl2(b) Light green → colourless (and a greyish deposit appears on the zinc). Zinc is more reactive than iron, so it displaces iron from ferrous sulphate.
Zn(s) + FeSO4(aq) → ZnSO4(aq) + Fe(s)(c) Reddish-brown → black. Copper is oxidised by atmospheric oxygen to black copper(II) oxide.
2Cu(s) + O2(g) →Heat 2CuO(s)- (a) Define a double displacement reaction.
- (b) Write the chemical equation of a double displacement reaction in which a precipitate is formed, and identify the precipitate.
(a) A double displacement reaction is a reaction in which two compounds exchange their ions (radicals) with each other to form two entirely new compounds. General form:
AB + CD → AD + CB(b)
Pb(NO3)2(aq) + 2KI(aq) → PbI2(s)↓ + 2KNO3(aq)The yellow precipitate is lead(II) iodide, PbI2.
Define a displacement reaction. Name a displacement reaction which is highly exothermic and has its use in joining railway tracks. Explain the process with a balanced chemical equation of the reaction that occurs.
Displacement reaction: a reaction in which a more reactive element displaces a less reactive element from its compound. General form: A + BC → AC + B.
Name: the Thermit reaction.
Fe2O3(s) + 2Al(s) → 2Fe(l) + Al2O3(s) + HeatProcess: Aluminium is more reactive than iron, so it displaces iron from ferric oxide. So much heat is liberated that the iron produced is in the molten state. This white-hot molten iron is poured straight into the gap between the two rails; on cooling it solidifies and welds the broken railway tracks (or cracked machine parts) together.
What is a chemical reaction? Describe one activity each to show that a chemical change has occurred in which (i) change of colour, and (ii) change in temperature has taken place.
- (i) Define a decomposition reaction. How can we say that (I) electrolysis of water, and (II) blackening of silver bromide when exposed to sunlight, are decomposition reactions? Mention the type of energy involved in each case.
- (ii) The types of reactions in which (I) calcium oxide is formed, and (II) calcium hydroxide is formed are opposite reactions to each other. Justify this statement with the help of balanced chemical equations.
Option (A). A chemical reaction is a process in which one or more substances (reactants) are converted into one or more entirely new substances (products) having different properties and composition.
(i) Activity showing change of colour — Take copper sulphate solution in a test tube and dip a clean iron nail in it. After about 20 minutes the blue colour of the solution fades to pale green and a reddish-brown layer of copper deposits on the nail.
Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s)(ii) Activity showing change in temperature — Take a small amount of quick lime in a beaker and add water slowly. Touch the beaker: it becomes distinctly hot, showing that heat has been released.
CaO(s) + H2O(l) → Ca(OH)2(aq) + HeatOption (B) (i). A decomposition reaction is a reaction in which a single compound splits into two or more simpler substances on absorbing energy.
(I) Electrolysis of water — one compound (water) breaks into two simpler substances (hydrogen and oxygen), so it is a decomposition. Energy involved: electrical energy.
2H2O(l) →Electricity 2H2(g) + O2(g)(II) Blackening of silver bromide — pale-yellow AgBr breaks into silver and bromine, so it is a decomposition. Energy involved: light (solar) energy.
2AgBr(s) →Sunlight 2Ag(s) + Br2(g)(ii) Calcium oxide is formed by decomposition, calcium hydroxide by combination — the two are exactly opposite processes.
(I) CaCO3(s) →Heat CaO(s) + CO2(g) decomposition, endothermic (II) CaO(s) + H2O(l) → Ca(OH)2(aq) + Heat combination, exothermicIn the first a single compound splits up and heat is absorbed; in the second two substances join into one and heat is released. Hence they are opposite in both respects.
When lead nitrate is heated strongly in a boiling tube, two gases are liberated and a solid residue is left behind in the test tube.
- (i) Name the type of chemical reaction and define it.
- (ii) Write the name and formula of the coloured gas liberated.
- (iii) Write the balanced chemical equation for the reaction.
- (iv) Name the residue left in the test tube and state the method of testing its nature (acidic/basic).
- (i) Write a balanced chemical equation for the following word equation:
Lead nitrate + Potassium iodide → Lead iodide + Potassium nitrate.
Is this a double displacement reaction? Justify. - (ii) What is observed and what type of reaction occurs when zinc granules are added to dilute sulphuric acid? Write the equation.
Option (A).
(i) Thermal decomposition reaction — a reaction in which a single compound breaks down into two or more simpler substances on heating.
(ii) The coloured gas is nitrogen dioxide, NO2 — it appears as dense brown fumes. (The other gas, oxygen, is colourless.)
(iii)
2Pb(NO3)2(s) →Heat 2PbO(s) + 4NO2(g) + O2(g)(iv) The residue is lead monoxide (PbO), a yellow solid. To test its nature, shake a little of the residue with water and test the suspension with red litmus paper — it turns blue, showing that PbO is basic (being a metal oxide). Alternatively, it dissolves in dilute HNO3 to give a salt and water.
Option (B) (i).
Pb(NO3)2(aq) + 2KI(aq) → PbI2(s)↓ + 2KNO3(aq)Yes, it is a double displacement reaction: Pb2+ and K+ exchange their partner ions (I− and NO3−), giving two new compounds. Since a yellow insoluble solid separates out, it is also a precipitation reaction.
(ii) Brisk effervescence is seen and the test tube becomes warm; colourless hydrogen gas is evolved, which burns with a pop sound when a burning splinter is brought near it. It is a displacement reaction (and exothermic).
Zn(s) + H2SO4(dil.) → ZnSO4(aq) + H2(g)↑Write chemical equations for the following:
- (a) Silver bromide is exposed to sunlight.
- (b) A piece of lead metal is dropped into copper(II) chloride solution.
- (c) Hydrogen gas is passed over heated copper oxide.
- (d) Methane gas is burnt in air.
- (e) Quick lime is mixed with water.
- (a) Study the experimental set-up shown in the diagram (lead nitrate heated in a boiling tube held with tongs) and write the chemical equation for the same. Name and define the type of reaction. Mention the colour of the products formed.
- (b) What type of reaction takes place when aqueous solutions of lead(II) nitrate and potassium iodide are mixed together? Write the chemical equation also.
Option (A).
(a) 2AgBr(s) →Sunlight 2Ag(s) + Br2(g) (b) Pb(s) + CuCl2(aq) → PbCl2(aq) + Cu(s) (c) CuO(s) + H2(g) →Heat Cu(s) + H2O(g) (d) CH4(g) + 2O2(g) → CO2(g) + 2H2O(g) + Heat (e) CaO(s) + H2O(l) → Ca(OH)2(aq) + HeatOption (B) (a).
2Pb(NO3)2(s) →Heat 2PbO(s) + 4NO2(g) + O2(g)Type: Thermal decomposition reaction — a reaction in which a single compound breaks down into two or more simpler substances on heating.
Colours of products: lead monoxide is a yellow solid residue, nitrogen dioxide comes off as reddish-brown fumes, and oxygen is colourless.
(b) A double displacement (precipitation) reaction takes place; a yellow precipitate of lead iodide appears.
Pb(NO3)2(aq) + 2KI(aq) → PbI2(s)↓ + 2KNO3(aq)Three metal samples of magnesium, aluminium and iron were taken and rubbed with sand paper. These samples were then put separately in test tubes containing dilute hydrochloric acid. Thermometers were also suspended in each test tube so that their bulbs dipped in the acid. The rate of formation of bubbles was observed. The activity was repeated with dilute nitric acid and the observations were recorded.
Answer the following questions:
- (a) When the activity was done with dilute hydrochloric acid, in which test tube was the rate of formation of bubbles the fastest, and why?
- (b) Why were the metal samples rubbed with sand paper before the activity?
- (c) What change was observed in the reading of the thermometers? What does it indicate?
- (d) Why is hydrogen gas generally not evolved when metals react with dilute nitric acid?
(a) The bubbles formed fastest in the test tube containing magnesium, because magnesium is the most reactive of the three metals (Mg > Al > Fe in the reactivity series), so it displaces hydrogen from the acid most vigorously.
Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)↑(b) To remove the oxide layer and dirt from the metal surfaces. A fresh, clean surface is exposed so the acid can react directly with the metal.
(c) The thermometer readings rose. This indicates that the reactions between the metals and the acid are exothermic — heat is released to the surroundings.
(d) Because dilute nitric acid is a strong oxidising agent. It oxidises the hydrogen produced into water, and itself gets reduced to oxides of nitrogen (N2O, NO or NO2). Hence hydrogen gas is normally not seen. (Exceptions: magnesium and manganese do give H2 with very dilute HNO3.)
Chapter 2 · ChemistryAcids, Bases and Salts
A few drops of turmeric solution are added to a colourless liquid. If the liquid becomes red, the liquid may be:
- (A) Hydrochloric acid
- (B) Distilled water
- (C) Ammonium hydroxide
- (D) Lemon juice
(C) Ammonium hydroxide
Turmeric is a natural indicator that stays yellow in acids and neutral liquids but turns reddish-brown in a base. Of the four, only NH4OH is basic. (This is why a turmeric stain on a shirt turns red when soap is rubbed on it.)
Match the natural source with the acid present in it. Which is correct?
| Source | Acid |
|---|---|
| Nettle sting | Methanoic acid |
| Tamarind | Tartaric acid |
| Tomato | Oxalic acid |
| Ant sting | Methanoic (formic) acid |
Asked in several forms: “The acid present in nettle sting is…”, “Juice of tamarind turns blue litmus red because of…”, “Acid present in tomato is…”
Nettle sting → methanoic acid; Tamarind →