NCERT Solutions – Chapter 2: Acids, Bases & Salts

NCERT Solutions – Chapter: Acids, Bases & Salts

In this chapter, we explore the fascinating chemistry of acids, bases, and salts — substances that play a crucial role in our daily lives. From the sourness of lemon juice to the effectiveness of antacids, this topic helps students understand how these substances react, neutralise each other, and form salts. The NCERT Class 10 Science Chapter 2 solutions provided below explain each exercise question in a clear, detailed, and conceptual manner to strengthen understanding and exam preparation.

Question 1

Q: A solution turns red litmus blue. Its pH is likely to be

Options: (a) 1   (b) 4   (c) 5   (d) 10

Answer & Explanation: Red litmus turns blue in basic solutions. A pH of 10 indicates a basic solution (pH > 7). Therefore, the most likely pH is 10. (d)

Question 2

Q: A solution reacts with crushed eggshells to give a gas that turns lime-water milky. The solution contains

Options: (a) NaCl   (b) HCl   (c) LiCl   (d) KCl

Answer & Explanation: Eggshells are mainly calcium carbonate (CaCO₃). An acid reacts with CaCO₃ to produce carbon dioxide (CO₂) which turns lime-water milky (forming CaCO₃ precipitate). Among the given salts, only HCl (hydrochloric acid) provides an acidic solution. Thus the solution contains HCl. (b)

Question 3

Q: 10 mL of a solution of NaOH is found to be completely neutralised by 8 mL of a given HCl solution. If we take 20 mL of the same NaOH solution, the amount of the same HCl solution required to neutralise it will be

Options: (a) 4 mL   (b) 8 mL   (c) 12 mL   (d) 16 mL

Answer & Explanation: Neutralisation volumes are proportional when concentrations remain the same. Doubling the volume of NaOH (from 10 mL to 20 mL) requires double the volume of HCl. So required HCl = 2 × 8 mL = 16 mL. (d)

Question 4

Q: Which one of the following types of medicines is used for treating indigestion?

Options: (a) Antibiotic   (b) Analgesic   (c) Antacid   (d) Antiseptic

Answer & Explanation: Indigestion is often caused by excess stomach acid. Antacids neutralise excess acid and relieve indigestion. Therefore the correct option is (c) Antacid.

Question 5

Q: Write word equations and then balanced equations for the reaction taking place when –

• dilute sulphuric acid reacts with zinc granules.
• dilute hydrochloric acid reacts with magnesium ribbon.
• dilute sulphuric acid reacts with aluminium powder.
• dilute hydrochloric acid reacts with iron filings.

Answer (word + balanced equations with explanation):

• 
  Word equation: Zinc + Dilute sulphuric acid → Zinc sulphate + Hydrogen gas.
  
  Chemical equation: Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g).
  
  Explanation: Zinc (a more reactive metal) displaces hydrogen from acid, producing hydrogen gas — a displacement reaction.
  
• 
  Word equation: Magnesium + Dilute hydrochloric acid → Magnesium chloride + Hydrogen gas.
  
  Chemical equation: Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g).
  
• 
  Word equation: Aluminium + Dilute sulphuric acid → Aluminium sulphate + Hydrogen gas.
  
  Chemical equation: 2Al(s) + 3H₂SO₄(aq) → Al₂(SO₄)₃(aq) + 3H₂(g).
  
• 
  Word equation: Iron + Dilute hydrochloric acid → Iron(II) chloride + Hydrogen gas.
  
  Chemical equation: Fe(s) + 2HCl(aq) → FeCl₂(aq) + H₂(g).
  

Teacher note: All these are single-displacement reactions and examples of redox processes (metal gets oxidised, H⁺ gets reduced to H₂).

Question 6

Q: Compounds such as alcohols and glucose also contain hydrogen but are not categorised as acids. Describe an activity to prove it.

Answer (Activity & Explanation):

• 
  Materials: Test tubes, distilled water, dilute HCl (or lemon juice), ethanol (or an alcohol sample), glucose solution, universal indicator or blue & red litmus paper.
  
• Procedure:
  1. Take three test tubes labeled A, B, C. In A put a small amount of dilute HCl (or lemon juice). In B put ethanol (or a small quantity of alcohol dissolved in water). In C put glucose solution.
  2. Add a little distilled water to each so solutions are comparable.
  3. Add a few drops of universal indicator (or dip litmus paper) to each test tube and observe colour changes.
• 
  Observation: Test tube A (HCl) turns the indicator to the acidic colour (red/orange). Test tubes B and C (alcohol, glucose) show little or no change — they remain near neutral (indicator stays green). Litmus test: blue litmus turns red for HCl, but not for alcohol or glucose solutions.
  
• 
  Conclusion: Although alcohols and glucose contain hydrogen atoms, they do not release H⁺ ions in water (or do so extremely weakly) like acids do. Therefore they do not show acidic behaviour and are not categorised as acids.
  

Conceptual note: Acidity is about proton (H⁺) donation in aqueous solution. Alcohols and many organic compounds do not ionise appreciably in water to produce H⁺, so they are not acidic in the Brønsted–Lowry sense.

Question 7

Q: Why does distilled water not conduct electricity, whereas rain water does?

Answer & Explanation: Pure distilled water contains almost no dissolved ions and therefore is a very poor conductor of electricity. Rainwater, however, contains dissolved gases (like CO₂) and impurities (dissolved salts, dust), which form ions (e.g., HCO₃⁻, H⁺, etc.) that can carry electric current. Hence rainwater conducts electricity while distilled water does not.

Question 8

Q: Why do acids not show acidic behaviour in the absence of water?

Answer & Explanation: Acidic behaviour requires the release of H⁺ ions into the medium. In the absence of water, many acids cannot ionise to give free H⁺ ions because water helps in the dissociation process (solvation stabilises ions). Without water there are no free H⁺ (hydronium) ions and so acids do not show their typical acidic properties.

Question 9

Q: Five solutions A, B, C, D and E when tested with universal indicator showed pH as 4, 1, 11, 7 and 9 respectively. Which solution is

• neutral?
• strongly alkaline?
• strongly acidic?
• weakly acidic?
• weakly alkaline?

Answer & Explanation:

• Neutral: pH 7 → Solution D.
• Strongly alkaline: pH 11 → Solution C.
• Strongly acidic: pH 1 → Solution B.
• Weakly acidic: pH 4 → Solution A.
• Weakly alkaline: pH 9 → Solution E.

Arrange the pH in increasing order of hydrogen-ion concentration:

Explanation: Hydrogen-ion concentration [H⁺] increases as pH decreases. Hence the increasing order of [H⁺] corresponds to decreasing pH values.

Order (increasing [H⁺]): pH 11 < pH 9 < pH 7 < pH 4 < pH 1.

Question 10

Q: Equal lengths of magnesium ribbons are taken in test tubes A and B. Hydrochloric acid (HCl) is added to test tube A, while acetic acid (CH₃COOH) is added to test tube B. Amount and concentration taken for both acids are same. In which test tube will fizzing occur more vigorously and why?

Answer & Explanation: Fizzing (evolution of H₂ gas) occurs when acid reacts with magnesium. HCl is a strong acid and ionises completely in water, producing a high concentration of H⁺ ions. Acetic acid (CH₃COOH) is a weak acid and ionises only partially. Therefore, reaction rate and hydrogen evolution will be greater in test tube A (with HCl) than in B. So fizzing is more vigorous in tube A.

Question 11

Q: Fresh milk has a pH of 6. How do you think the pH will change as it turns into curd? Explain your answer.

Answer & Explanation: During curd formation, lactose in milk is converted by bacteria (lactic acid bacteria) into lactic acid. The production of lactic acid increases the H⁺ concentration and thus lowers the pH. Therefore, pH will decrease as milk turns into curd (becomes more acidic).

Question 12

Q: A milkman adds a very small amount of baking soda to fresh milk.

• Why does he shift the pH of fresh milk from 6 to slightly alkaline?
• Why does this milk take a long time to set as curd?

Answer & Explanation:

• Why pH shifts to slightly alkaline: Baking soda (sodium bicarbonate, NaHCO₃) is a basic salt. Adding a small amount neutralises some of the acid or provides basic ions, shifting the pH slightly upward (toward alkaline).
• Why milk takes long to set: Formation of curd requires acidic conditions for lactic acid bacteria to act effectively. Slight alkalinity inhibits bacterial activity and slows down fermentation, hence milk takes longer to set into curd.

Question 13

Q: Plaster of Paris should be stored in a moisture-proof container. Explain why?

Answer & Explanation: Plaster of Paris (calcium sulphate hemihydrate, CaSO₄·½H₂O) reacts with water to form gypsum (calcium sulphate dihydrate, CaSO₄·2H₂O), which hardens:

CaSO4·½H2O + 1½H2O → CaSO4·2H2O

If Plaster of Paris is exposed to moisture it will hydrate and set prematurely. Therefore it must be stored in a moisture-proof container to keep it dry and usable.

Question 14

Q: What is a neutralisation reaction? Give two examples.

Answer & Explanation: A neutralisation reaction is a reaction in which an acid and a base react to form a salt and water. Heat may be released (many neutralisations are exothermic).

• Example 1: HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)
• Example 2: H₂SO₄(aq) + 2NaOH(aq) → Na₂SO₄(aq) + 2H₂O(l)

Question 15

Q: Give two important uses of washing soda and baking soda.

Answer & Explanation:

• Washing soda (sodium carbonate, Na₂CO₃):
  1. Used as a water softener in laundries — it removes Mg²⁺ and Ca²⁺ ions from hard water and helps soap to lather better.
  2. Used in the manufacture of glass and as an important industrial cleaning agent (removes grease and stains).
• Baking soda (sodium hydrogen carbonate, NaHCO₃):
  1. Used in baking as a leavening agent — it produces CO₂ on decomposition, making the dough rise.
  2. Used as an antacid to neutralise excess stomach acid (for relief from acidity and indigestion).

1. Chemical Reactions and Equations
2. Acids, Bases and Salts
3. Metals and Non‑Metals
4. Carbon and Its Compounds
5. Life Processes
6. Control and Coordination
7. How Do Organisms Reproduce?
8. Heredity and Evolution
9. Light – Reflection and Refraction
10. The Human Eye and the Colourful World
11. Electricity
12. Magnetic Effects of Electric Current
13. Our Environment

Conclusion:
The chapter “Acids, Bases and Salts” helps students connect chemistry with real-life experiences — from the sourness of curd to the fizz in soft drinks, from cleaning agents to medicines. Understanding concepts like pH scale, neutralisation, and acid–base reactions builds the foundation for higher classes and practical applications. These NCERT Solutions provide a detailed, step-by-step understanding that promotes conceptual clarity and exam confidence.