CBSE Class 10 Science HOTs Chemical Reactions and Equations Set 05

This is Set 05 of higher-order thinking (HOTS) questions for CBSE Class 10 Science Chapter 1, Chemical Reactions and Equations, with step-by-step answers and a Teacher's Note for each question. The questions test reactivity of metals, correcting wrong equations and conclusions, identifying unknown substances from observations and the meaning of oxidising agents.

Very Short Answer Type HOTS Questions

Question 1. The following chemical equation does not represent a chemical reaction that can take place. 3Fe(s) + 4H₂O(l) → Fe₃O₄(s) State what needs to be changed in the equation above for it to represent the correct reaction between Fe and H₂O.
Answer:
Step 1: Count atoms of hydrogen. The left side has 8 H atoms but the right side has none, so the equation is not balanced. Hydrogen gas must be a product.
Step 2: Iron does not react with cold or hot liquid water. It reacts only with steam, so water must be shown as (g).
Correct equation: 3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g) (Fe: 3 = 3, O: 4 = 4, H: 8 = 8).
In simple words: Add the missing product H₂ and write steam instead of liquid water.

Teacher's Note:
1. Check every element on both sides before accepting an equation.
2. Remember that the physical state of water matters in this reaction.

Question 2. X, Y and Z are three elements which undergo chemical reactions in the following way:
X₂O₃ + 2Y → Y₂O₃ + 2X
3ZSO₄ + 2Y → Y₂(SO₄)₃ + 3Z
3ZO + 2X → X₂O₃ + 3Z
Based on the above chemical reactions, answer the following questions:
(A) Which element is most reactive out of X, Y and Z?
(B) What is the least reactive element in X, Y and Z?
Answer:
Reaction 1: Y displaces X from X₂O₃, so Y is more reactive than X.
Reaction 2: Y displaces Z from ZSO₄, so Y is more reactive than Z.
Reaction 3: X displaces Z from ZO, so X is more reactive than Z.
(a) The most reactive element is Y, because it displaces both X and Z.
(b) The least reactive element is Z, because it is displaced by both X and Y.
Order of reactivity: Y > X > Z.
In simple words: The element that pushes out the other two is the most reactive; the one that gets pushed out by both is the least reactive.

Teacher's Note:
1. A more reactive element displaces a less reactive one from its compound.
2. Write the three comparisons one by one and then join them into a single order.

Question 3. Rani was not able to understand how to find out which substance is reduced and which one is oxidised in a chemical reaction. Her friend Seema told her that in a chemical reaction, an oxidising agent always gets reduced. Do you agree with this explanation? Support Seema's view by justifying her statement and provide an explanation by giving an example for the same.
Answer:
Yes, Seema is right. An oxidising agent oxidises another substance (by supplying oxygen or removing hydrogen from it) and in doing so it is itself reduced.
Example: CuO + H₂ → Cu + H₂O
Hydrogen gains oxygen, so H₂ is oxidised (it is the reducing agent). CuO loses oxygen, so CuO is reduced; it is the oxidising agent because it oxidised hydrogen.
Thus the oxidising agent (CuO) is the substance that gets reduced.
In simple words: The substance that oxidises another is itself reduced, and the substance that reduces another is itself oxidised.

Teacher's Note:
1. Oxidation and reduction always occur together in a redox reaction.
2. Identify the agent by asking which substance does the job on the other.

Short Answer Type HOTS Questions

Question 4. A student poured 100 mL of water in a bottle and added 40 mL vinegar to it. A balloon was filled with 20 g baking soda and was fixed at the mouth of the bottle. Slowly, the shape of the balloon changed, as shown. The student claims that a chemical change happened when the two substances were mixed. Is the claim made by the student correct?
(a) Yes, as a new substance was formed in the form of a gas.
(b) Yes, as the mass remains the same throughout the experiment.
(c) No, as the formation of bubbles in the mixture shows a physical change.
(d) No, as the change in the shape and size of the balloon shows a physical change.
Answer:
Correct option: (a). The claim is correct because a new substance, carbon dioxide gas, is formed and it inflates the balloon.
NaHCO₃ + CH₃COOH → CH₃COONa + H₂O + CO₂
Option (b) is wrong because the total mass stays the same in physical changes too. Options (c) and (d) are wrong because bubbles of a new gas show a chemical change.
In simple words: The gas formed in the bottle fills the balloon, so the change is chemical.

Teacher's Note:
1. Look for a new substance (gas, precipitate, colour, temperature change).
2. Constant mass cannot prove a chemical change.

Question 5. Tanya added magnesium into a test tube containing dil. HCl acid. She saw some gas coming out of it. She took a burning match stick near the mouth of the test tube and heard a popping sound while the match stick extinguished. Tanya concluded that the gas evolved is hydrogen and it is not combustible. Find the error in her conclusion and support your answer with reason.
Answer:
Reaction: Mg + 2HCl → MgCl₂ + H₂
The identification of the gas as hydrogen is correct, because hydrogen burns with a characteristic pop sound.
The error: the pop sound itself is hydrogen burning in air, so hydrogen is combustible. What it does not do is support burning, which is why the matchstick goes out.
Correct conclusion: hydrogen is a combustible gas, but it is not a supporter of combustion.
In simple words: Hydrogen burns (combustible) but it cannot keep another flame burning (not a supporter of combustion).

Teacher's Note:
1. Do not confuse 'combustible' with 'supporter of combustion'.
2. The popping sound is the standard test for hydrogen.

Question 6. Aluminium is a reactive metal but is still used for packing food articles. Why?
Answer:
Aluminium reacts with oxygen of the air to form a very thin, tough, unreactive layer of aluminium oxide: 4Al + 3O₂ → 2Al₂O₃
This oxide layer sticks firmly to the surface and does not allow air and moisture to reach the metal below. It stops further corrosion.
Because the layer is stable and does not react with food, the foil does not spoil and does not contaminate the food.
In simple words: A thin protective coat of aluminium oxide keeps the metal underneath safe, so aluminium can be used for packing food.

Teacher's Note:
1. Reactivity alone does not decide whether a metal corrodes quickly.
2. Write the equation for the oxide layer to get full marks.

Question 7. An aluminium can is used to store ferrous sulphate solution. It is observed that in few days holes appeared in the can. Explain the observation and write chemical equation to support your answer.
Answer:
Aluminium is more reactive than iron. It displaces iron from ferrous sulphate solution, so the metal of the can slowly dissolves and holes appear.
Equation: 2Al(s) + 3FeSO₄(aq) → Al₂(SO₄)₃(aq) + 3Fe(s)
The green colour of the solution gradually fades because colourless aluminium sulphate is formed, and iron is deposited as a grey solid. This is a displacement reaction.
In simple words: A more reactive metal (Al) pushes out a less reactive metal (Fe) from its salt, eating away the can.

Teacher's Note:
1. Use the reactivity series to decide whether a displacement reaction will occur.
2. Mention the colour change of the solution as the evidence.

Question 8. H₂ is a highly inflammable gas and O₂ being a supporter of combustion, yet water which is a compound made up of hydrogen and oxygen is used to extinguish fire. Give reason for same.
Answer:
Water is a compound, and its properties are completely different from those of hydrogen and oxygen, the elements it is made of.
Water is the product obtained when hydrogen has already burnt in oxygen: 2H₂ + O₂ → 2H₂O. It cannot burn again and it does not support burning.
Water also cools the burning material and its vapour keeps oxygen away from the fire.
In simple words: Water is already 'burnt hydrogen', so it can neither burn nor feed a fire.

Teacher's Note:
1. A compound does not keep the properties of its elements.
2. Mention at least two reasons: it cannot burn and it cools the fuel.

Question 9. Give reasons: (A) All decomposition reactions are endothermic reactions.
Answer:
No. Most decomposition reactions need heat, light or electricity, so they are endothermic. Examples: CaCO₃ →^heat CaO + CO₂ and 2H₂O →^electricity 2H₂ + O₂.
But some decomposition reactions release energy. The decomposition of vegetable matter into compost is a slow decomposition in which heat is given out, so it is exothermic.
Thus the statement is not true for all decomposition reactions.
In simple words: Decomposition only means breaking down; heat may be taken in or given out.

Teacher's Note:
1. Separate the idea of 'breaking down' from the idea of 'energy change'.
2. Quote compost formation as the standard exothermic example.

Long Answer Type HOTS Questions

Question 10. When the powder of a common metal is heated in an open china dish, its colour turns black. However, when hydrogen is passed over the hot black substance so formed, it regains its original colour. Based on the above information answer the following questions:
(i) What type of chemical reaction takes place in each of the two given steps?
(ii) Name the metal initially taken in the powder form. Write balanced chemical equations for both the reactions.
Answer:
(i) In the first step the metal gains oxygen, so it is oxidation (a combination reaction). In the second step the black oxide loses oxygen to hydrogen, so it is reduction. Together they are redox reactions.
(ii) The metal is copper. The reddish-brown powder turns black because black copper(II) oxide forms.
2Cu(s) + O₂(g) →^heat 2CuO(s)
CuO(s) + H₂(g) →^heat Cu(s) + H₂O(l)
In simple words: Copper turns black by gaining oxygen and turns back to copper by losing it to hydrogen.

Teacher's Note:
1. Mention the colours: reddish-brown, black, reddish-brown.
2. Show which substance is oxidised and which is reduced.

Question 11. Compound A when dissolved in water gives compound B and liberates heat. Compound A is used in white washing. Compound B reacts with CO₂ to form a white precipitate of compound C. Identify compounds A, B and C and also write the equations involved.
Answer:
(a) A is calcium oxide (quicklime), CaO; B is calcium hydroxide (slaked lime), Ca(OH)₂; C is calcium carbonate, CaCO₃.
(b) CaO(s) + H₂O(l) → Ca(OH)₂(aq) + heat
Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)
(c) The milky solution becomes clear again because calcium carbonate reacts with water and excess carbon dioxide to form soluble calcium hydrogencarbonate: CaCO₃ + H₂O + CO₂ → Ca(HCO₃)₂
In simple words: Quicklime plus water gives slaked lime; slaked lime plus CO₂ gives milky chalk-like calcium carbonate, which clears with excess CO₂.

Teacher's Note:
1. The release of heat on adding water is the clue for quicklime.
2. Do not forget the follow-up reaction with excess CO₂.

Question 12. In a quiz, the teacher gave hints in the following table:
(i) Substance 'A' emits brown fumes of gas 'G' on thermal decomposition.
(ii) The product 'P' of substance 'B' is used in the manufacture of cement.
(iii) Substance 'C' is green in colour and changes to white on heating.
(iv) The decomposition reaction of substance 'D' is used in black and white photography.
Based on the above hints, answer the following questions:
(A) Identify the substances A, B, C and D.
(B) Illustrate with the help of balanced chemical equations, the changes that take place in: (i) Substance 'A'. Name the gas 'G'. (ii) Substance 'B'. Name the product 'P'.
(C) Take example of a chemical reaction showing decomposition of substance 'D' to justify that the chemical equation is balanced.
(D) Explain with the help of chemical equation, the various colour changes that take place on heating substance 'C'.
Answer:
(a) A = lead nitrate, Pb(NO₃)₂; B = calcium carbonate (limestone), CaCO₃; C = hydrated ferrous sulphate, FeSO₄·7H₂O; D = silver chloride, AgCl.
(b) 2Pb(NO₃)₂(s) →^heat 2PbO(s) + 4NO₂(g) + O₂(g); G is nitrogen dioxide.
CaCO₃(s) →^heat CaO(s) + CO₂(g); P is calcium oxide (quicklime).
(c) 2AgCl(s) →^sunlight 2Ag(s) + Cl₂(g). Atoms on the left: Ag 2, Cl 2. Atoms on the right: Ag 2, Cl 2. The numbers are equal, so the equation is balanced.
(d) Green FeSO₄·7H₂O first loses its water of crystallisation and becomes white (anhydrous FeSO₄). On further strong heating it decomposes to reddish-brown Fe₂O₃ with the smell of burning sulphur (SO₂): FeSO₄·7H₂O → FeSO₄ + 7H₂O and 2FeSO₄ →^heat Fe₂O₃ + SO₂ + SO₃
In simple words: Brown fumes mean NO₂ from a nitrate, cement means limestone, a green salt turning white is hydrated ferrous sulphate, and photography means silver halide.

Teacher's Note:
1. Take each hint separately and match it with a substance you have studied.
2. Copper nitrate also gives brown fumes, but lead nitrate is the usual answer.

Case Based HOTS Questions

Question 13. Amit took two boiling tubes, he added about 2 grams of a green coloured metal salt 'A' in the first tube and 2 grams of a white coloured metal salt 'B' in the second tube. Both the tubes were heated by holding them with a pair of tongs. Smell of burning sulphur was observed in first test tube whereas brown gas was emitted in the second test tube.
(A) The salts 'A' and 'B' are:
(a) ferrous nitrate and lead sulphate respectively.
(b) ferric oxide and lead nitrate respectively.
(c) ferrous sulphate and lead nitrate respectively.
(d) ferric oxide and lead sulphate respectively.
(C) What are the products formed when green coloured metal salt 'A' is heated?
(D) On heating white coloured metal salt 'B', two gases are evolved, one is colourless and the other is brown in colour. Which gases are they?
Answer:
(A) Option (c): A is ferrous sulphate (green) and B is lead nitrate (white).
(C) 2FeSO₄ →^heat Fe₂O₃ + SO₂ + SO₃. The products are ferric oxide (reddish-brown solid), sulphur dioxide and sulphur trioxide. The smell of burning sulphur is due to SO₂.
(D) 2Pb(NO₃)₂ →^heat 2PbO + 4NO₂ + O₂. The brown gas is nitrogen dioxide and the colourless gas is oxygen.
In simple words: Heating breaks each salt into simpler substances: SO₂ smell in one case and brown NO₂ in the other.

Teacher's Note:
1. Connect colour and smell clues with the compound.
2. Both changes are thermal decomposition reactions.

Question 14. A 2 cm long thin ribbon of a metal 'X' was taken and first cleaned with a sandpaper. It was then burnt using a spirit lamp or burner by holding it with a pair of tongs. The ribbon burnt with a dazzling white flame and formed a powder 'Y' which was collected in a watch glass.
(A) Which option correctly identifies both X and Y?
(a) X Magnesium, Y Magnesium carbonate (b) X Aluminium, Y Aluminium oxide (c) X Magnesium, Y Magnesium oxide (d) X Iron, Y Iron oxide
(B) The colour of powder or ash formed when a magnesium ribbon is burnt in air: (a) grey (b) black (c) white (d) yellow
(C) Why should a magnesium ribbon be cleaned before burning in air?
(D) Is burning of magnesium ribbon a physical change or a chemical change? Justify your answer.
Answer:
(A) Option (c): X is magnesium and Y is magnesium oxide, 2Mg + O₂ → 2MgO.
(B) Option (c), white.
(C) Magnesium kept in air gets a layer of magnesium oxide. This layer would stop burning, so the ribbon is cleaned with sandpaper.
(D) It is a chemical change because a new substance, MgO, with different properties is formed.
In simple words: Clean magnesium burns with a dazzling white flame and turns into white magnesium oxide.

Teacher's Note:
1. Do not look at the flame directly.
2. A new substance means a chemical change.

Question 15. Following sequence of reaction is given:
X + H₂SO₄ → Na₂SO₄ + Y + H₂O; Y reacts with Ca(OH)₂ to give a milky precipitate Z. Answer the following questions on the basis of the above reaction sequence.
1. Write the chemical formula of X. (a) Na₂CO₃ (b) NaOH (c) NaHCO₃ (d) NaCl
2. Write the name of product Y. (a) Sodium carbonate (b) Carbon dioxide (c) Hydrogen (d) Hydrogen peroxide
3. Write the chemical name of Z.
Answer:
1. Option (a) Na₂CO₃: it supplies the two sodium atoms of one Na₂SO₄.
2. Option (b) carbon dioxide, the gas that turns lime water milky.
3. Z is calcium carbonate.
Na₂CO₃ + H₂SO₄ → Na₂SO₄ + CO₂ + H₂O and Ca(OH)₂ + CO₂ → CaCO₃ + H₂O
In simple words: A carbonate with an acid gives CO₂, and CO₂ turns lime water milky as calcium carbonate forms.

Teacher's Note:
1. Milky lime water is the test for carbon dioxide.
2. Balance Na, S, C, H and O on both sides.