Class 10 Science Chapter 1 Chemical Reactions & Equations

Q1. Which of the following is NOT required for balancing a chemical equation?
(a) Counting atoms of each element
(b) Changing the formulae of reactants
(c) Using coefficients
(d) Following law of conservation of mass

Answer: (b) Changing the formulae of reactants
Explanation: While balancing a chemical equation, we can only change the coefficients (numbers before formulae). We must never change the formulae of reactants or products as this would represent a different chemical substance.

Q2. The decomposition of vegetable matter to form compost is an example of:
(a) Endothermic reaction
(b) Exothermic reaction
(c) Displacement reaction
(d) Combination reaction

Answer: (b) Exothermic reaction
Explanation: Decomposition of vegetable matter to compost releases heat energy, making it warm. Hence it is an exothermic reaction. This is why compost pits feel warm.

Q3. Which of the following represents the correct product of the reaction between calcium hydroxide and carbon dioxide?
(a) CaO + H₂O
(b) CaCO₃ + H₂O
(c) Ca(HCO₃)₂
(d) CaCl₂ + H₂O

Answer: (b) CaCO₃ + H₂O
Explanation: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. Calcium hydroxide reacts with carbon dioxide to form
calcium carbonate (used in whitewashing) and water.

Q4. When water is added to quick lime (CaO), which observation is made?
(a) Temperature decreases
(b) Temperature increases
(c) No change in temperature
(d) Precipitate is formed

Answer: (b) Temperature increases
Explanation: CaO + H₂O → Ca(OH)₂ + Heat. This is an exothermic reaction. The beaker becomes hot due
to release of heat energy.

Q5. The reaction Fe₂O₃ + 2Al → Al₂O₃ + 2Fe is an example of:
(a) Combination reaction
(b) Double displacement reaction
(c) Decomposition reaction
(d) Displacement reaction

Answer: (d) Displacement reaction
Explanation: Aluminium being more reactive displaces iron from iron oxide (Fe₂O₃). This is a displacement reaction (also called thermite reaction).

Q6. Food items become rancid due to:
(a) Reduction
(b) Oxidation
(c) Corrosion
(d) Displacement

Answer: (b) Oxidation
Explanation: Rancidity occurs when fats and oils in food items get oxidized by oxygen present in air. This changes their taste and smell. Oxidation is the main cause of rancidity.

Q7. In the reaction MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂, which element gets oxidized?
(a) Mn
(b) Cl
(c) H
(d) O

Answer: (b) Cl
Explanation: HCl is oxidized to Cl₂. Chlorine changes from -1 oxidation state in HCl to 0 in Cl₂. Loss of hydrogen (or gain of positive charge) is oxidation.

Q8. The type of reaction represented by: AB → A + B is:
(a) Combination
(b) Decomposition
(c) Displacement
(d) Double displacement

Answer: (b) Decomposition
Explanation: In decomposition reactions, a single compound (AB) breaks down into two or more simpler substances (A and B). This is the general form of decomposition.

Q9. Which of the following methods can be used to prevent rusting of iron?
(a) Galvanization
(b) Painting
(c) Oiling
(d) All of the above

Answer: (d) All of the above
Explanation: All three methods - galvanization (zinc coating), painting, and oiling - prevent rusting by stopping contact of iron with oxygen and moisture, which are necessary for rusting.

Q10. The formation of slaked lime from quick lime is:
(a) Exothermic and displacement
(b) Endothermic and combination
(c) Exothermic and combination
(d) Endothermic and decomposition

Answer: (c) Exothermic and combination
Explanation: CaO + H₂O → Ca(OH)₂ + Heat. Two substances combine to form one product (combination)
and heat is released (exothermic).

Q11. What is meant by a chemical equation? Why is it necessary to balance it?

Answer: Chemical Equation: A chemical equation is a symbolic representation of a chemical reaction in which the formulae of reactants are written on the left side and formulae of products on the right side, separated by an arrow. Necessity of balancing:
• To follow the Law of Conservation of Mass
• To ensure that the number of atoms of each element remains the same before and after the reaction
• To represent the correct stoichiometric ratios of reactants and products
• To make the equation scientifically correct and meaningful

Q12. When a green iron salt is heated, it changes to brown and a gas having smell of burning sulphur is evolved. Name the iron salt and write the balanced equation.

Answer: Name of iron salt: Ferrous sulphate (FeSO₄.7H₂O) - also called Green vitriol Observations:
• Green crystals change to brown (ferric oxide)
• Smell of burning sulphur (due to SO₂ and SO₃ gases) Balanced equation:
2FeSO (s) Heat→ Fe O (s) + SO (g) + SO (g)
4 2 3 2 3 (Green) (Brown)

Q13. Give one example each of thermal, electrolytic and photolytic decomposition with balanced equations.

Answer: 1. Thermal Decomposition: (Decomposition by heat)
CaCO (s) Heat→ CaO(s) + CO (g)
3 2 Calcium carbonate decomposes on heating to form calcium oxide and carbon dioxide.
2. Electrolytic Decomposition: (Decomposition by electricity)
2H O(l) Electricity→ 2H (g) + O (g)
2 2 2 Water decomposes into hydrogen and oxygen when electric current is passed through it.
3. Photolytic Decomposition: (Decomposition by light)
2AgCl(s) Suⁿlight→ 2Ag(s) + Cl (g)
2 Silver chloride decomposes in sunlight to form silver and chlorine gas.

Q14. Define:
(a) Oxidation
(b) Reduction

Answer: (a) Oxidation: Oxidation is a chemical reaction in which:
• A substance gains oxygen, OR
• A substance loses hydrogen
Example: 2Cu + O₂ → 2CuO (Copper gains oxygen)
(b) Reduction: Reduction is a chemical reaction in which:
• A substance loses oxygen, OR
• A substance gains hydrogen
Example: CuO + H₂ → Cu + H₂O (Copper oxide loses oxygen)

Q15. What is corrosion? How does it damage iron articles? Suggest three methods to prevent corrosion of iron.

Answer: Corrosion: Corrosion is the process in which metals are gradually eaten away or damaged by the action of substances around them such as moisture, acids, and air. How corrosion damages iron:
• Iron reacts with oxygen and moisture present in air
• It forms hydrated iron(III) oxide (Fe₂O₃.xH₂O) called rust
• Rust is reddish-brown, soft and porous
• It weakens the iron structure and causes holes
• The article eventually breaks down and loses its strength
• Every year enormous amount of money is spent to replace damaged iron Methods to prevent corrosion:
1. Galvanization:
• Coating iron with a layer of zinc
• Zinc is more reactive and gets oxidized first
• This protects iron from rusting
2. Painting:
• Applying paint creates a protective layer
• Prevents contact of iron with air and moisture
• Regular painting keeps iron protected
3. Making Alloys:
• Mixing iron with chromium and nickel forms stainless steel
• Stainless steel does not rust
• Used in cutlery, utensils, and surgical instruments

Q16. (a) What is a precipitation reaction? Give two examples.
(b) Why are precipitation reactions important in everyday life?

Answer: (a) Precipitation Reaction: A precipitation reaction is a type of double displacement reaction in which two solutions react to form an insoluble solid (precipitate) that separates from the solution. Examples:
1. AgNO₃(aq) + NaCl(aq) → AgCl(s)↓ + NaNO₃(aq)
White precipitate of silver chloride is formed
2. Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s)↓ + 2KNO₃(aq)
Yellow precipitate of lead iodide is formed
(b) Importance in everyday life:
• Water purification: Precipitation is used to remove unwanted ions from water
• Medical diagnosis: Certain tests use precipitation to detect diseases
• Photography: Silver halides precipitate in photographic films
• Manufacturing: Production of various chemicals and pigments
• Qualitative analysis: Identifying ions present in a solution

Q17. Explain the activity to show that "In a chemical reaction, mass is neither created nor destroyed." Draw the diagram and write the conclusion.

Answer: Activity to demonstrate Law of Conservation of Mass: Materials Required:
• Conical flask with cork
• Small test tube
• Barium chloride solution
• Sodium sulphate solution
• Physical balance Procedure:
1. Take a conical flask and pour barium chloride solution into it
2. Take sodium sulphate solution in a small test tube
3. Carefully place the test tube inside the conical flask
4. Cork the flask tightly so that nothing can escape
5. Weigh the flask with its contents on a physical balance and record the mass (M₁)
6. Now tilt the flask so that the two solutions mix
7. White precipitate of barium sulphate is formed
8. Weigh the flask again and record the mass (M₂) Observation:
Mass before reaction (M₁) = Mass after reaction (M₂)
Chemical Equation:
BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s)↓ + 2NaCl(aq)
Conclusion:
• The total mass remains constant before and after the chemical reaction
• This proves the Law of Conservation of Mass
• Mass is neither created nor destroyed in a chemical reaction
• Atoms are only rearranged during a chemical reaction

Q18. (a) What are redox reactions? Explain with suitable examples.
(b) Identify the substances oxidized and reduced in the following reactions:
(i) ZnO + C → Zn + CO
(ii) CuO + H₂ → Cu + H₂O
(iii) 4Na + O₂ → 2Na₂O

Answer: (a) Redox Reactions: Redox reactions are chemical reactions in which oxidation and reduction occur simultaneously. The term "redox" is derived from "reduction-oxidation".
• One substance loses oxygen or gains hydrogen (gets reduced)
• Another substance gains oxygen or loses hydrogen (gets oxidized)
• Both processes occur together in the same reaction Example 1:
CuO(s) + H₂(g) → Cu(s) + H₂O(l)
• CuO loses oxygen → Reduction
• H₂ gains oxygen → Oxidation
Example 2:
ZnO(s) + C(s) → Zn(s) + CO(g)
• ZnO loses oxygen → Reduction
• C gains oxygen → Oxidation
(b) Identifying oxidized and reduced substances:
(i) ZnO + C → Zn + CO
• Reduced substance: ZnO (loses oxygen to form Zn)
• Oxidized substance: C (gains oxygen to form CO)
(ii) CuO + H₂ → Cu + H₂O
• Reduced substance: CuO (loses oxygen to form Cu)
• Oxidized substance: H₂ (gains oxygen to form H₂O)
(iii) 4Na + O₂ → 2Na₂O
• Reduced substance: O₂ (gains electrons from Na, though this is better explained as gaining sodium)
• Oxidized substance: Na (gains oxygen to form Na₂O)

Q19. Case Study 1: During a laboratory experiment, a student mixed solutions of lead nitrate and potassium iodide in a beaker. He observed the formation of a yellow precipitate which settled at the bottom. The other product remained dissolved in the solution. Answer the following:
(a) Name the yellow precipitate formed. (1 mark)
(b) Write the balanced chemical equation for this reaction. (1 mark)
(c) What type of chemical reaction is this? (1 mark)
(d) Why is the reaction also called a precipitation reaction? (1 mark)

Answer: (a) The yellow precipitate formed is Lead iodide (PbI₂).
(b) Balanced chemical equation:
Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s)↓ + 2KNO₃(aq)
(Lead nitrate) (Potassium iodide) (Yellow ppt) (Potassium nitrate)
(c) Type of reaction: This is a double displacement reaction. In this reaction, Pb²⁺ and K⁺ exchange their anions (NO ⁻ and I⁻) to form new compounds. 3
(d) This reaction is called a precipitation reaction because:
• One of the products (PbI₂) is insoluble in water
• It separates out as a solid precipitate
• The precipitate settles at the bottom of the container
• Any double displacement reaction that produces an insoluble product is called a precipitation reaction

Q20. Case Study 2: A biology teacher was explaining to students about how plants prepare their food through photosynthesis. She told them that this process requires sunlight, carbon dioxide from air and water from soil. The process produces glucose (food) and oxygen. She also mentioned that this reaction is essential for life on Earth as it provides oxygen and food. Based on this information, answer:
(a) Write the balanced chemical equation for photosynthesis. (1 mark)
(b) Is photosynthesis an exothermic or endothermic process? Justify. (1 mark)
(c) Name the catalyst involved in this reaction. (1 mark)
(d) What is the source of oxygen released during photosynthesis? (1 mark) Made with ❤ by Sumeet Sahu Website: uniquestudyonline.com

Answer: (a) Balanced equation for photosynthesis:
6CO (g) + 6H O(l) Suⁿlight→ C H O (aq) + 6O (g)
2 2 Chlorophyll 6 12 6 2 (Carbon dioxide) (Water) (Glucose) (Oxygen)
(b) Photosynthesis is an endothermic process. Justification:
• It requires energy in the form of sunlight to proceed
• Light energy is absorbed by chlorophyll
• This energy is used to break chemical bonds in CO₂ and H₂O
• Energy is stored in the glucose molecules formed
• Without continuous supply of light energy, the reaction stops
• Energy is absorbed from surroundings (sunlight), not released
(c) The catalyst involved is Chlorophyll (the green pigment present in leaves).
(d) The source of oxygen released during photosynthesis is water (H₂O).
• Water molecules are split during photosynthesis
• Oxygen from water is released as O₂ gas
• Hydrogen from water is used to make glucose
• This has been proved by experiments using water labeled with oxygen-18 isotope Made with ❤ by Sumeet Sahu Website: uniquestudyonline.com