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NCERT Class 10 Science Acids, Bases and Salts: Exercise Solutions

Step-by-step working for every question of End-of-chapter exercises in NCERT's Class 10 Science book, chapter “Acids, Bases and Salts”, written by Shishya. Each question is in our own short words; open the NCERT chapter for the exact wording and figures.

1. Exercises Q1

   A solution turns red litmus blue. Its pH is likely to be (a) 1 (b) 4 (c) 5 (d) 10.

   1. A solution that turns red litmus blue is basic in nature.
   2. Basic solutions have a pH greater than 7.
      why?

      pH below 7 is acidic, 7 is neutral and above 7 is basic; the lower the pH, the more hydrogen ions in the solution.

   3. Options (a) 1, (b) 4, and (c) 5 are all less than 7, so they are acidic.
   4. Option (d) 10 is greater than 7, so it is basic.

   Answer: (d) 10

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2. Exercises Q2

   A solution reacts with crushed egg-shells to give a gas that turns lime-water milky. It contains (a) NaCl (b) HCl (c) LiCl (d) KCl.

   1. Crushed eggshells are mainly made of calcium carbonate (CaCO₃).
   2. The gas that turns lime water milky is carbon dioxide (CO₂).
   3. Only acids react with carbonates to produce carbon dioxide gas.
   4. Among the options, HCl is the only acid; NaCl, LiCl, and KCl are salts.
   5. Therefore, the solution must contain HCl to react with the eggshells and release CO₂.

   Answer: (b) HCl

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3. Exercises Q3

   10 mL of an NaOH solution is neutralised by 8 mL of an HCl solution. How much of the same HCl neutralises 20 mL of the NaOH? (a) 4 mL (b) 8 mL (c) 12 mL (d) 16 mL.

   1. The neutralisation reaction between NaOH and HCl is NaOH + HCl → NaCl + H₂O.
      why?

      An acid and a base react to give a salt and water (neutralisation): the H⁺ ions of the acid and the OH⁻ ions of the base join to form water.

   2. From the balanced equation, 1 mole of NaOH reacts with 1 mole of HCl, so the ratio of moles is 1:1.
      why?

      A balanced equation has the same number of atoms of each element on both sides, because atoms are neither created nor destroyed in a chemical reaction (law of conservation of mass). Only the numbers in front (coefficients) change, never the formulas.

   3. Using the dilution formula M₁V₁ = M₂V₂ for the first case, we have M_NaOH × 10 mL = M_HCl × 8 mL.
   4. This gives the relationship M_NaOH = (8/10) M_HCl or M_NaOH = 0.8 M_HCl.
   5. For the second case, let V be the volume of HCl needed to neutralise 20 mL of the same NaOH solution.
      why?

      An acid and a base react to give a salt and water (neutralisation): the H⁺ ions of the acid and the OH⁻ ions of the base join to form water.

   6. Applying the formula again: M_NaOH × 20 mL = M_HCl × V.
   7. Substitute M_NaOH = 0.8 M_HCl into the equation: 0.8 M_HCl × 20 mL = M_HCl × V.
   8. Cancel M_HCl from both sides to get 0.8 × 20 = V.
   9. Calculate V = 16 mL.

   Answer: 16 mL, (d)

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4. Exercises Q4

   Which medicine treats indigestion: (a) antibiotic (b) analgesic (c) antacid (d) antiseptic?

   1. Indigestion is often caused by excess stomach acid, which leads to acidity or heartburn.
   2. Antibiotics are medicines used to treat bacterial infections, not acid-related issues.
   3. Analgesics are pain relievers that reduce pain but do not neutralize stomach acid.
      why?

      An acid and a base react to give a salt and water (neutralisation): the H⁺ ions of the acid and the OH⁻ ions of the base join to form water.

   4. Antiseptics are applied externally to prevent infection on wounds or skin.
   5. Antacids contain weak bases that neutralize the excess acid in the stomach.
      why?

      An acid and a base react to give a salt and water (neutralisation): the H⁺ ions of the acid and the OH⁻ ions of the base join to form water.

   6. Therefore, antacids are the correct medicine to treat indigestion caused by acidity.

   Answer: (c) antacid

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5. Exercises Q5

   Write word equations and balanced equations when (a) dilute sulphuric acid reacts with zinc granules (b) dilute hydrochloric acid reacts with magnesium ribbon (c) dilute sulphuric acid reacts with aluminium powder (d) dilute hydrochloric acid reacts with iron filings.

   1. (a) Zinc reacts with dilute sulphuric acid to form zinc sulphate and hydrogen gas.
   2. Zn + H2SO4 → ZnSO4 + H2
   3. (b) Magnesium reacts with dilute hydrochloric acid to form magnesium chloride and hydrogen gas.
   4. Mg + 2HCl → MgCl2 + H2
   5. (c) Aluminium reacts with dilute sulphuric acid to form aluminium sulphate and hydrogen gas.
   6. 2Al + 3H2SO4 → Al2(SO4)3 + 3H2
   7. (d) Iron reacts with dilute hydrochloric acid to form iron(II) chloride and hydrogen gas.
   8. Fe + 2HCl → FeCl2 + H2

   Answer: (a) Zn + H2SO4 → ZnSO4 + H2; (b) Mg + 2HCl → MgCl2 + H2; (c) 2Al + 3H2SO4 → Al2(SO4)3 + 3H2; (d) Fe + 2HCl → FeCl2 + H2

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6. Exercises Q6

   Alcohols and glucose contain hydrogen but are not acids. Describe an activity to show this.

   1. Acids are substances that give H⁺ ions in water, and a solution with ions conducts electricity. So test whether solutions of glucose and alcohol conduct.
   2. Fix two iron nails in a cork, place the cork in a 100 mL beaker, and connect the nails through a 6 V battery and a bulb with a switch.
   3. Pour dilute hydrochloric acid into the beaker and switch on: the bulb glows, because the acid gives ions (H⁺ and Cl⁻) that carry the current.
   4. Repeat with dilute sulphuric acid: the bulb glows again.
   5. Now repeat with a glucose solution and then with alcohol (ethanol): the bulb does not glow.
   6. So glucose and alcohol do not give H⁺ ions in water, even though they contain hydrogen; that is why they are not acids.

   Answer: In the conductivity test, acid solutions light the bulb but glucose and alcohol solutions do not, so glucose and alcohol do not give H⁺ ions and are not acids.

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7. Exercises Q7

   Why does distilled water not conduct electricity while rain water does?

   1. Distilled water is pure H₂O and contains no dissolved ions.
   2. Electricity in water is conducted by the movement of charged particles called ions.
   3. Since distilled water has no ions, it cannot carry an electric current.
   4. Rain water falls through the atmosphere and picks up dissolved gases and impurities.
   5. These impurities, such as carbon dioxide, react with water to form weak acids like carbonic acid.
   6. This reaction produces small amounts of hydrogen and bicarbonate ions in the rain water.
   7. The presence of these ions allows rain water to conduct electricity.
   8. Therefore, the difference in conductivity is due to the presence of ions in rain water and their absence in distilled water.

   Answer: Distilled water does not conduct electricity because it lacks ions, while rain water does conduct because it contains dissolved ions from atmospheric impurities.

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8. Exercises Q8

   Why does dry HCl gas not show acidic behaviour without water?

   1. Acids show acidic behaviour only when they release hydrogen ions (H⁺) in a solution.
   2. In the presence of water, HCl molecules dissociate to form H⁺ ions and Cl⁻ ions.
   3. Dry HCl gas does not have water molecules to accept the proton or facilitate dissociation.
   4. Without water, HCl remains as neutral molecules and cannot produce H⁺ ions.
   5. Since no H⁺ ions are formed, dry HCl gas does not turn litmus red or show acidic properties.

   Answer: Dry HCl gas does not show acidic behaviour because it cannot produce H⁺ ions without water.

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9. Exercises Q9

   Solutions A, B, C, D, E have pH 4, 1, 11, 7, 9. Which is (a) neutral (b) strongly alkaline (c) strongly acidic (d) weakly acidic (e) weakly alkaline? Arrange the pH values in increasing order of hydrogen-ion concentration.

   1. (a) Solution D has a pH of 7, which is the neutral point on the pH scale.
      why?

      pH below 7 is acidic, 7 is neutral and above 7 is basic; the lower the pH, the more hydrogen ions in the solution.

   2. (b) Solution C has a pH of 11, which is the highest value, indicating it is strongly alkaline.
      why?

      pH below 7 is acidic, 7 is neutral and above 7 is basic; the lower the pH, the more hydrogen ions in the solution.

   3. (c) Solution B has a pH of 1, which is the lowest value, indicating it is strongly acidic.
      why?

      pH below 7 is acidic, 7 is neutral and above 7 is basic; the lower the pH, the more hydrogen ions in the solution.

   4. (d) Solution A has a pH of 4, which is acidic but closer to 7 than B, making it weakly acidic.
      why?

      pH below 7 is acidic, 7 is neutral and above 7 is basic; the lower the pH, the more hydrogen ions in the solution.

   5. (e) Solution E has a pH of 9, which is alkaline but closer to 7 than C, making it weakly alkaline.
      why?

      pH below 7 is acidic, 7 is neutral and above 7 is basic; the lower the pH, the more hydrogen ions in the solution.

   6. Hydrogen-ion concentration is inversely proportional to pH, so lower pH means higher concentration.
      why?

      pH below 7 is acidic, 7 is neutral and above 7 is basic; the lower the pH, the more hydrogen ions in the solution.

   7. The order of increasing hydrogen-ion concentration runs from the highest pH (fewest H⁺ ions) to the lowest pH (most H⁺ ions): 11 (C), 9 (E), 7 (D), 4 (A), 1 (B), that is C < E < D < A < B.
      why?

      pH below 7 is acidic, 7 is neutral and above 7 is basic; the lower the pH, the more hydrogen ions in the solution.

   Answer: (a) D; (b) C; (c) B; (d) A; (e) E; C < E < D < A < B

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10. Exercises Q10

    Equal magnesium ribbons are put in test tube A with HCl and test tube B with acetic acid of the same amount and concentration. Where is the fizzing more vigorous, and why?

    1. Equal amounts of magnesium ribbon are placed in test tube A containing HCl and test tube B containing acetic acid.
    2. Both acids have the same concentration and volume, so the number of moles of acid is equal in both tubes.
    3. HCl is a strong acid and dissociates completely in water, releasing a high concentration of H+ ions.
    4. Acetic acid is a weak acid and dissociates only partially, resulting in a lower concentration of H+ ions.
    5. The reaction between magnesium and acid produces hydrogen gas, which causes fizzing.
    6. Since HCl has a higher concentration of H+ ions, it reacts faster with magnesium than acetic acid does.
    7. Therefore, the rate of hydrogen gas production is higher in test tube A.
    8. This leads to more vigorous fizzing in test tube A compared to test tube B.

    Answer: Fizzing is more vigorous in test tube A because HCl is a strong acid that provides more H+ ions for a faster reaction with magnesium.

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11. Exercises Q11

    Fresh milk has pH 6. How does the pH change as it turns into curd? Explain.

    1. Fresh milk has a pH of 6, which is slightly acidic.
       why?

       pH below 7 is acidic, 7 is neutral and above 7 is basic; the lower the pH, the more hydrogen ions in the solution.

    2. When milk is left to stand, bacteria present in it start multiplying.
    3. These bacteria produce lactic acid through the fermentation of lactose in the milk.
    4. The accumulation of lactic acid increases the concentration of hydrogen ions in the milk.
    5. An increase in hydrogen ion concentration causes the pH value to decrease.
       why?

       pH below 7 is acidic, 7 is neutral and above 7 is basic; the lower the pH, the more hydrogen ions in the solution.

    6. So as milk turns into curd, its pH falls below 6.
       why?

       pH below 7 is acidic, 7 is neutral and above 7 is basic; the lower the pH, the more hydrogen ions in the solution.

    7. This drop in pH confirms that the medium has become more acidic due to the formation of lactic acid.
       why?

       pH below 7 is acidic, 7 is neutral and above 7 is basic; the lower the pH, the more hydrogen ions in the solution.

    Answer: The pH decreases as fresh milk turns into curd because lactic acid is produced by bacteria, making the solution more acidic.

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12. Exercises Q12

    A milkman adds a little baking soda to fresh milk. (a) Why does he make it slightly alkaline? (b) Why does this milk take longer to set as curd?

    1. (a) Fresh milk is slightly acidic due to lactic acid produced by bacteria.
    2. (a) Baking soda is a base, so adding it neutralizes the acid and makes the milk slightly alkaline.
       why?

       An acid and a base react to give a salt and water (neutralisation): the H⁺ ions of the acid and the OH⁻ ions of the base join to form water.

    3. (a) This alkaline environment slows down the growth of bacteria that spoil the milk.
    4. (b) Curd forms when lactic acid bacteria convert milk sugar into lactic acid.
    5. (b) The added baking soda neutralizes the lactic acid as it is produced.
       why?

       An acid and a base react to give a salt and water (neutralisation): the H⁺ ions of the acid and the OH⁻ ions of the base join to form water.

    6. (b) Since the acid is neutralized, the pH of the milk does not drop enough to coagulate the milk proteins.
       why?

       An acid and a base react to give a salt and water (neutralisation): the H⁺ ions of the acid and the OH⁻ ions of the base join to form water.

    7. (b) Therefore, it takes longer for the milk to set as curd.

    Answer: (a) To make the milk slightly alkaline and slow down bacterial spoilage; (b) Because the baking soda neutralizes the lactic acid required for curdling.

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13. Exercises Q13

    Why should Plaster of Paris be stored in a moisture-proof container?

    1. Plaster of Paris is calcium sulphate hemihydrate, CaSO₄·½H₂O.
    2. When it comes into contact with moisture, it reacts with water to form gypsum.
    3. This reaction causes the plaster to harden and set into a solid mass.
    4. If stored in a non-moisture-proof container, it will absorb atmospheric moisture and harden prematurely.
    5. Therefore, it must be kept in a moisture-proof container to prevent this unwanted setting.

    Answer: Plaster of Paris should be stored in a moisture-proof container to prevent it from absorbing moisture and hardening prematurely.

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14. Exercises Q14

    What is a neutralisation reaction? Give two examples.

    1. A neutralisation reaction is a chemical reaction between an acid and a base that produces salt and water.
       why?

       An acid and a base react to give a salt and water (neutralisation): the H⁺ ions of the acid and the OH⁻ ions of the base join to form water.

    2. The general equation for this reaction is: Acid + Base → Salt + Water.
    3. Example 1: Hydrochloric acid reacts with sodium hydroxide to form sodium chloride and water.
    4. The equation is: HCl + NaOH → NaCl + H₂O.
    5. Example 2: Sulphuric acid reacts with potassium hydroxide to form potassium sulphate and water.
    6. The equation is: H₂SO₄ + 2KOH → K₂SO₄ + 2H₂O.

    Answer: A neutralisation reaction is the reaction between an acid and a base to form salt and water. Examples: HCl + NaOH → NaCl + H₂O and H₂SO₄ + 2KOH → K₂SO₄ + 2H₂O.

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15. Exercises Q15

    Give two important uses each of washing soda and baking soda.

    1. Washing soda is used in the manufacture of glass, soap, and paper.
    2. Washing soda is also used to remove the permanent hardness of water and as a cleaning agent in homes.
    3. Baking soda is used in baking to make cakes and breads soft and spongy.
    4. Baking soda is used in fire extinguishers to extinguish fires.

    Answer: Washing soda: in making glass, soap and paper; to remove permanent hardness of water; as a household cleaner. Baking soda: in baking (as part of baking powder) to make cakes soft and spongy; in soda-acid fire extinguishers (also as an antacid).

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