Class 10 Science Chapter 2 Acids, Bases and Salts Practice Paper 3

Class: X Subject: Science Session: 2025-26 Chapter: 02 - Acids, Bases and Salts Time: 1½ Hours Max. Marks: 40

General Instructions:

1. All questions are compulsory.

2. This question paper contains 20 questions divided into five sections A, B, C, D and E.

3. Section A contains 10 MCQs of 1 mark each.

4. Section B contains 4 questions of 2 marks each.

5. Section C contains 3 questions of 3 marks each.

6. Section D contains 1 question of 5 marks.

7. Section E contains 2 Case Study Based questions of 4 marks each.

SECTION A - Multiple Choice Questions (1 mark each)

Q1. Which of the following is used for whitewashing walls?
(a) Calcium oxide
(b) Calcium hydroxide
(c) Calcium carbonate
(d) Calcium chloride

Q2. Turmeric stain on white shirt turns reddish-brown when washed with soap because:
(a) Soap is acidic
(b) Soap is basic
(c) Soap is neutral
(d) Turmeric changes color naturally

Q3. Which of the following gases is produced when dilute HCl reacts with sodium carbonate?
(a) Hydrogen
(b) Oxygen
(c) Carbon dioxide
(d) Sulphur dioxide

Q4. The pH of lemon juice is approximately:
(a) 2.2
(b) 7.0
(c) 10.5
(d) 14.0

Q5. Which of the following is a use of washing soda?
(a) Making antacids
(b) Removing permanent hardness of water
(c) Making plaster for fractured bones
(d) Making baking powder

Q6. Sodium hydroxide solution is:
(a) Soapy to touch and bitter in taste
(b) Sour in taste
(c) Sweet in taste
(d) Saltish in taste

Q7. Metal compound A reacts with dilute HCl to produce effervescence. The gas evolved extinguishes a burning candle. The metal compound A is:
(a) Metal oxide
(b) Metal carbonate
(c) Metal sulphate
(d) Metal chloride

Q8. When concentrated sulphuric acid is added to water, the process is:
(a) Endothermic
(b) Exothermic
(c) Neither exothermic nor endothermic
(d) Sometimes exothermic, sometimes endothermic

Q9. Acid rain occurs when the pH of rainwater is:
(a) More than 7
(b) Equal to 7
(c) Less than 5.6
(d) Between 6 and 7

Q10. Number of water molecules in plaster of Paris is:
(a) 2
(b) 1
(c) ½
(d) 5

SECTION B - Short Answer Questions (2 marks each)

Q11. While diluting an acid, why is it recommended that acid should be added to water and not water to acid?

Q12. How is the concentration of hydroxide ions affected when excess base is dissolved in a solution of sodium hydroxide?

Q13. Name the gas evolved when dilute HCl reacts with sodium hydrogencarbonate. How will you test for this gas?

Q14. What is the common name of Ca(ClO)₂? Give one use of this compound.

SECTION C - Short Answer Questions (3 marks each)

Q15. A farmer has a field where the soil is acidic. Suggest two chemical compounds which he can add to neutralize the excess acid. Explain your answer with equations.

Q16. How is washing soda prepared from baking soda? Write the chemical equations involved. Give two uses of washing soda.

Q17. What is meant by dilution? Explain why mixing concentrated acid or base with water is an exothermic process.

SECTION D - Long Answer Question (5 marks)

Q18.
(a) Explain why metallic oxides are called basic oxides and non-metallic oxides are called acidic oxides. Give one example of each.
(b) Write the balanced chemical equations for: (i) Copper oxide reacting with dilute HCl (ii) Carbon dioxide reacting with calcium hydroxide

SECTION E - Case Study Based Questions (4 marks each)

Q19. Case Study 1: Limestone, chalk and marble are different forms of calcium carbonate (CaCO₃). All metal carbonates and hydrogencarbonates react with acids to give corresponding salt, carbon dioxide and water. When carbon dioxide is passed through lime water, it turns milky. On passing excess CO₂, the milky color disappears due to formation of soluble calcium hydrogencarbonate.
(a) What are the different forms of calcium carbonate? (1 mark)
(b) Write the equation for reaction of calcium carbonate with HCl. (1 mark)
(c) Why does lime water turn milky when CO₂ is passed through it? (1 mark)
(d) Why does the milkiness disappear on passing excess CO₂? Write the equation. (1 mark)

Q20. Case Study 2: Bee sting and nettle sting contain methanoic acid which causes pain and irritation. Traditional remedies for such stings include rubbing the affected area with baking soda solution or the leaf of dock plant. These remedies work on the principle of neutralization. Plants require specific pH range for their healthy growth.
(a) Which acid is present in bee sting and nettle sting? (1 mark)
(b) Why does baking soda solution provide relief from bee sting? (1 mark)
(c) What is the nature of dock plant leaf? (1 mark)
(d) Why do plants require specific pH range for growth? (1 mark) DETAILED ANSWER KEY - PAPER 03

SECTION A - Answers to MCQs

Ans 1.
(b) Calcium hydroxide Calcium hydroxide [Ca(OH)₂], also called slaked lime, is used for whitewashing walls. It reacts with CO₂ from air to form calcium carbonate (CaCO₃) which gives a shiny white finish: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O Ans 2.
(b) Soap is basic Turmeric is a natural indicator. It is yellow in acidic and neutral medium but turns reddish-brown in basic medium. Since soap is basic in nature, it turns turmeric stain reddish-brown. This demonstrates that turmeric can be used as an acid-base indicator.

Ans 3.
(c) Carbon dioxide When dilute HCl reacts with sodium carbonate, carbon dioxide gas is evolved: Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂↑ This CO₂ can be tested by passing it through lime water which turns milky. Ans 4.
(a) 2.2 Lemon juice contains citric acid and is highly acidic with pH around 2.2. This is much less than 7, indicating high H⁺ ion concentration. Other acidic solutions include gastric juice (pH ~1.2) and vinegar (pH ~2.4). Ans 5.
(b) Removing permanent hardness of water Washing soda (Na₂CO₃.10H₂O) is used to remove permanent hardness of water by precipitating calcium and magnesium salts. It is also used in glass, soap and paper industries, and in manufacturing sodium compounds like borax.

Ans 6.
(a) Soapy to touch and bitter in taste Sodium hydroxide (NaOH) is a strong base. All bases are soapy to touch and bitter in taste. However, we should never taste or touch strong bases as they are corrosive and can cause severe burns. Ans 7.
(b) Metal carbonate Metal carbonates react with dilute acids to produce effervescence (bubbles) due to CO₂ evolution. CO₂ extinguishes burning candle. Example: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂↑. This is a test for carbonates. Ans 8.
(b) Exothermic Dissolving concentrated sulphuric acid in water is a highly exothermic process that releases large amount of heat. This is why acid should always be added slowly to water with stirring, never water to acid, to prevent dangerous splashing and container breakage.

Ans 9.
(c) Less than 5.6 Normal rainwater is slightly acidic (pH ~5.6) due to dissolved CO₂. When pH falls below 5.6 due to dissolved acidic gases like SO₂ and NO₂ from pollution, it is called acid rain. It is harmful for aquatic life, buildings, and plants. Ans 10.
(c) ½ Plaster of Paris has the formula CaSO₄.½H₂O. The ½ water molecule means two formula units of CaSO₄ share one molecule of water. It is obtained by heating gypsum (CaSO₄.2H₂O) at 373 K.

SECTION B - Answers to Short Answer Questions

Ans 11. Dilution of Acid - Safety Precaution Acid should always be added to water and never water to acid because:

1. Highly exothermic process: Dissolving concentrated acid in water releases large amount of heat

2. If water is added to acid: • The small amount of water gets heated instantly to very high temperature • This can cause the mixture to splash out violently • The splashing acid can cause severe burns • The glass container may break due to excessive local heating

3. If acid is added to water: • Heat is absorbed by large volume of water • Temperature rise is gradual and controlled • Safe dilution occurs without splashing Warning: Always look for the warning sign on containers of concentrated acids and bases. Ans 12. Effect on OH⁻ Ion Concentration When excess base is dissolved in a solution of sodium hydroxide:

1. More NaOH molecules are added to the solution

2. NaOH dissociates completely: NaOH → Na⁺ + OH⁻

3. This increases the number of OH⁻ ions in the solution

4. The concentration of hydroxide ions (OH⁻) increases

5. As OH⁻ concentration increases, H⁺ concentration decreases

6. The pH of solution increases (becomes more basic)

7. The solution becomes more alkaline Ans 13. Gas Evolution and Test Gas evolved: Carbon dioxide (CO₂) Chemical Equation: NaHCO₃(s) + HCl(aq) → NaCl(aq) + H₂O(l) + CO₂(g) Test for CO₂ gas:

1. Pass the evolved gas through lime water [Ca(OH)₂ solution]

2. If the gas is CO₂, lime water will turn milky

3. This happens due to formation of insoluble calcium carbonate: Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l) (Lime water) (White precipitate - milky)

4. On passing excess CO₂, milkiness disappears due to formation of soluble calcium hydrogencarbonate Ans 14. Common Name and Use Common name: Ca(ClO)₂ is commonly called Bleaching powder Chemical name: Calcium hypochlorite (though actual composition is complex) One important use: Bleaching powder is used for disinfecting drinking water to make it free from germs. When added to water, it releases chlorine which kills harmful bacteria and other microorganisms. Other uses: Bleaching cotton and linen in textile industry, bleaching wood pulp in paper factories, as an oxidizing agent in chemical industries.

SECTION C - Answers to Short Answer Questions

Ans 15. Neutralizing Acidic Soil Two chemical compounds to neutralize acidic soil:

1. Slaked lime [Calcium hydroxide - Ca(OH)₂]: • It is a base that neutralizes acids in soil • Reaction: Ca(OH)₂ + 2H⁺ → Ca²⁺ + 2H₂O • Being a strong base, it effectively raises soil pH • Easily available and economical

2. Quick lime (Calcium oxide - CaO): • Reacts with water to form Ca(OH)₂: CaO + H₂O → Ca(OH)₂ • Then Ca(OH)₂ neutralizes soil acidity • Overall reaction: CaO + 2H⁺ → Ca²⁺ + H₂O Alternative - Chalk (Calcium carbonate - CaCO₃): • Reacts with acid to neutralize it • CaCO₃ + 2H⁺ → Ca²⁺ + H₂O + CO₂↑ • Milder than CaO or Ca(OH)₂ • Safer to use in large quantities Process called: This treatment of soil is called liming. It is essential because most plants require pH between 6-7 for optimal growth. Acidic soil (pH < 6) hampers nutrient availability and plant growth.

Ans 16. Preparation of Washing Soda Preparation from Baking Soda: Step 1: Baking soda (sodium hydrogencarbonate) is heated: Heat 2NaHCO₃(s) → Na₂CO₃(s) + H₂O(l) + CO₂(g) (Sodium hydrogencarbonate) → (Sodium carbonate) + Water + Carbon dioxide Step 2: Sodium carbonate is dissolved in water and recrystallized: Na₂CO₃(s) + 10H₂O(l) → Na₂CO₃.10H₂O(s) (Sodium carbonate) → (Washing soda) The 10H₂O represents water of crystallization. Two Uses of Washing Soda:

1. Removing permanent hardness of water: • Reacts with calcium and magnesium salts in hard water • Precipitates them as insoluble carbonates • Makes water soft for washing and other purposes

2. Manufacturing glass: • Important raw material in glass industry • Used in making soda-lime glass • Also used in soap and paper industries Ans 17. Dilution and Exothermic Nature What is Dilution: Dilution is the process of decreasing the concentration of ions (H₃O⁺ or OH⁻) per unit volume by adding more water to an acid or base solution. When an acid or base is diluted: • The number of H₃O⁺ ions (in acid) or OH⁻ ions (in base) remains same • But they are distributed in larger volume of water • Therefore, concentration (ions per unit volume) decreases • The acid becomes less acidic and base becomes less basic Why Mixing is Exothermic:

Mixing concentrated acid or base with water is a highly exothermic process because:

1. Ion-dipole interaction: • Acid or base molecules interact strongly with water molecules • When H⁺ ions from acid (or OH⁻ from base) get hydrated, strong bonds form • Example: H⁺ + H₂O → H₃O⁺ (hydronium ion formation)

2. Energy release: • The process of hydration releases large amount of energy • This energy is released as heat • The heat generated can be so much that water may boil

3. Breaking of molecular structure: • Concentrated acids/bases have organized molecular structure • Adding water breaks this structure • This breaking and new bond formation releases heat Safety Note: Due to this exothermic nature, concentrated acid must always be added slowly to water with constant stirring, never water to acid.

SECTION D - Answer to Long Answer Question

Ans 18. Metallic and Non-metallic Oxides
(a) Why Metallic Oxides are Basic and Non-metallic Oxides are Acidic: Metallic Oxides - Basic Oxides: Metallic oxides are called basic oxides because they react with acids to give salt and water, similar to how bases react with acids: Metal oxide + Acid → Salt + Water Example: Copper oxide (CuO) • CuO is a metallic oxide • It reacts with acids like a base: CuO + 2HCl → CuCl₂ + H₂O CuO + H₂SO₄ → CuSO₄ + H₂O • This behavior is characteristic of bases • Hence, metallic oxides are basic in nature • Other examples: Na₂O, MgO, Al₂O₃, ZnO, Fe₂O₃ Non-metallic Oxides - Acidic Oxides:

Non-metallic oxides are called acidic oxides because they react with bases to give salt and water, similar to how acids react with bases: Non-metal oxide + Base → Salt + Water Example: Carbon dioxide (CO₂) • CO₂ is a non-metallic oxide • It reacts with bases like an acid: CO₂ + 2NaOH → Na₂CO₃ + H₂O CO₂ + Ca(OH)₂ → CaCO₃ + H₂O • This behavior is characteristic of acids • Hence, non-metallic oxides are acidic in nature • Other examples: SO₂, SO₃, NO₂, P₄O₁₀
(b) Balanced Chemical Equations: (i) Copper oxide reacting with dilute HCl:

CuO(s) + 2HCl(aq) → CuCl₂(aq) + H₂O(l) (Black copper oxide) + (Hydrochloric acid) → (Blue-green copper chloride solution) + Water Observation: Black copper oxide dissolves in HCl to form blue-green solution of copper(II) chloride. This demonstrates that metallic oxides react with acids like bases do. (ii) Carbon dioxide reacting with calcium hydroxide: CO₂(g) + Ca(OH)₂(aq) → CaCO₃(s) + H₂O(l) (Carbon dioxide) + (Lime water) → (Calcium carbonate - white precipitate) + Water Observation: CO₂ gas turns lime water milky due to formation of insoluble calcium carbonate. This is used as a test for CO₂. This demonstrates that non-metallic oxides react with bases like acids do.

SECTION E - Answers to Case Study Based Questions

Ans 19. Case Study 1 - Calcium Carbonate Forms
(a) Different forms of calcium carbonate: Calcium carbonate (CaCO₃) exists in three main natural forms:

1. Limestone - Sedimentary rock form

2. Chalk - Soft, porous form

3. Marble - Metamorphic rock form (hardest and most crystalline) All three have the same chemical composition (CaCO₃) but different physical properties.
(b) Reaction of calcium carbonate with HCl: CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g) (Calcium carbonate) + (Hydrochloric acid) → (Calcium chloride) + Water + (Carbon dioxide) This reaction is common to all metal carbonates and is used to test for the presence of carbonate ions.
(c) Why lime water turns milky: Lime water turns milky when CO₂ is passed through it because:

1. CO₂ reacts with calcium hydroxide in lime water

2. Reaction: Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)

3. Calcium carbonate (CaCO₃) formed is insoluble in water

4. It forms a white precipitate suspended in water

5. This white suspension makes the solution appear milky

6. This is used as a confirmatory test for CO₂ gas
(d) Disappearance of milkiness: When excess CO₂ is passed through milky lime water, the milkiness disappears because: Chemical Equation: CaCO₃(s) + H₂O(l) + CO₂(g) → Ca(HCO₃)₂(aq) (Calcium carbonate - insoluble) → (Calcium hydrogencarbonate - soluble) Explanation: • The white precipitate CaCO₃ reacts with excess CO₂ and water • Forms calcium hydrogencarbonate [Ca(HCO₃)₂] • Ca(HCO₃)₂ is soluble in water • As the insoluble precipitate dissolves, milkiness disappears • Solution becomes clear again Ans 20. Case Study 2 - Neutralization in Nature
(a) Acid in bee sting and nettle sting:

Methanoic acid (also called formic acid, HCOOH) is present in both bee sting and nettle sting. When the stinging hair of nettle plant or the sting of a bee pierces the skin, this acid is injected, causing burning pain and irritation.
(b) Why baking soda provides relief: Baking soda (sodium hydrogencarbonate - NaHCO₃) provides relief from bee sting because:

1. Baking soda is a mild base (alkaline substance)

2. Methanoic acid from the sting is acidic

3. When baking soda is applied, neutralization reaction occurs: HCOOH + NaHCO₃ → HCOONa + H₂O + CO₂ (Acid from sting) + (Baking soda - base) → (Salt) + Water + Carbon dioxide

4. The acid is neutralized and converted to harmless salt and water

5. This reduces the acidic effect and provides relief from pain

6. The principle used is: Base neutralizes acid
(c) Nature of dock plant leaf: The dock plant leaf is basic (alkaline) in nature . That's why it provides relief when rubbed on nettle sting - it neutralizes the methanoic acid from the sting. This is a natural example of neutralization. Dock plants often grow near nettle plants in nature, providing a natural remedy.
(d) Why plants require specific pH range: Plants require specific pH range for healthy growth because:

1. Nutrient availability: Different nutrients are available to plants at different pH levels. If pH is too high or too low, essential nutrients become unavailable

2. Enzyme activity: Plant enzymes work best at specific pH. Wrong pH affects metabolic activities

3. Microbial activity: Beneficial soil microorganisms that help in nitrogen fixation and decomposition thrive at specific pH

4. Root growth: Extreme pH can damage root cells and inhibit root growth

5. Toxic elements: At very low pH (highly acidic), toxic elements like aluminum become soluble and can harm plants

6. Optimal range: Most plants grow best in slightly acidic to neutral soil (pH 6-7), though some plants like azaleas prefer more acidic soil