CBSE Class 10 Science HOTs Chemical Reactions and Equations Set 07

This is Set 07 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 every question. It tests why some metals displace others, how to read observations of heating carbonates and nitrates, how to explain colours of precipitates and how to balance a fertiliser explosion.

Short Answer Type HOTS Questions

Question 1. An iron object is immersed in a dark blue solution of CuSO₄ in a beaker. After half an hour the colour of the solution has changed to light green. Now a platinum object is immersed in CuSO₄ solution in another beaker. Is a similar change possible in the second beaker too? Explain.
Answer:
First beaker: iron is more reactive than copper, so it displaces copper from copper sulphate. The blue colour fades as light green ferrous sulphate forms: Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s).
Second beaker: platinum is less reactive than copper, so it cannot displace copper. No change is seen.
In simple words: Only a more reactive metal can displace copper.

Teacher's Note:
1. Check the reactivity series before predicting.
2. A change in colour shows a displacement reaction.

Question 2. Iron filings are put into an aqueous solution of HCl and a colourless gas which burns with a pop sound is evolved. Will the same gas be evolved if platinum filings are used instead of iron in the same solution? Why?
Answer:
Iron is more reactive than hydrogen and displaces it from HCl: Fe + 2HCl → FeCl₂ + H₂↑. The colourless gas is hydrogen.
Platinum is less reactive than hydrogen, so it cannot displace hydrogen from the acid and no gas is evolved.
In simple words: Metals above hydrogen in the reactivity series release H₂ from acids.

Teacher's Note:
1. Hydrogen burns in oxygen to form water.
2. Copper, silver and gold do not react with dilute acids.

Question 3. A piece of aluminium and a piece of zinc, dropped separately into green ferrous sulphate solution, show a colour change, but no change is seen when a piece of copper is dropped in it. Explain with equations.
Answer:
Aluminium and zinc are more reactive than iron, so they displace iron from its salt solution and the green colour fades.
2Al(s) + 3FeSO₄(aq) → Al₂(SO₄)₃(aq) + 3Fe(s)
Zn(s) + FeSO₄(aq) → ZnSO₄(aq) + Fe(s)
Copper is less reactive than iron, so Cu + FeSO₄ gives no reaction and the colour remains green.
In simple words: Al and Zn push iron out; Cu cannot.

Teacher's Note:
1. Order of reactivity: Al > Zn > Fe > Cu.
2. Colour change is a sign of a displacement reaction.

Question 4. Explain the difference in the colour of the precipitate formed when H₂S gas is passed through CuSO₄ solution and when barium chloride solution is added to CuSO₄ solution.
Answer:
Both are double displacement reactions but they give different precipitates.
H₂S(g) + CuSO₄(aq) → CuS(s)↓ + H₂SO₄(aq). Copper sulphide is a black precipitate.
CuSO₄(aq) + BaCl₂(aq) → BaSO₄(s)↓ + CuCl₂(aq). Barium sulphate is a white precipitate.
In simple words: Different products give different colours: CuS is black, BaSO₄ is white.

Teacher's Note:
1. The colour tells which precipitate is formed.
2. Name the precipitate in every answer.

Question 5. A water-insoluble substance X, when added to dilute hydrochloric acid, gives a colourless and odourless gas. When the gas is passed through lime water, it turns milky. Write the chemical formula of X and the chemical equations involved.
Answer:
Step 1: The gas that turns lime water milky is carbon dioxide, so X is a carbonate. Being insoluble in water, X is calcium carbonate, CaCO₃.
Step 2: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂↑
Step 3: Ca(OH)₂ + CO₂ → CaCO₃↓ + H₂O (lime water turns milky)
In simple words: A carbonate with acid gives CO₂, which turns lime water milky.

Teacher's Note:
1. The milkiness is due to insoluble CaCO₃.
2. Do not confuse CO₂ with SO₂, which smells.

Question 6. Solutions of ferrous sulphate, zinc sulphate, copper sulphate and aluminium sulphate were separately taken in four test tubes and some iron nails were placed in each of the solutions. The colour of which solution changes? Give a reason.
Answer:
Only the copper sulphate solution changes colour (blue to light green) because iron is more reactive than copper and displaces it: Fe + CuSO₄ → FeSO₄ + Cu.
Iron cannot displace zinc or aluminium because it is less reactive than both. With ferrous sulphate there is nothing to displace. So these solutions show no change.
In simple words: Iron displaces only the less reactive copper.

Teacher's Note:
1. Order of reactivity: Al > Zn > Fe > Cu.
2. Look for a brown deposit of copper on the nails.

Question 7. During the reaction of some metals with dilute hydrochloric acid, the following observations were made. (i) Silver metal does not show any change. (ii) Some bubbles of a gas are seen when lead is reacted with the acid. (iii) The reaction of sodium metal is found to be highly explosive. (iv) The temperature of the reaction mixture rises when aluminium is added to the acid. Explain these observations giving appropriate reasons.
Answer:
(i) Silver is less reactive than hydrogen and cannot displace it from the acid, so there is no reaction.
(ii) Lead is slightly more reactive than hydrogen and displaces it slowly, so bubbles of hydrogen are seen: Pb + 2HCl → PbCl₂ + H₂.
(iii) Sodium is very reactive, so 2Na + 2HCl → 2NaCl + H₂ is violent and highly exothermic.
(iv) The reaction 2Al + 6HCl → 2AlCl₃ + 3H₂ is exothermic, so heat is released and the temperature rises.
In simple words: The more reactive the metal, the faster and more violent the reaction.

Teacher's Note:
1. Reactivity order: Na > Al > Pb > H > Ag.
2. Bubbles show evolution of hydrogen gas.

Question 8. On adding a drop of barium chloride solution to an aqueous solution of sodium sulphite, a white precipitate is obtained. (i) Write a balanced chemical equation of the reaction involved. (ii) What other name can be given to this precipitation reaction? (iii) On adding dilute hydrochloric acid to the reaction mixture, the white precipitate disappears. Why?
Answer:
(i) Na₂SO₃(aq) + BaCl₂(aq) → BaSO₃(s)↓ + 2NaCl(aq)
(ii) It is also a double displacement reaction.
(iii) Barium sulphite reacts with the acid to give soluble barium chloride, water and sulphur dioxide gas: BaSO₃ + 2HCl → BaCl₂ + H₂O + SO₂↑. As the solid is used up, the precipitate disappears.
In simple words: The sulphite precipitate dissolves in acid by giving off SO₂.

Teacher's Note:
1. Sulphur dioxide has the smell of burning sulphur.
2. Sulphates, unlike sulphites, do not dissolve in dilute HCl.

Question 9. On heating a blue coloured powder of copper(II) nitrate in a boiling tube, black copper oxide, oxygen gas and a brown gas X are formed. (i) Identify the type of reaction and the gas X. (ii) Write a balanced chemical equation for the reaction. (iii) Write the pH range of an aqueous solution of gas X.
Answer:
(i) A single compound breaks into several substances, so it is a thermal decomposition reaction. The brown gas X is nitrogen dioxide, NO₂.
(ii) 2Cu(NO₃)₂(s) →^heat 2CuO(s) + 4NO₂(g) + O₂(g)
(iii) NO₂ is an oxide of a non-metal and forms an acidic solution in water, so its pH is less than 7.
In simple words: Blue copper nitrate gives black CuO, brown NO₂ and oxygen.

Teacher's Note:
1. Brown fumes also come from heating lead nitrate.
2. Non-metal oxides are acidic.

Long Answer Type HOTS Questions

Question 10. Ammonium nitrate, a colourless ionic solid used as a fertiliser, explodes when it is heated above 300 °C. The products are three gases: molecular nitrogen, molecular oxygen and steam (water vapour). Find the type of chemical reaction and write a balanced equation for the explosion of ammonium nitrate.
Answer:
Step 1: One compound gives three simpler substances, so it is a decomposition reaction.
Step 2: Skeleton: NH₄NO₃ → N₂ + O₂ + H₂O
Step 3: Balance H by taking 4H₂O for 2NH₄NO₃ (8 H). Then N: 4 on the left, so 2N₂. O: 6 on the left; right side has 4 (from water) + 2 (from O₂) = 6.
2NH₄NO₃(s) → 2N₂(g) + O₂(g) + 4H₂O(g)
In simple words: Heat splits ammonium nitrate into nitrogen, oxygen and steam.

Teacher's Note:
1. Count each atom: N 4, H 8, O 6 on both sides.
2. Handle fertilisers with care since they can explode.

Case Based HOTS Questions

Question 11. Two boiling tubes A and B are marked. Copper(II) carbonate (green) is put in tube A and tube B is filled with lime water. On heating, the green CuCO₃ turns black. At first the lime water in tube B turns milky and, after prolonged heating, it becomes colourless again. Explain.
Answer:
Step 1: CuCO₃ decomposes on heating: CuCO₃(s) → CuO(s) + CO₂(g). CuO is black.
Step 2: The CO₂ formed turns lime water milky because of insoluble calcium carbonate: Ca(OH)₂ + CO₂ → CaCO₃↓ + H₂O.
Step 3: With excess CO₂, the milkiness disappears because soluble calcium hydrogencarbonate forms: CaCO₃ + H₂O + CO₂ → Ca(HCO₃)₂.
In simple words: A little CO₂ makes lime water milky; plenty of CO₂ makes it clear again.

Teacher's Note:
1. Heating a carbonate is a thermal decomposition reaction.
2. The change from milky to clear is a good board question.

Question 12. In the set-up of the previous question, a piece of coal is used in place of CuCO₃ and heated in air. Will a similar change be observed in tube B? Give a reason with the equation.
Answer:
Yes. Coal is carbon, which burns in air to form carbon dioxide: C(s) + O₂(g) → CO₂(g).
This is a combination reaction. The CO₂ passes into tube B and turns the lime water milky, as before.
In simple words: Burning coal also gives CO₂, so lime water turns milky.

Teacher's Note:
1. Here a combination, not a decomposition, gives CO₂.
2. Carbon dioxide is the gas that turns lime water milky.

Question 13. An unknown compound XCO₃ is heated in test tube A and a milky appearance is observed in test tube B containing lime water. When water is added to the residue in test tube A, a colourless solution is obtained. What is the element X?
Answer:
Step 1: Milky lime water shows that CO₂ is formed, so XCO₃ decomposes: XCO₃ → XO + CO₂.
Step 2: The residue XO dissolves in water to give a colourless solution, which fits calcium oxide: CaO + H₂O → Ca(OH)₂.
So the compound is CaCO₃ and the element X is calcium (Ca).
In simple words: The carbonate that gives CO₂ and a soluble oxide is calcium carbonate.

Teacher's Note:
1. CuCO₃ would leave a black, insoluble residue.
2. Quicklime reacts with water giving slaked lime.