NCERT Solutions Class 10 Science Chapter 2 Acids, Bases and Salts

Access comprehensive NCERT Solutions Class 10 Science Chapter 2 Acids, Bases and Salts designed to help CBSE students score top marks in their board examinations. This chapter forms the foundation of chemical reactions and everyday chemistry in the Class 10 Science curriculum. In this in-depth guide, you will find step-by-step NCERT Science Class 10 solutions for all in-text questions and end-of-chapter textbook exercises, complete with balanced chemical equations, clear scientific reasoning, and practical exam tips.

Chapter Overview

Chapter 2 of Class 10 Science, Acids, Bases and Salts, explores the chemical behaviour of acidic and basic substances, the pH scale, indicators, and the commercial preparation and utility of essential salts. In the CBSE Class 10 Board Examination, this chapter carries a significant weightage of 6 to 8 marks under the "Chemical Substances — Nature and Behaviour" unit.

The key concepts covered in this chapter include:

• Indicators and Classification: Natural indicators (litmus, turmeric, red cabbage), synthetic indicators (phenolphthalein, methyl orange), and olfactory indicators (onion, vanilla, clove oil).
• Chemical Properties of Acids and Bases: Reactions with active metals, metal carbonates, metal hydrogen carbonates, metallic oxides, and non-metallic oxides.
• Ionisation & Dilution: Role of water in releasing H⁺(aq) / H₃O⁺(aq) and OH⁻(aq) ions, and the strictly exothermic process of acid dilution.
• The pH Scale: Measuring hydrogen ion concentration from 0 (strongly acidic) to 14 (strongly basic), with 7 being neutral.
• Importance of pH in Everyday Life: Digestive system, dental health, self-defence in plants and animals, and soil testing.
• Chemicals from Common Salt (NaCl): Industrial preparation, properties, and applications of Sodium Hydroxide (Chlor-Alkali process), Bleaching Powder, Baking Soda, Washing Soda, and Plaster of Paris.

Detailed NCERT Solutions: In-Text and Exercise Questions

Below are the comprehensive, step-by-step CBSE NCERT solutions for all textbook questions, structured to meet CBSE marking criteria.

In-Text Questions (Page 18)

Question 1: Why should curd and sour substances not be kept in brass and copper vessels?

Answer:

Curd and other sour food substances contain organic acids (such as lactic acid). When these acidic substances are kept in brass or copper containers, the acids react with the metals to produce toxic and poisonous metallic salts and liberate hydrogen gas:

Acid + Metal → Toxic Metal Salt + Hydrogen Gas

Consuming food containing these toxic compounds can lead to severe food poisoning. Hence, curd and sour items must always be stored in glass, ceramic, or food-grade stainless steel containers.

Question 2: Which gas is usually liberated when an acid reacts with a metal? Illustrate with an example. How will you test for the presence of this gas?

Answer:

1. Gas Liberated: Hydrogen gas (H₂) is liberated when an acid reacts with an active metal.
2. Example & Balanced Chemical Equation: When dilute sulphuric acid is added to zinc granules in a test tube, zinc sulphate and hydrogen gas are formed:
   Zn (s) + H₂SO₄ (aq) → ZnSO₄ (aq) + H₂ (g)↑
3. Test for Hydrogen Gas: Pass the evolved gas through a soap solution so that gas bubbles are formed. Bring a burning candle near the gas-filled soap bubble. The gas burns with a characteristic 'pop' sound, confirming the presence of hydrogen.

Question 3: Metal compound A reacts with dilute hydrochloric acid to produce effervescence. The gas evolved extinguishes a burning candle. Write a balanced chemical equation for the reaction if one of the compounds formed is calcium chloride.

Answer:

• The gas that extinguishes a burning candle and turns lime water milky is Carbon Dioxide (CO₂).
• Since one of the products is calcium chloride (CaCl₂) and the gas evolved is CO₂, compound A must be Calcium Carbonate (CaCO₃).
• Balanced Chemical Equation:
  CaCO₃ (s) + 2HCl (aq) → CaCl₂ (aq) + H₂O (l) + CO₂ (g)↑

In-Text Questions (Page 22)

Question 4: Why do HCl, HNO₃, etc., show acidic characters in aqueous solutions while solutions of compounds like alcohol and glucose do not show acidic character?

Answer:

Acids like hydrochloric acid (HCl) and nitric acid (HNO₃) ionise in the presence of water to release free hydronium ions (H₃O⁺ or H⁺(aq)), which are solely responsible for acidic properties:

HCl (aq) + H₂O (l) → H₃O⁺ (aq) + Cl⁻ (aq)

On the other hand, compounds like glucose (C₆H₁₂O₆) and ethyl alcohol (C₂H₅OH) contain hydrogen atoms bound covalently. They do not dissociate or ionise in aqueous solutions to produce H⁺(aq) ions. Therefore, they do not exhibit acidic characteristics and do not conduct electricity in solution.

Question 5: Why does an aqueous solution of an acid conduct electricity?

Answer:

Electrical conductivity in liquids requires the presence of mobile charged particles (ions). When an acid dissolves in water, it dissociates into hydrogen ions (H⁺/H₃O⁺) and corresponding anions (e.g., Cl⁻, SO₄²⁻). These freely moving ions act as charge carriers, allowing electric current to flow through the aqueous solution.

Question 6: Why does dry HCl gas not change the colour of dry litmus paper?

Answer:

Acidic behaviour is manifested only when an acid dissociates into hydrogen ions (H⁺). Dry HCl gas does not contain H⁺ ions because the covalent bond between hydrogen and chlorine cannot break in the absence of moisture. Similarly, dry litmus paper contains no water to induce ionisation. In the absence of H⁺(aq) ions, no acidic reaction occurs, and the dry litmus paper does not change colour.

Question 7: While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?

Answer:

The dissolution of a concentrated acid (especially concentrated H₂SO₄ or HNO₃) in water is an extremely exothermic reaction. If water is added to concentrated acid:

1. A massive amount of heat is generated instantaneously in a small volume.
2. This localised boiling can cause the corrosive acid mixture to splash out onto your face or hands, causing severe chemical burns.
3. The extreme local heat may also cause the glass container to crack or shatter.

Therefore, acid must always be added slowly to water with constant stirring. The large volume of water absorbs the dissipated heat safely.

In-Text Questions (Page 28)

Question 8: You have two solutions, A and B. The pH of solution A is 6 and pH of solution B is 8. Which solution has more hydrogen ion concentration? Which of this is acidic and which is basic?

Answer:

• The pH value is inversely proportional to hydrogen ion concentration: pH ∝ 1 / [H⁺]. Lower pH corresponds to higher [H⁺].
• Solution A (pH = 6) has a higher hydrogen ion concentration than Solution B (pH = 8).
• Nature of solutions:
  • Solution A (pH = 6 < 7): Acidic
  • Solution B (pH = 8 > 7): Basic / Alkaline

Question 9: Under what soil condition do you think a farmer would treat the soil of his fields with quick lime (calcium oxide) or slaked lime (calcium hydroxide) or chalk (calcium carbonate)?

Answer:

Quick lime (CaO), slaked lime (Ca(OH)₂), and chalk (CaCO₃) are basic substances. A farmer treats the soil with these bases when the soil becomes overly acidic (pH < 6.5) due to acid rain, excessive chemical fertiliser application, or organic matter decay. The basic compounds neutralise excess soil acidity, restoring the optimum pH range (6.0 to 7.5) necessary for healthy crop growth.

In-Text Questions (Page 33)

Question 10: What is the common name of the compound CaOCl₂?

Answer: The common name of Calcium Oxychloride (CaOCl₂) is Bleaching Powder.

Question 11: Name the substance which on treatment with chlorine yields bleaching powder.

Answer: Dry slaked lime [Calcium Hydroxide, Ca(OH)₂] reacts with chlorine gas to yield bleaching powder:

Ca(OH)₂ (s) + Cl₂ (g) → CaOCl₂ (s) + H₂O (l)

Question 12: Name the sodium compound which is used for softening hard water.

Answer: Washing Soda (Sodium Carbonate Decahydrate, Na₂CO₃·10H₂O) is used to remove both temporary and permanent hardness of water.

Question 13: What will happen if a solution of sodium hydrogen carbonate is heated? Give the equation of the reaction involved.

Answer:

When sodium hydrogen carbonate (baking soda) solution is heated, it undergoes thermal decomposition to form sodium carbonate, water, and carbon dioxide gas:

2NaHCO₃ (aq) ± Δ → Na₂CO₃ (s) + H₂O (l) + CO₂ (g)↑

Question 14: Write an equation to show the reaction between Plaster of Paris and water.

Answer:

When Plaster of Paris (POP) reacts with water, it rehydrates to form a hard, solid mass called Gypsum:

CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O
(Plaster of Paris) + (Water) → (Gypsum - Hard solid)

NCERT Exercise Solutions (End of Chapter)

Question 1: A solution turns red litmus blue, its pH is likely to be:

(a) 1     (b) 4     (c) 5     (d) 10

Answer: (d) 10

Reason: Bases turn red litmus blue. Basic solutions have a pH greater than 7. Among the given options, only 10 is greater than 7.

Question 2: A solution reacts with crushed egg-shells to give a gas that turns lime-water milky. The solution contains:

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

Answer: (b) HCl

Reason: Crushed eggshells are composed primarily of Calcium Carbonate (CaCO₃). When CaCO₃ reacts with hydrochloric acid (HCl), it releases carbon dioxide (CO₂) gas, which turns lime water milky due to the formation of insoluble calcium carbonate:

CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂↑

Question 3: 10 mL of a solution of NaOH is found to be completely neutralised by 8 mL of a given solution of HCl. If we take 20 mL of the same solution of NaOH, the amount HCl solution (the same solution as before) required to neutralise it will be:

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

Answer: (d) 16 mL

Method:
Given: 10 mL NaOH solution requires = 8 mL HCl solution
Therefore, 1 mL NaOH solution requires = (8 / 10) mL HCl
For 20 mL NaOH solution, HCl required = (8 / 10) × 20 = 16 mL

Question 4: Which one of the following types of medicines is used for treating indigestion?

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

Answer: (c) Antacid

Reason: Indigestion is caused by the excess production of hydrochloric acid in the stomach. Antacids are mild basic substances (such as Magnesium Hydroxide / Milk of Magnesia or Sodium Hydrogen Carbonate) that neutralise excess stomach acid.

Question 5: Write word equations and balanced equations for the reactions taking place when:

1. Dilute sulphuric acid reacts with zinc granules.
2. Dilute hydrochloric acid reacts with magnesium ribbon.
3. Dilute sulphuric acid reacts with aluminium powder.
4. Dilute hydrochloric acid reacts with iron filings.

Answer:

1. Word Equation: Zinc + Dilute Sulphuric acid → Zinc sulphate + Hydrogen gas
   Balanced Equation: Zn (s) + H₂SO₄ (aq) → ZnSO₄ (aq) + H₂ (g)↑
2. Word Equation: Magnesium + Dilute Hydrochloric acid → Magnesium chloride + Hydrogen gas
   Balanced Equation: Mg (s) + 2HCl (aq) → MgCl₂ (aq) + H₂ (g)↑
3. Word Equation: Aluminium + Dilute Sulphuric acid → Aluminium sulphate + Hydrogen gas
   Balanced Equation: 2Al (s) + 3H₂SO₄ (aq) → Al₂(SO₄)₃ (aq) + 3H₂ (g)↑
4. Word Equation: Iron + Dilute Hydrochloric acid → Iron(II) chloride + Hydrogen gas
   Balanced Equation: Fe (s) + 2HCl (aq) → FeCl₂ (aq) + H₂ (g)↑

Question 6: Compounds such as alcohols and glucose also contain hydrogen but are not categorized as acids. Describe an Activity to prove it.

Answer:

1. Apparatus Setup: Fix two iron nails on a rubber cork and place it in a 100 mL beaker. Connect the nails to the two terminals of a 6-volt battery through a bulb and a plug key.
2. Procedure:
   • Pour dilute HCl solution into the beaker and switch on the current. The bulb glows brightly because HCl dissociates into H⁺ and Cl⁻ ions.
   • Rinse the beaker thoroughly and repeat the experiment separately with glucose (C₆H₁₂O₆) solution and alcohol (C₂H₅OH) solution.
3. Observation: The bulb does not glow in the case of glucose or alcohol solutions, and the ammeter indicates no electric current.
4. Conclusion: Although glucose and alcohol contain hydrogen, they do not ionise to produce free H⁺(aq) ions in solution. Hence, they do not show acidic properties or conduct electricity.

Question 7: Why does distilled water not conduct electricity, whereas rain water does?

Answer:

Distilled water is a pure chemical substance consisting entirely of neutral H₂O molecules without any dissolved mineral salts or ions. In the absence of free ions, it acts as an electrical insulator.

Rainwater, while falling through the atmosphere, dissolves atmospheric gases such as carbon dioxide (CO₂), sulphur dioxide (SO₂), and nitrogen oxides. Dissolved CO₂ forms carbonic acid (H₂CO₃), which ionises in water:

H₂CO₃ (aq) ↔ 2H⁺ (aq) + CO₃²⁻ (aq)

The presence of these mobile ions enables rainwater to conduct electricity.

Question 8: Why do acids not show acidic behaviour in the absence of water?

Answer:

The acidic behaviour of any substance is specifically due to the formation of hydrogen ions (H⁺) or hydronium ions (H₃O⁺). Hydrogen ions cannot exist in isolation; they are released only when water molecules pull them away from polar covalent acid molecules through hydration:

HA + H₂O → H₃O⁺ + A⁻

In the complete absence of water, acids cannot ionise. Without free H⁺/H₃O⁺ ions, acids cannot change the colour of indicators, react with metals to liberate H₂, or exhibit any acidic traits.

Question 9: 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:

• (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.

Answer:

• (a) Neutral: Solution D (pH = 7)
• (b) Strongly alkaline: Solution C (pH = 11)
• (c) Strongly acidic: Solution B (pH = 1)
• (d) Weakly acidic: Solution A (pH = 4)
• (e) Weakly alkaline: Solution E (pH = 9)

Increasing order of hydrogen-ion concentration [H⁺]:
Lower hydrogen-ion concentration corresponds to higher pH value.
pH 11 < pH 9 < pH 7 < pH 4 < pH 1
Therefore: Solution C < Solution E < Solution D < Solution A < Solution B

Question 10: 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 the same. In which test tube will the fizzing occur more vigorously and why?

Answer:

Fizzing will occur much more vigorously in test tube A containing hydrochloric acid (HCl).

Scientific Reason:

1. HCl is a strong acid that ionises completely in aqueous solution, yielding a very high concentration of H⁺ ions.
2. CH₃COOH (acetic acid) is a weak acid that only partially ionises, resulting in a significantly lower concentration of H⁺ ions.
3. Because the rate of hydrogen gas evolution depends on the concentration of H⁺ ions available to react with magnesium, the reaction proceeds much faster in test tube A, causing vigorous fizzing.

Question 11: Fresh milk has a pH of 6. How do you think the pH will change as it turns into curd? Explain your answer.

Answer:

When fresh milk turns into curd, its pH decreases (becomes less than 6).

Explanation: During the curdling process, Lactobacillus bacteria convert the lactose sugar present in milk into lactic acid. The production of lactic acid increases the concentration of free H⁺ ions in the medium. Since pH is inversely related to hydrogen ion concentration, the pH drops into the acidic range (typically around 4.5 to 5.0).

Question 12: A milkman adds a very small amount of baking soda to fresh milk.

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

Answer:

1. Reason for shifting pH: Fresh milk naturally turns sour when lactic acid is produced by bacteria. By adding baking soda (NaHCO₃, a mild base), the milkman makes the milk slightly alkaline. This basic medium neutralises initial traces of lactic acid, preventing the milk from souring quickly during warm weather and transport.
2. Reason for delay in curd setting: Before curd can form, the lactic acid produced by Lactobacillus must first neutralise the added alkali (baking soda). Only after the alkaline base is completely consumed can the pH drop below 6 into the acidic range required for milk protein coagulation. Hence, it takes a much longer time to set as curd.

Question 13: Plaster of Paris should be stored in a moisture-proof container. Explain why?

Answer:

Plaster of Paris (Calcium Sulphate Hemihydrate, CaSO₄·½H₂O) is a hygroscopic white powder. When exposed to even small amounts of atmospheric moisture or water, it rapidly absorbs water molecules and sets into a hard, non-mouldable mass of Gypsum (CaSO₄·2H₂O):

CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O (Hard Solid)

Once set, it permanently loses its setting property and usability. Storing it in airtight, moisture-proof containers preserves its powder form.

Question 14: What is a neutralisation reaction? Give two examples.

Answer:

A chemical reaction in which an acid and a base react together quantitatively to form a salt and water with the evolution of heat is called a neutralisation reaction:

Acid + Base → Salt + Water + Heat

Examples:

1. Reaction between hydrochloric acid and sodium hydroxide:
   HCl (aq) + NaOH (aq) → NaCl (aq) + H₂O (l)
2. Reaction between sulphuric acid and potassium hydroxide:
   H₂SO₄ (aq) + 2KOH (aq) → K₂SO₄ (aq) + 2H₂O (l)

Question 15: Give two important uses of washing soda and baking soda.

Answer:

Two Important Uses of Washing Soda (Na₂CO₃·10H₂O):

1. Used in the manufacture of glass, soap, and paper products, as well as sodium compounds like borax.
2. Used as a cleansing agent for domestic laundry and for removing permanent hardness of water.

Two Important Uses of Baking Soda (NaHCO₃):

1. Used as an ingredient in baking powder (a mixture of NaHCO₃ and mild tartaric acid) to make cakes, bread, and dhoklas soft and spongy via the liberation of CO₂ gas.
2. Used as an active antacid to neutralise excess stomach acidity and in soda-acid fire extinguishers.

Important Formulas, Chemical Equations, and Concepts

Here is a quick reference summary of all core chemical reactions and formulas in Chapter 2:

Key Concept: Water of Crystallisation

Water of crystallisation is the fixed number of water molecules chemically attached to one formula unit of a salt in its crystalline form. Common examples include:

• Hydrated Copper Sulphate: CuSO₄·5H₂O (Blue crystals turn white anhydrous CuSO₄ on heating).
• Hydrated Ferrous Sulphate: FeSO₄·7H₂O (Green crystals).
• Gypsum: CaSO₄·2H₂O.

Common Mistakes and Tips

• Dilution Error: Never write "add water to acid". Always specify adding acid slowly to water with continuous stirring. Mentioning the exothermic nature scores full marks.
• Baking Soda vs. Baking Powder: Baking soda is pure NaHCO₃. Baking powder is a mixture of NaHCO₃ and an edible acid (such as tartaric acid). Tartaric acid neutralises the bitter sodium carbonate produced on heating, preventing bitter taste.
• Incomplete Chemical States: In board exams, always write state symbols: (s) for solid, (l) for liquid, (g) for gas, and (aq) for aqueous solutions.
• Plaster of Paris Temperature: Gypsum must be heated at exactly 373 K (100°C). If heated above 373 K, all water of crystallisation is lost, forming dead burnt plaster (anhydrous CaSO₄) which does not set with water.

Board Exam Relevance

In the CBSE Class 10 Board Exam, Chapter 2 is evaluated across diverse question formats:

• 1-Mark MCQs & Assertion-Reason: Questions regarding indicator color changes, pH comparison of common liquids, and identification of gases (CO₂ vs H₂).
• 2 & 3-Mark Short Answer Questions: Explaining why dry HCl doesn't affect dry litmus, reasons for adding baking soda to milk, and Chlor-Alkali reaction products at anode and cathode (Cl₂ at anode, H₂ at cathode, NaOH near cathode).
• 4-Mark Case-Based Questions: Paragraphs detailing pH in digestive health or industrial preparation of sodium compounds with sub-questions on compound identification ('X', 'Y', 'Z').

More NCERT Solutions and Practice

To master Class 10 Science and secure a 100% score in your board exams, practicing with official pattern questions is crucial. Explore chapter-wise test series, previous year solved papers (PYQs), assertion-reason banks, and competency-based case studies tailored strictly to the latest CBSE syllabus at qptool.theorify.in.