CBSE Class 10 Chemistry Chemical Reactions And Equations Worksheet Set 10

This is Printable Worksheet Set 10 for CBSE Class 10 Chemistry Chapter 1, Chemical Reactions and Equations, with 19 short questions and answers that print well on two or three pages. It covers definitions, differences between reaction types, tests for common gases, signs of a chemical reaction and completing, classifying and balancing equations.

Short Questions with Answers

Question 1. Why is it advisable to show the physical states of substances when writing a chemical equation?
Answer:
Physical states (s), (l), (g) and (aq) tell us in which form each substance takes part in the reaction.
They make the equation more informative and show the conditions under which the reaction occurs, for example BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq).
In simple words: State symbols give extra information about the reaction.

Teacher's Note:
1. (aq) means the substance is dissolved in water.
2. Write the state symbol immediately after the formula.

Question 2. Why is a displacement reaction also known as a single displacement reaction?
Answer:
In this reaction only one element (a metal or non-metal) replaces another element in a compound.
For example, in Zn + CuSO₄ → ZnSO₄ + Cu only zinc replaces copper, so it is a single displacement.
In simple words: Only one element is replaced.

Teacher's Note:
1. Compare it with double displacement, where two compounds exchange ions.
2. Use the reactivity series to predict it.

Question 3. What is a combination reaction? Give one example.
Answer:
A reaction in which two or more reactants combine to form a single product is called a combination reaction.
Example: 2Mg + O₂ → 2MgO.
In simple words: Many reactants give one product.

Teacher's Note:
1. Combination reactions are usually exothermic.
2. The reverse of it is a decomposition reaction.

Question 4. What is the difference between an exothermic and an endothermic reaction?
Answer:
Exothermic reaction: heat is released. Example: CaO + H₂O → Ca(OH)₂ + heat.
Endothermic reaction: heat is absorbed. Example: CaCO₃ → CaO + CO₂ (heat supplied).
In simple words: Exothermic gives out heat; endothermic takes in heat.

Teacher's Note:
1. Respiration is exothermic; photosynthesis is endothermic.
2. The reaction vessel becomes hot in an exothermic reaction.

Question 5. What is a reactant and what is a product in a chemical reaction?
Answer:
Reactants are the substances that take part in a chemical reaction and are written on the left side of the arrow.
Products are the new substances formed in the reaction and are written on the right side of the arrow.
In simple words: Reactants go in; products come out.

Teacher's Note:
1. Example: in 2H₂ + O₂ → 2H₂O, H₂ and O₂ are reactants and H₂O is the product.
2. The arrow means 'gives'.

Question 6. What do you understand by a double displacement reaction? Give an example.
Answer:
A reaction in which two compounds exchange their ions to form two new compounds is a double displacement reaction.
Example: AgNO₃(aq) + NaCl(aq) → AgCl(s)↓ + NaNO₃(aq).
In simple words: Two compounds swap their partners.

Teacher's Note:
1. If an insoluble solid is formed it is also a precipitation reaction.
2. Neutralisation is a double displacement reaction.

Question 7. Balance the equation for the reaction between sodium hydroxide and sulphuric acid to form sodium sulphate and water.
Answer:
Skeleton equation: NaOH + H₂SO₄ → Na₂SO₄ + H₂O
Balance sodium by putting 2 before NaOH: 2NaOH + H₂SO₄ → Na₂SO₄ + H₂O
Balance hydrogen by putting 2 before H₂O.
2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O (Na 2, S 1, H 4, O 6 on both sides)
In simple words: Two NaOH neutralise one H₂SO₄.

Teacher's Note:
1. Balance the metal first and the hydrogen and oxygen last.
2. This is a neutralisation reaction.

Question 8. Give an example of a reaction which is a displacement reaction as well as a redox reaction.
Answer:
Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)
Iron displaces copper, so it is a displacement reaction. Iron is oxidised to Fe²⁺ and Cu²⁺ is reduced to Cu, so it is also a redox reaction.
In simple words: Replacing a metal in a salt also moves electrons.

Teacher's Note:
1. Every displacement reaction involves oxidation and reduction.
2. ZnO + C → Zn + CO is another example.

Question 9. Define rusting.
Answer:
Rusting is the slow corrosion of iron in which iron reacts with oxygen and moisture of air to form a reddish-brown coating of hydrated iron(III) oxide called rust.
4Fe + 3O₂ + xH₂O → 2Fe₂O₃·xH₂O
In simple words: Iron slowly turns brown in damp air.

Teacher's Note:
1. Both air and moisture are necessary for rusting.
2. Painting, oiling and galvanising prevent rusting.

Question 10. Define (i) corrosion and (ii) combination reaction.
Answer:
(i) Corrosion: the gradual eating up of a metal by the action of air, moisture or chemicals on its surface, for example the rusting of iron.
(ii) Combination reaction: a reaction in which two or more substances combine to form a single product, for example C + O₂ → CO₂.
In simple words: Corrosion damages metals; combination joins substances.

Teacher's Note:
1. Give an example with each definition.
2. Silver turns black and copper turns green on corrosion.

Question 11. State the following: (i) the type of reaction used in photography; (ii) an example of an endothermic reaction; (iii) a reaction that takes place during the digestion of food in our body; (iv) the substance used in white washing; (v) the gas evolved when Zn reacts with hydrochloric acid; (vi) whether burning of a candle is a physical or a chemical change.
Answer:
(i) Photochemical decomposition of silver bromide (or silver chloride) in light.
(ii) Photosynthesis (or thermal decomposition of CaCO₃).
(iii) Decomposition reaction.
(iv) Calcium hydroxide (slaked lime), made from quicklime.
(v) Hydrogen gas.
(vi) Both: melting of wax is a physical change but burning of wax vapour is a chemical change, so burning of a candle involves a chemical change.
In simple words: A quick set of one-line facts from the chapter.

Teacher's Note:
1. Learn these facts as a short list.
2. Photosynthesis absorbs light energy, so it is endothermic.

Question 12. State one characteristic of the chemical reaction which takes place when: (i) dilute hydrochloric acid is added to sodium carbonate; (ii) lemon juice is added gradually to potassium permanganate solution; (iii) dilute sulphuric acid is added to barium chloride solution; (iv) quicklime is treated with water; (v) wax is burned in the form of a candle.
Answer:
(i) Evolution of a gas (carbon dioxide) with brisk effervescence.
(ii) Change in colour: purple colour disappears.
(iii) Formation of a white precipitate of barium sulphate.
(iv) Change in temperature: a lot of heat is released.
(v) Change in state: solid wax changes to liquid and gaseous products.
In simple words: Each reaction shows one clear sign of change.

Teacher's Note:
1. The five signs are state, colour, temperature, gas and precipitate.
2. Match the sign to the reaction.

Question 13. Give the characteristic tests for the following gases: (i) CO₂ (ii) SO₂ (iii) O₂ (iv) H₂.
Answer:
(i) CO₂: turns lime water milky: Ca(OH)₂ + CO₂ → CaCO₃↓ + H₂O.
(ii) SO₂: has a smell of burning sulphur and turns acidified potassium permanganate solution from purple to colourless.
(iii) O₂: a burning candle burns more brightly in it, as oxygen supports combustion.
(iv) H₂: a burning matchstick brought near it makes a pop sound as the gas burns.
In simple words: Each gas gives its own simple test.

Teacher's Note:
1. Lime water turns milky with CO₂.
2. Hydrogen is the gas that burns with a pop sound.

Question 14. Which among the following changes are exothermic and which are endothermic? (i) Decomposition of ferrous sulphate (ii) Dilution of sulphuric acid (iii) Dissolution of sodium hydroxide in water (iv) Dissolution of ammonium chloride in water
Answer:
(i) Decomposition of ferrous sulphate: endothermic, as heat is absorbed.
(ii) Dilution of sulphuric acid: exothermic, as heat is released.
(iii) Dissolution of sodium hydroxide in water: exothermic, as heat is released.
(iv) Dissolution of ammonium chloride in water: endothermic, as heat is absorbed and the vessel feels cold.
In simple words: Heat given out means exothermic; heat taken in means endothermic.

Teacher's Note:
1. Always add acid to water slowly during dilution.
2. A cold container shows an endothermic change.

Question 15. Complete the following reactions: (i) NaOH + HCl → (ii) AgNO₃ + NaCl → (iii) Na₂O + H₂O → (iv) Mg + O₂ →
Answer:
(i) NaOH + HCl → NaCl + H₂O
(ii) AgNO₃ + NaCl → AgCl↓ + NaNO₃
(iii) Na₂O + H₂O → 2NaOH
(iv) 2Mg + O₂ → 2MgO
In simple words: Write the products and balance each equation.

Teacher's Note:
1. Check atoms on both sides.
2. Part (i) is neutralisation and part (ii) is a precipitation reaction.

Question 16. Classify the following reactions: (i) Reaction of magnesium and oxygen to form magnesium oxide. (ii) Heating of calcium carbonate. (iii) Reaction of potassium hydroxide and nitric acid to form potassium nitrate and water. (iv) Reaction of zinc oxide and carbon to form zinc and carbon monoxide.
Answer:
(i) Combination reaction: 2Mg + O₂ → 2MgO.
(ii) Decomposition reaction: CaCO₃ → CaO + CO₂.
(iii) Neutralisation (double displacement) reaction: KOH + HNO₃ → KNO₃ + H₂O.
(iv) Displacement reaction, which is also a redox reaction: ZnO + C → Zn + CO.
In simple words: Look at how many reactants and products there are.

Teacher's Note:
1. One product: combination. One reactant: decomposition.
2. An element replacing another: displacement.

Question 17. Identify the type of reaction in each of the following equations: (i) CH₄ + 2O₂ → CO₂ + 2H₂O (ii) Pb(NO₃)₂ + 2KI → PbI₂ + 2KNO₃ (iii) CaO + H₂O → Ca(OH)₂ (iv) CuSO₄ + Zn → ZnSO₄ + Cu
Answer:
(i) Combustion (oxidation) reaction.
(ii) Double displacement and precipitation reaction.
(iii) Combination reaction.
(iv) Displacement reaction.
In simple words: Decide by what happens to the reactants.

Teacher's Note:
1. Write the correct formula ZnSO₄ for zinc sulphate.
2. A yellow precipitate of PbI₂ forms in (ii).

Question 18. Write balanced chemical equations for the following reactions: (i) Calcium hydroxide + carbon dioxide → calcium carbonate + water. (ii) Aluminium + copper chloride → aluminium chloride + copper.
Answer:
(i) Ca(OH)₂ + CO₂ → CaCO₃ + H₂O (already balanced)
(ii) 2Al + 3CuCl₂ → 2AlCl₃ + 3Cu
In simple words: Write formulae first, then balance.

Teacher's Note:
1. Aluminium is above copper in the reactivity series.
2. Part (ii) is a displacement reaction.

Question 19. Complete and balance the following chemical equations: (i) CaCO₃ + HCl → (ii) Al + HCl → (iii) MnO₂ + HCl →
Answer:
(i) CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂
(ii) 2Al + 6HCl → 2AlCl₃ + 3H₂
(iii) MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂
In simple words: Predict the products and balance.

Teacher's Note:
1. Metal + acid gives salt and hydrogen.
2. Carbonate + acid gives salt, water and carbon dioxide.