NCERT Class 10 Science Chapter 1 Chemical Reactions and Equations
Q1. Why should a magnesium ribbon be cleaned before burning in the air?
Answer: Magnesium gets covered with a layer of magnesium oxide when kept in the air for a long time. This layer hinders the burning of magnesium. Hence, it is to be cleaned before burning.
Q2. Write the balanced equation for the following chemical reactions.
(i) Hydrogen + Chlorine → Hydrogen chloride
Ans: (i) H2 + Cl2 → 2HCl
(ii) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride
Ans: 3 BaCl2 + Al2(SO4)3 → BaSO4 + 2 AlCl3
(iii) Sodium + Water → Sodium hydroxide + Hydrogen
Ans: (iii) 2Na + 2H2O → 2NaOH + H2
Q3. Write a balanced chemical equation with state symbols for the following reactions:
(i) Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and the solution of sodium chloride.
Ans: BaCl2 (aq) + Na2SO4 (aq) → BaSO4(s) + 2NaCl (aq)
(ii) Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water.
Ans: (ii) NaOH (aq) + HCl(aq) → NaCl(aq) + H2O(l)
Q. A solution of a substance ‘X’ is used for whitewashing.
(i) Name the substance ‘X’ and write its formula.
Ans: calcium oxide (or quick lime). Its formula is CaO.
(ii) Write the reaction of the substance ‘X’ named in (i) above with water.
Q. Why does the colour of copper sulphate solution change when an iron nail is dipped in it?
Answer: When an iron nail is dipped in copper sulphate solution, the displacement reaction takes place. The colour of copper sulphate solution fades due to the formation of a light green solution of iron sulphate.
Q. Identify the substances that are oxidised and the substances which are reduced in the following reactions.
(i) 4Na(s) + O2(g) → 2Na2O(s)
ans: Na is oxidised and O2 is reduced.
(ii) CuO (s) + H2(g) → Cu (s) + H2O(l)
ans: Cu is reduced and H2 is oxidised.
Q. Which of the statements about the reaction below are incorrect?
2 PbO(s) + C(s) → 2Pb (s) + CO2(g)
(a) Lead is getting reduced.
(b) Carbon dioxide is getting oxidised.
(c) Carbon is getting oxidised.
(d) Lead oxide is getting reduced.
(i) (a) and (b)
(ii) (a) and (c)
(iii) (a), (b) and (c)
Answer: (i) (a) and (b)
Q. Fe2O3 + 2Al → Al2O3 + 2Fe
The above reaction is an example of a
(a) combination reaction
(b) double displacement reaction
(c) decomposition reaction
(d) displacement reaction
Answer: (d) Displacement reaction.
Q. What happens when dilute hydrochloric acid is added to iron filings? Tick the correct answer:
(a) Hydrogen gas and iron chloride are produced.
(b) Chlorine gas and iron hydroxide are produced.
(c) No reaction takes place.
(d) Iron salt and water are produced.
Answer: (a) Hydrogen gas and iron chloride are produced.
Q. What is a balanced chemical equation? Why should chemical equations be balanced?
Answer: A balanced equation is a type of reaction in which the number of different atoms on both the reactant and product sides are equal.
The chemical equations should be balanced to satisfy the law of conservation of mass.
Q. Translate the following statements into chemical equations and then balance them.
(a) Hydrogen gas combines with nitrogen to form ammonia.
(b) Hydrogen sulphide gas burns in air to give water and sulphur dioxide.
(c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.
(d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.
(a) 3H2 (g) + N2 (g) → 2NH3 (g)
(b) H2S (g) + 3O2 (g) → SO2 (g) + 2H2O(l)
(c) 3BaCl2 (aq) + Al2(SO4)3 (aq) → 2AlCl3 (aq) + 3BaSO4 ↓(s)
(d) 2K (s) + 2H2O (l) → 2KOH (aq) + H2 (g)
Q. Balance the following chemical equations :
(a) HNO3 + Ca (OH)2 → Ca (NO3)2 + H2O
(b) NaOH + H2SO4 → Na2SO4 + H2O
(c) NaCl + AgNO3 → AgCl + NaNO3
(d) BaCl2 + H2SO4 → BaSO4 + HCl
(a) 2HNO3 + Ca(OH)2 → Ca(NO3)2 + 2H2O
(b) 2NaOH + H2SO4 → Na2SO4 + 2H2O
(c) NaCl + AgNO3 → AgCl + NaNO3
(d) BaCl2 + H2SO4 → BaSO4 + 2HCl
Q. Write the balanced chemical equations for the following reactions:
(a) Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water
(b) Zinc + Silver nitrate → Zinc nitrate + Silver
(c) Aluminium + Copper chloride → Aluminium chloride + Copper
(d) Barium chloride + Potassium sulphate → Barium sulphate + Potassium chloride
(a) Ca (OH)2 + CO2 → CaCO3 + H2O
(b) Zn + 2AgNO3 → Zn(NO3)2 + 2 Ag
(c) 2Al + 3 CuCl2 → 2AlCl3 + 3 Cu
(d) BaCl2 + K2SO4 → BaSO4 + 2KCl
Q: Write the balanced chemical equation for the following and identify the type of reaction in each case:
(a) Potassium bromide (aq) + Barium iodide (aq) → Potassium iodide (aq) + Barium
(b) Zinc carbonate(s) → Zinc oxide (s) + Carbon dioxide (g)
(c) Hydrogen (g) + Chloride (g) → Hydrogen chloride (g)
(d) Magnesium (s) + Hydrochloric acid (aq) → Magnesium chloride (aq) + Hydrogen (g)
(a) 2KBr (aq) + Bal2(aq) → 2Kl(aq) + BaBr2(s)
Type : Double displacement reaction
(b) ZnCO3 (s) → ZnO (s) + CO2 (g)
Type : Decomposition reaction
(c) H2 (g) + Cl2 (g) → 2HCl(g)
Type : Combination reaction
(d) Mg (s) + 2HCl (aq) → MgCl2 (aq) + H2 (g)
Type : Displacement reaction
Q9: What does one mean by exothermic and endothermic reactions ? Give examples.
Exothermic reactions: Those reactions in which heat is evolved are known as exothermic reactions.
(i) C (s) + O2 (g) → CO2 (g) + Heat
(ii) N2 (g) + 3H2 (g) → 2NH3 (g) + Heat
Endothermic reactions: Those reactions in which heat is absorbed are known as endothermic reactions.
(i) C (s) + 2S (s) + Heat → CS2 (l)
(ii) N2 (g) + O2 (g) + Heat → 2NO(g)
Q10. Why is respiration considered an exothermic reaction? Explain
Ans: A chemical reaction in which heat/energy is produced along with the products is called an exothermic reaction.
During digestion, food is broken down into simpler substances called glucose.
Glucose combines with the oxygen present in our body and provides energy. This is known as respiration.
Since it involves the release of energy along with the products, it is an exothermic process.
The chemical reaction involved in respiration is
C 6 H 12 O 6 (aq) + 6O 2 (aq) → 6CO 2 (aq) + 6H 2 O(l) + energy
Q11. Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.
Ans: During the decomposition reaction, a single substance decomposes and forms two or many substances. Whereas in a combination reaction, two or more reactants combine and form a single substance. Hence, a decomposition reaction is called the opposite of a combination reaction.
Examples of decomposition and combination reactions are given below:
Decomposition Reaction: CaCO3(S) + CuSO4(aq) →ZnSO4(aq) + Cu(S)
Combination Reaction: CaO(S) + H2O (l)→Ca(OH)2 (aq)
Q12: Write one equation each for the decomposition reactions where energy is supplied in the form of heat, light or electricity.
Q13. What is the difference between displacement and double displacement reactions? Write equations for these reactions.
|Displacement reaction||Double displacement reaction|
|In a displacement reaction, a more reactive element replaces a less reactive element from a compound.||In a double displacement reaction, two atoms or a group of atoms switch places to form new compounds.|
|General equation: |
A + BX → AX + B
|General equation: |
AB + CD → AD + CB
|The precipitate is not formed.||The precipitate is formed.|
Q: In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the reaction involved.
Q. What do you mean by a precipitation reaction? Explain by giving examples.
Ans: A reaction in which an insoluble solid precipitate is formed is called Precipitation Reaction
- When Sodium Sulphate solution is mixed with Barium Chloride solution it forms Barium Sulphate and Sodium Chloride solution. In this reaction, a white precipitate of Barium Sulphate is formed.
Na 2 SO 4 (aq) + BaCl 2 (aq) → BaSO 4 (s) + 2NaCl (aq)
- When Potassium Iodide solution is added to Lead Nitrate solution, a yellow precipitate of Lead Iodide is formed along with Potassium Nitrate solution
KI + Pb(NO 3 ) 2 → KNO 3 + PbI 2