Acids and Bases (Case Study) (Class 10 Science)

Real-world scenarios involving pH, neutralization, and industrial acid-base reactions. Students apply concepts to agriculture, medicine, and household chem

Acids and Bases (Case Study) (Class 10 Science)

Real-world scenarios involving pH, neutralization, and industrial acid-base reactions. Students apply concepts to agriculture, medicine, and household chem

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TL;DR: Real-world scenarios involving pH, neutralization, and industrial acid-base reactions. Students apply concepts to agriculture, medicine, and household…

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Real-world scenarios involving pH, neutralization, and industrial acid-base reactions. Students apply concepts to agriculture, medicine, and household chem

Acids and Bases (Case Study) MCQs with Answers & Explanations

Q1. A farmer has acidic soil (pH 5) and needs to raise it to neutral (pH 7) for optimal crop growth. Which substance should be added?

  1. Hydrochloric acid
  2. Ammonium nitrate
  3. Lime (CaCO3) or caustic soda ✓ (correct)
  4. Sulfuric acid

Explanation: Basic substances like lime (CaCO3) or NaOH neutralize excess H+ ions in acidic soil, raising pH toward neutral. Acids would worsen the condition.

Q2. A person suffers from heartburn due to excess HCl in the stomach. Which type of medicine would relieve the symptoms?

  1. Antibiotic
  2. Diuretic
  3. Laxative
  4. Antacid (e.g., MgO or Al(OH)3) ✓ (correct)

Explanation: Antacids are weak bases that neutralize excess stomach acid. MgO and Al(OH)3 are common antacids that react with HCl to form salt and water.

Q3. In the neutralization reaction HCl + NaOH -> NaCl + H2O, what is the molar ratio when 100 mL of 1 M HCl reacts with NaOH solution?

  1. 1:1, requiring 100 mL of 1 M NaOH ✓ (correct)
  2. 1:2, requiring 200 mL of 1 M NaOH
  3. Cannot determine without knowing concentration
  4. 2:1, requiring 50 mL of 1 M NaOH

Explanation: HCl and NaOH react in 1:1 molar ratio. Moles of HCl = 0.1 L * 1 M = 0.1 mol. For complete neutralization, 0.1 mol NaOH is needed, which is 100 mL of 1 M solution.

Q4. A solution has a pH of 3. What is the concentration of H+ ions and is the solution acidic or basic?

  1. 10^-7 M, neutral
  2. 10^-3 M, acidic ✓ (correct)
  3. 10^-3 M, basic
  4. 10^-11 M, basic

Explanation: pH = -log[H+]. If pH = 3, then [H+] = 10^-3 M = 0.001 M. Since pH < 7, the solution is acidic.

Q5. A student tests a colorless solution with universal indicator paper and observes a blue color. What can be concluded?

  1. pH is between 1-6, acidic
  2. pH is between 7-8, weakly basic
  3. pH is between 8-14, basic ✓ (correct)
  4. pH is exactly 7, neutral

Explanation: Universal indicator turns blue in basic (alkaline) solutions with pH between 8-14. A blue color indicates a pH in this range.

Q6. Milk of magnesia (Mg(OH)2) is a weak base used as an antacid and laxative. Why is it considered weak?

  1. It has low density
  2. It cannot neutralize acids
  3. It has a low molecular weight
  4. It only partially dissolves in water and partially ionizes ✓ (correct)

Explanation: Weak bases partially ionize in water and have low solubility. Mg(OH)2 dissolves only slightly, providing limited OH- ions but sufficient for gentle antacid action.

Q7. In a lab, 50 mL of 2 M H2SO4 is diluted with water to 500 mL. What is the molarity of the diluted solution?

  1. 0.2 M ✓ (correct)
  2. 0.5 M
  3. 2 M
  4. 1 M

Explanation: Dilution conserves moles: M₁V₁ = M₂V₂, so 2 x 50 = M₂ x 500 and M₂ = 100/500 = 0.2 M.

Q8. Sodium carbonate (Na2CO3) solution is basic even though it contains no OH- ions directly. Why?

  1. Water molecules dissociate more in salt solutions
  2. Carbonate ions hydrolyze to form OH- ions ✓ (correct)
  3. Sodium ions make it basic
  4. Carbon dioxide escapes, leaving a basic residue

Explanation: Carbonate ions (CO3^2-) are weak bases. They react with water: CO3^2- + H2O <-> HCO3^- + OH-, producing hydroxide ions and making the solution basic.

Frequently Asked Questions

What is the difference between strong and weak acids?

Strong acids (e.g., HCl, H2SO4) completely ionize in water, releasing all hydrogen ions. Weak acids (e.g., acetic acid, citric acid) only partially ionize, establishing an equilibrium with their undissociated molecules. This means weak acids have fewer H+ ions in solution at the same concentration.

Why does adding salt to pure water not change its pH significantly?

The pH of water at 25°C is 7 because [H+][OH-] = Kw = 10^-14. Adding neutral salt (like NaCl) that does not hydrolyze does not consume or produce H+ or OH- ions, so [H+] and [OH-] remain equal and pH stays at 7. However, salts from weak acids or bases do affect pH through hydrolysis.

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