CBSE Class 10 Science Chemical Reactions and Equations Assignment Set 05

This is Assignment Set 05 for CBSE Class 10 Science Chapter 1, Chemical Reactions and Equations, with 18 questions carrying 42 marks in all. It practises balancing of equations, types of reactions, displacement based on the reactivity series, redox terms and the signs that show a chemical change, each with a step-by-step answer.

Very Short Answer Questions (1 Mark)

Question 1. What is a balanced chemical equation? or Define a balanced chemical equation.
Answer:
A chemical equation in which the number of atoms of each element is the same on the reactant side and on the product side is called a balanced chemical equation.
In simple words: Equal atoms of every element on both sides.

Teacher's Note:
1. Mention atoms of each element.
2. The balancing follows the law of conservation of mass.

Question 2. Name the type of chemical reaction which takes place when quicklime is added to water.
Answer:
CaO + H₂O → Ca(OH)₂ + heat
It is a combination reaction and it is also exothermic.
In simple words: Two substances join to form one product and heat is released.

Teacher's Note:
1. Give both the type and the heat nature.
2. Quicklime is calcium oxide.

Question 3. Give the values of x, y, z for the equation: x HCl + O₂ → y H₂O + z Cl₂
Answer:
Oxygen atoms: 2 on the left, so y = 2. Then hydrogen: x = 2y = 4. Chlorine: z = x/2 = 2.
4HCl + O₂ → 2H₂O + 2Cl₂
x = 4, y = 2, z = 2
In simple words: Balance oxygen first, then hydrogen, then chlorine.

Teacher's Note:
1. Start with the element appearing in one compound on each side.
2. Check all atoms at the end.

Question 4. Which one is a chemical change: fermentation of fruit juice or diluting fruit juice?
Answer:
Fermentation of fruit juice is a chemical change because new substances (such as alcohol) are formed. Diluting juice only mixes it with water and is a physical change.
In simple words: New substance formed means chemical change.

Teacher's Note:
1. Ask whether a new substance is produced.
2. Dilution can be reversed by removing water.

Question 5. What is the brown coloured gas evolved when lead nitrate crystals are heated in a dry test tube?
Answer:
2Pb(NO₃)₂ →^heat 2PbO + 4NO₂ + O₂
The brown gas is nitrogen dioxide (NO₂).
In simple words: Heating lead nitrate gives brown NO₂ fumes.

Teacher's Note:
1. Remember the yellow residue is lead oxide.
2. Perform this activity in a fume cupboard.

Question 6. Arrange iron, zinc and copper in the order of their increasing reactivity.
Answer:
From the reactivity series: Copper < Iron < Zinc.
In simple words: Zinc is the most reactive of the three and copper the least.

Teacher's Note:
1. Use the activity series.
2. A more reactive metal displaces a less reactive one.

Short Answer Questions (2 Marks)

Question 7. 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 reason to justify your answer.
Answer:
3BaCl₂(aq) + Al₂(SO₄)₃(aq) → 2AlCl₃(aq) + 3BaSO₄(s)
Ions are exchanged between the two reactants and an insoluble solid forms, so it is a double displacement (precipitation) reaction.
In simple words: Ions swap partners and white barium sulphate settles out.

Teacher's Note:
1. Write formulae first, then balance.
2. Name both double displacement and precipitation.

Question 8. What do you understand by skeletal and balanced chemical equation?
Answer:
A skeletal equation shows reactants and products with formulae but the numbers of atoms are not equal on both sides. Example: Mg + HCl → MgCl₂ + H₂
A balanced equation has equal numbers of atoms of each element on both sides. Example: Mg + 2HCl → MgCl₂ + H₂
In simple words: Skeletal is unbalanced; balanced has equal atoms.

Teacher's Note:
1. Give one example of each.
2. Balancing is done by hit and trial.

Question 9. (i) Rewrite the following equation in a balanced form: Fe + O₂ → Fe₂O₃
(ii) Rewrite the following equation indicating that a solution of lime in water has been used and that a precipitate is formed in the reaction. Ca(OH)₂ + CO₂ → CaCO₃ + H₂O
Answer:
(i) 4Fe + 3O₂ → 2Fe₂O₃
(ii) Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s)↓ + H₂O(l)
In simple words: Use coefficients 4, 3, 2 and write the states.

Teacher's Note:
1. Check Fe: 4 = 4 and O: 6 = 6.
2. (aq) means lime water, (s) the milky precipitate.

Question 10. Identify the substance oxidised, substance reduced, oxidising agent and reducing agent. MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂
Answer:
MnO₂ loses oxygen, so it is reduced. HCl loses hydrogen (to form Cl₂), so it is oxidised.
Substance oxidised: HCl; substance reduced: MnO₂.
Oxidising agent: MnO₂; reducing agent: HCl.
In simple words: The one that loses oxygen is reduced; the one that loses hydrogen is oxidised.

Teacher's Note:
1. The oxidising agent is the substance reduced.
2. The reducing agent is the substance oxidised.

Question 11. Reverse of the following chemical reaction is not possible: Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s). Justify this statement with reason.
Answer:
Copper is less reactive than zinc.
A less reactive metal cannot displace a more reactive metal from its salt solution, so Cu cannot displace Zn from ZnSO₄ solution.
In simple words: Copper cannot push out the more reactive zinc.

Teacher's Note:
1. Use the reactivity series.
2. State clearly which metal is more reactive.

Short Answer Questions (3 Marks)

Question 12. A thin zinc plate was kept in a glass container having CuSO₄ solution. On examining it was found that the blue colour of the solution is getting lighter and lighter. After a few days when the zinc plate was taken out of the solution, a number of small holes were noticed in it. State the reason and write chemical equation of the reaction.
Answer:
Zinc is more reactive than copper and displaces it from copper sulphate solution.
Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s)
Zinc dissolves as colourless zinc sulphate, so holes appear in the plate; blue CuSO₄ is used up, so the blue colour fades.
In simple words: Zinc dissolves and copper is deposited.

Teacher's Note:
1. It is a displacement reaction.
2. Colour fades because blue Cu²⁺ ions are used up.

Question 13. Identify the type of each of the following reactions stating reason for your answers:
(a) Fe₂O₃ + 2Al → Al₂O₃ + 2Fe + heat
(b) Pb(NO₃)₂ + 2KI → PbI₂ + 2KNO₃
(c) ZnCO₃ →^heat ZnO + CO₂
Answer:
(a) Displacement reaction: more reactive Al displaces Fe from Fe₂O₃; it also releases a lot of heat.
(b) Double displacement (precipitation) reaction: ions are exchanged and yellow PbI₂ is formed as a precipitate.
(c) Thermal decomposition reaction: one compound breaks into two simpler ones on heating.
In simple words: Look at how many reactants and products there are and what changes places.

Teacher's Note:
1. Give a reason with every type.
2. Precipitate formation points to double displacement.

Question 14. In the electrolysis of water: (a) Name the gases liberated at anode and cathode. (b) Why is it that the volume of gas collected on one electrode is two times that on the other electrode? (c) What would happen if dil. H₂SO₄ is not added to water?
Answer:
(a) Anode: oxygen; cathode: hydrogen.
(b) 2H₂O(l) →^electricity 2H₂(g) + O₂(g). Water has two hydrogen atoms for every oxygen atom, so hydrogen is collected in double the volume.
(c) Pure water is a poor conductor, so electrolysis would be extremely slow. Dilute sulphuric acid makes water conduct electricity.
In simple words: Hydrogen : oxygen = 2 : 1 by volume.

Teacher's Note:
1. The test tube with double volume holds hydrogen.
2. The acid is only to help conduction; it is not used up.

Question 15. Balance the following skeleton equations: (i) KClO₃ → KCl + O₂ (ii) Na₂CO₃ + HCl → NaCl + H₂O + CO₂ (iii) CH₄ + O₂ → CO₂ + H₂O (iv) NaCl + H₂SO₄ → Na₂SO₄ + HCl (v) Fe + H₂O → Fe₃O₄ + H₂ (vi) H₂S + O₂ → H₂O + S (vii) H₂ + N₂ → NH₃
Answer:
(i) 2KClO₃ → 2KCl + 3O₂
(ii) Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂
(iii) CH₄ + 2O₂ → CO₂ + 2H₂O
(iv) 2NaCl + H₂SO₄ → Na₂SO₄ + 2HCl
(v) 3Fe + 4H₂O → Fe₃O₄ + 4H₂
(vi) 2H₂S + O₂ → 2H₂O + 2S
(vii) N₂ + 3H₂ → 2NH₃
In simple words: Adjust coefficients until each element has equal atoms on both sides.

Teacher's Note:
1. Never change the subscripts of a formula.
2. Balance the most complicated formula first.

Long Answer Questions (5 Marks)

Question 16. With the help of an appropriate example, justify that some of the chemical reactions are determined by (a) Change in temperature, (b) Evolution of a gas, and (c) Change in colour. Give chemical equation for the reaction involved in each case.
Answer:
(a) Temperature change: CaO + H₂O → Ca(OH)₂ + heat. The mixture becomes hot.
(b) Evolution of gas: Zn + H₂SO₄ → ZnSO₄ + H₂. Bubbles of hydrogen, which burns with a pop sound.
(c) Colour change: Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) + 2KNO₃(aq). A colourless mixture gives a yellow precipitate. (Also Fe + CuSO₄: blue changes to pale green.)
In simple words: Heat, bubbles and colour show that a chemical reaction has taken place.

Teacher's Note:
1. Give one equation for each part.
2. Add the observation in words.

Question 17. Name the type of following reactions: (i) Fe(s) + CuSO₄(aq) → Cu(s) + FeSO₄(aq) (ii) 2NH₃(g) → N₂(g) + 3H₂(g) (iii) NH₄Cl(s) → NH₃(g) + HCl(g) (iv) 2H₂(g) + O₂(g) → 2H₂O(g) (v) CuO(s) + H₂(g) → Cu(s) + H₂O(l) (vi) NH₄NO₂(s) → 2H₂O(g) + N₂(g) (vii) Mg(s) + H₂SO₄(aq) → MgSO₄(aq) + H₂(g) (viii) 2NH₃(g) + H₂SO₄(aq) → (NH₄)₂SO₄(aq)
Answer:
(i) Displacement
(ii) Decomposition
(iii) Decomposition (thermal)
(iv) Combination
(v) Displacement (hydrogen displaces copper from CuO; it is also a redox reaction)
(vi) Decomposition
(vii) Displacement
(viii) Combination
In simple words: One reactant, many products is decomposition; many reactants, one product is combination.

Teacher's Note:
1. Count reactants and products, then see what is replaced.
2. Do not mix up displacement with double displacement.

Case Based Question (4 Marks)

Question 18. When two or more substances react and form some new substance, it is called a chemical reaction. Chemical equation is represented in terms of symbols, molecular formulas, moles, states, etc. As we know, all chemical reaction obeys law of chemical combination. Therefore, chemical reactions need to be balanced. It is done by hit and trial method. The chemical reactions can be classified into different types such as combination reaction, decomposition reaction, displacement reaction, double displacement reaction. The reactions take place in solution is precipitation reactions and neutralisation reactions.
(i) Define a chemical equation. (ii) Which law is followed by all chemical reactions? (iii) Name four types of chemical reactions. (iv) Give example of precipitation reactions.
Answer:
(i) A chemical equation is the shorthand representation of a chemical reaction using symbols and formulae of the substances.
(ii) The law of conservation of mass.
(iii) Combination, decomposition, displacement and double displacement reactions.
(iv) AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq), where white AgCl is the precipitate.
In simple words: An equation is a short way to write a reaction, and it must obey conservation of mass.

Teacher's Note:
1. The source key named a different law; the correct one is conservation of mass.
2. Include the states while writing the example.