Chemical Reactions and Equations NCERT Solutions (Class 10)
Get complete NCERT In-text and Exercise Solutions for Class 10 Science Chapter 1. These answers are written as per CBSE marking scheme to help students score full marks in exams.
NCERT In-text Questions (Page 5)
Q1. Why should a magnesium ribbon be cleaned before burning in air?
Magnesium ribbon is cleaned to remove the layer of magnesium oxide (MgO) formed on its surface due to reaction with oxygen in air. This layer prevents proper burning, so cleaning ensures smooth combustion with a bright white flame.
NCERT In-text Questions (Page 6)
Q2. Write the balanced equation for the following chemical reactions:
(i) Hydrogen + Chlorine → Hydrogen chloride
(ii) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride
(iii) Sodium + Water → Sodium hydroxide + Hydrogen
(i) H₂ + Cl₂ → 2HCl
(ii) 3BaCl₂ + Al₂(SO₄)₃ → 3BaSO₄ + 2AlCl₃
(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 sodium chloride solution.
(ii) Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water.
(i) BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq)
(ii) NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)
NCERT In-text Questions (Page 10)
Q1. A solution of a substance ‘X’ is used for whitewashing.
(i) Name the substance ‘X’ and write its formula.
(ii) Write the reaction of the substance ‘X’ named in (i) above with water.
(i) The substance ‘X’ is Calcium Oxide (quicklime). Its chemical formula is CaO.
(ii) When calcium oxide reacts with water, it forms calcium hydroxide (slaked lime) and releases heat (exothermic reaction):
CaO(s) + H₂O(l) → Ca(OH)₂ + Heat
Q2. Why is the amount of gas collected in one of the test tubes in Activity 1.7 double of the amount collected in the other? Name this gas.
In Activity 1.7 (electrolysis of water), water decomposes into hydrogen and oxygen gases.
2H₂O → 2H₂ + O₂
From the equation, hydrogen gas is produced in double the amount compared to oxygen gas.
Therefore, the gas collected in double amount is Hydrogen (H₂).
NCERT In-text Questions (Page 13)
Q1. Why does the colour of copper sulphate solution change when an iron nail is dipped in it?
When an iron nail is dipped in copper sulphate solution, a displacement reaction takes place. Iron is more reactive than copper, so it displaces copper from the solution.
Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)
As a result, the blue colour of copper sulphate solution fades and turns green due to the formation of iron(II) sulphate (FeSO₄).
Q2. Give an example of a double displacement reaction other than the one given in Activity 1.10.
An example of a double displacement reaction is:
Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq)
In this reaction, exchange of ions takes place and a white precipitate of barium sulphate is formed.
Q3. Identify the substances that are oxidised and the substances that are reduced in the following reactions:
(i) 4Na(s) + O₂(g) → 2Na₂O(s)
(ii) CuO(s) + H₂(g) → Cu(s) + H₂O(l)
(i) Sodium (Na) is oxidised because it gains oxygen.
Oxygen (O₂) is reduced.
(ii) Hydrogen (H₂) is oxidised because it gains oxygen.
Copper oxide (CuO) is reduced to copper (Cu).
NCERT Exercise Solutions
Q1. Which of the statements about the reaction below are incorrect?
2PbO(s) + C(s) → 2Pb(s) + CO₂(g)
Correct Answer: (ii) (a) and (b)
Explanation:
(a) Lead is getting reduced ✘ (Incorrect — PbO is reduced, not Pb)
(b) Carbon dioxide is getting oxidised ✘ (Incorrect — CO₂ is already fully oxidised)
(c) Carbon is getting oxidised ✔
(d) Lead oxide is getting reduced ✔
Therefore, incorrect statements are (a) and (b).
Q2. Fe₂O₃ + 2Al → Al₂O₃ + 2Fe
The above reaction is an example of:
Correct Answer: Displacement Reaction
Explanation:
Aluminium displaces iron from iron oxide because aluminium is more reactive than iron. Hence, it is a displacement reaction.
Q3. What happens when dilute hydrochloric acid is added to iron filings?
Correct Answer: (a) Hydrogen gas and iron chloride are produced.
Explanation:
When iron reacts with dilute hydrochloric acid, hydrogen gas is evolved and iron(II) chloride is formed.
Fe + 2HCl → FeCl₂ + H₂↑
Q4. What is a balanced chemical equation? Why should chemical equations be balanced?
A balanced chemical equation is one in which the number of atoms of each element is equal on both sides of the equation.
Reason:
Chemical equations must be balanced to satisfy the Law of Conservation of Mass, which states that mass can neither be created nor destroyed in a chemical reaction.
Q5. Translate the following statements into chemical equations and balance them:
(a) Hydrogen gas combines with nitrogen to form ammonia:
N₂ + 3H₂ → 2NH₃
(b) Hydrogen sulphide gas burns in air to give water and sulphur dioxide:
2H₂S + 3O₂ → 2H₂O + 2SO₂
(c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and barium sulphate:
3BaCl₂ + Al₂(SO₄)₃ → 3BaSO₄ + 2AlCl₃
(d) Potassium reacts with water to give potassium hydroxide and hydrogen gas:
2K + 2H₂O → 2KOH + H₂
Q6. Balance the following chemical equations:
(a) HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + H₂O
= 2HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + 2H₂O
(b) NaOH + H₂SO₄ → Na₂SO₄ + H₂O
= 2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O
(c) NaCl + AgNO₃ → AgCl + NaNO₃
= NaCl + AgNO₃ → AgCl + NaNO₃
(d) BaCl₂ + H₂SO₄ → BaSO₄ + HCl
= BaCl₂ + H₂SO₄ → BaSO₄ + 2HCl
Q7. Write the balanced chemical equations for the following reactions:
(a) Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water
= Ca(OH)₂ + CO₂ → CaCO₃ + H₂O
(b) Zinc + Silver nitrate → Zinc nitrate + Silver
= Zn + 2AgNO₃ → Zn(NO₃)₂ + 2Ag
(c) Aluminium + Copper chloride → Aluminium chloride + Copper
= 2Al + 3CuCl₂ → 2AlCl₃ + 3Cu
(d) Barium chloride + Potassium sulphate → Barium sulphate + Potassium chloride
= BaCl₂ + K₂SO₄ → BaSO₄ + 2KCl
Q8. Write the balanced chemical equation and identify the type of reaction:
(a) 2KBr + BaI₂ → 2KI + BaBr₂
(Type: Double Displacement Reaction)
(b) ZnCO₃ → ZnO + CO₂
(Type: Decomposition Reaction)
(c) H₂ + Cl₂ → 2HCl
(Type: Combination Reaction)
(d) Mg + 2HCl → MgCl₂ + H₂
(Type: Displacement Reaction)
Q9. What does one mean by exothermic and endothermic reactions? Give examples.
Exothermic reactions: Reactions in which heat is released.
Example: CH₄ + 2O₂ → CO₂ + 2H₂O + heat
Endothermic reactions: Reactions in which heat is absorbed.
Example: CaCO₃ → CaO + CO₂
Q10. Why is respiration considered an exothermic reaction?
Respiration is exothermic because energy is released when glucose reacts with oxygen.
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy
This energy is used for life processes like movement and growth.
Q11. Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.
Decomposition reactions are called the opposite of combination reactions because:
- In a combination reaction, two or more substances combine to form a single product.
- In a decomposition reaction, a single compound breaks down into two or more simpler substances.
Example of Combination Reaction:
CaO + H₂O → Ca(OH)₂
Example of Decomposition Reaction:
CaCO₃ → CaO + CO₂
Q12. Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.
(i) Thermal Decomposition (Heat):
CaCO₃ → CaO + CO₂
(ii) Photochemical Decomposition (Light):
2AgCl → 2Ag + Cl₂
(iii) Electrolytic Decomposition (Electricity):
2H₂O → 2H₂ + O₂
Q13. What is the difference between displacement and double displacement reactions? Write equations for these reactions.
Displacement Reaction: A more reactive element displaces a less reactive element from its compound.
Example: Zn + CuSO₄ → ZnSO₄ + Cu
Double Displacement Reaction: Two compounds exchange their ions to form two new compounds.
Example: Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl
Q14. In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the reaction involved.
Cu + 2AgNO₃ → Cu(NO₃)₂ + 2Ag
In this reaction, copper displaces silver from silver nitrate solution, and pure silver is obtained.
Q15. What do you mean by a precipitation reaction? Explain by giving examples.
A precipitation reaction is a reaction in which two aqueous solutions react to form an insoluble solid called a precipitate.
Example:
BaCl₂ + Na₂SO₄ → BaSO₄↓ + 2NaCl
Here, BaSO₄ is a white insoluble precipitate.
Q16. Explain the following in terms of gain or loss of oxygen with two examples each: (a) Oxidation (b) Reduction
(a) Oxidation: It is the process in which oxygen is gained by a substance.
Examples:
1. 2Mg + O₂ → 2MgO (Magnesium gains oxygen)
2. C + O₂ → CO₂ (Carbon gains oxygen)
(b) Reduction: It is the process in which oxygen is removed from a substance.
Examples:
1. CuO + H₂ → Cu + H₂O (Copper oxide loses oxygen)
2. Fe₂O₃ + 3CO → 2Fe + 3CO₂ (Iron oxide loses oxygen)
Q17. A shiny brown coloured element ‘X’ on heating in air becomes black in colour. Name the element ‘X’ and the black coloured compound formed.
Element ‘X’ is Copper (Cu).
On heating in air, copper reacts with oxygen to form black coloured copper(II) oxide (CuO).
Reaction:
2Cu + O₂ → 2CuO
Q18. Why do we apply paint on iron articles?
Paint is applied on iron articles to prevent rusting.
The paint layer acts as a protective barrier and prevents contact of iron with air (oxygen) and moisture, which are necessary for rusting.
Q19. Oil and fat containing food items are flushed with nitrogen. Why?
Oil and fat containing food items are flushed with nitrogen to prevent rancidity.
Nitrogen is an inert gas and prevents oxidation of fats and oils, thereby increasing the shelf life of food.
Q20. Explain the following terms with one example each: (a) Corrosion (b) Rancidity
(a) Corrosion: It is the gradual destruction of metals due to reaction with air, moisture or chemicals.
Example: Rusting of iron (formation of Fe₂O₃·xH₂O)
(b) Rancidity: It is the oxidation of fats and oils in food leading to unpleasant smell and taste.
Example: Spoiling of butter or oil when left exposed to air for a long time.
