Navigating through the CBSE Class 10 Science curriculum requires an in-depth understanding of chemical properties, ionic dissociation, pH dynamics, and the commercial preparation of industrial salts. Chapter 2 of Class 10 Chemistry, “Acids, Bases and Salts”, forms the foundation of chemical reactivity in secondary education. It investigates how acids and bases interact with metals, carbonates, and indicators; explores the role of water in generating hydronium (H₃O⁺) and hydroxide (OH⁻) ions; and details the commercial manufacture of daily chemical compounds like Bleaching Powder, Baking Soda, Washing Soda, and Plaster of Paris. To help students master every aspect of this high-weightage chapter, this comprehensive solutions guide offers textbook-accurate, highly structured, and pedagogically sound responses strictly aligned with the latest CBSE evaluation standards.
Every question presented in the official NCERT textbook—ranging from all five in-text question sets and chapter-end exercises to balancing chemical equations, indicator color charts, chemical salt tables, and an expanded set of 15 board-level FAQs—has been solved with exhaustive detail. Short-answer conceptual responses follow the official 30–40 word limit, while 3-mark and 5-mark reactions use structured chemical equations with states of matter. Key scoring terms, balanced reactions, and practical safety protocols have been highlighted to ensure students secure maximum marks in their CBSE Board Examinations.
Master Formula & Concept Summary Tables
1. Indicators and Their Colour Changes
| Indicator | Original Colour | Colour in Acidic Medium | Colour in Basic Medium |
|---|---|---|---|
| Blue Litmus Paper / Solution | Blue | Red | Remains Blue |
| Red Litmus Paper / Solution | Red | Remains Red | Blue |
| Phenolphthalein | Colourless | Colourless | Deep Pink |
| Methyl Orange | Orange | Red / Pinkish-Red | Yellow |
| Turmeric (Natural Indicator) | Yellow | Remains Yellow | Reddish-Brown |
| Vanilla Essence / Clove Oil | Characteristic Smell | Smell Retained | Smell Destroyed / Lost |
2. Important Chemical Salts Master Table
| Chemical Compound | Common Name | Chemical Formula | Preparation Reaction | Key Applications / Uses |
|---|---|---|---|---|
| Sodium Hydroxide | Caustic Soda | NaOH | 2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g) (Chlor-Alkali Process) | Making soaps, detergents, paper manufacturing, oil refining. |
| Calcium Oxychloride | Bleaching Powder | CaOCl₂ | Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O | Disinfecting drinking water, bleaching cotton and linen. |
| Sodium Hydrogen Carbonate | Baking Soda | NaHCO₃ | NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃ | Antacid in medicine, baking powder for cakes, fire extinguishers. |
| Sodium Carbonate Decahydrate | Washing Soda | Na₂CO₃·10H₂O | Na₂CO₃ + 10H₂O → Na₂CO₃·10H₂O | Softening permanent hard water, manufacturing glass, soap, and paper. |
| Calcium Sulphate Hemihydrate | Plaster of Paris (POP) | CaSO₄·½H₂O | CaSO₄·2H₂O —(373 K)→ CaSO₄·½H₂O + 1½H₂O | Setting fractured bones in casts, making decorative statues, false ceilings. |
| Calcium Sulphate Dihydrate | Gypsum | CaSO₄·2H₂O | CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O | Setting agent in cement manufacturing, soil conditioning. |
3. Natural Sources of Organic Acids
| Natural Source | Organic Acid Present |
|---|---|
| Vinegar | Acetic Acid (CH₃COOH) |
| Orange / Lemon | Citric Acid |
| Tamarind | Tartaric Acid |
| Tomato | Oxalic Acid |
| Sour Milk / Curd | Lactic Acid |
| Ant Sting / Nettle Sting | Methanoic Acid / Formic Acid (HCOOH) |
NCERT In-Text Questions: Set 1 (Page No. 18)
Question 1 You have been provided with three test tubes. One contains distilled water and the other two contain an acidic solution and a basic solution, respectively. If you are given only red litmus paper, how will you identify the contents of each test tube? [BOARD EXAM FAVORITE]
Answer: We can identify the contents of the three test tubes using only red litmus paper through the following step-by-step procedure:
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IDENTIFICATION PROTOCOL USING ONLY RED LITMUS PAPER:
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STEP 1: Dip the red litmus paper into a small sample taken from each test tube.
• The test tube whose solution turns the RED litmus paper BLUE is the
BASIC solution (Sodium Hydroxide / Alkali).
STEP 2: Take this newly formed BLUE litmus paper and dip it into the remaining
two test tubes.
• The test tube whose solution turns the BLUE litmus paper back to RED
is the ACIDIC solution (Hydrochloric acid / dilute acid).
STEP 3: The remaining third test tube will have NO EFFECT on either red or blue
litmus paper. This confirms it is DISTILLED WATER (neutral substance).
================================================================================
NCERT In-Text Questions: Set 2 (Page No. 22)
Question 1 Why should curd and sour substances not be kept in brass and copper vessels? [BOARD EXAM FAVORITE]
Answer: Curd and sour foodstuffs contain organic acids (such as lactic acid).
When stored in brass or copper containers, the acids react with the active metals (copper and zinc) to produce toxic, poisonous metallic salts and release hydrogen gas. Consuming food contaminated with these toxic metallic compounds causes severe food poisoning and gastrointestinal damage.
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? [BOARD EXAM FAVORITE]
Answer: Hydrogen gas (H₂) is usually liberated when an acid reacts with a reactive metal.
================================================================================ CHEMICAL REACTION & TESTING PROTOCOL: -------------------------------------------------------------------------------- Reaction Example: Dilute Sulphuric Acid + Zinc Granules → Zinc Sulphate + Hydrogen Gas H₂SO₄(aq) + Zn(s) → ZnSO₄(aq) + H₂(g)↑ Test for Hydrogen Gas (Pop Sound Test): 1. Pass the evolved gas through a soap bubble solution. Soap bubbles filled with hydrogen rise into the air. 2. Bring a burning candle near the soap bubble. The bubble bursts and 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. [BOARD EXAM FAVORITE]
Answer:
- The metal compound A is Calcium Carbonate (CaCO₃) (limestone/marble).
- The gas evolved that extinguishes a burning candle with effervescence is Carbon Dioxide (CO₂).
================================================================================ BALANCED CHEMICAL EQUATION: -------------------------------------------------------------------------------- Calcium Carbonate + Hydrochloric Acid → Calcium Chloride + Water + Carbon Dioxide CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)↑ ================================================================================
NCERT In-Text Questions: Set 3 (Page No. 25)
Question 1 Why do HCl, HNO₃, etc., show acidic characters in aqueous solutions while solutions of compounds like alcohol and glucose do not show acidic character? [BOARD EXAM FAVORITE]
Answer: Acidic behavior is caused solely by the presence of free-floating Hydrogen ions (H⁺) or Hydronium ions (H₃O⁺) in aqueous solution:
- Mineral acids like HCl and HNO₃ completely dissociate (ionize) in water to release free H⁺(aq) ions, which turn blue litmus red and conduct electricity.
- Compounds like glucose (C₆H₁₂O₆) and ethyl alcohol (C₂H₅OH) contain hydrogen atoms bound by covalent bonds. They do not ionize in water to produce H⁺ ions, and therefore show no acidic character.
Question 2 Why does an aqueous solution of an acid conduct electricity?
Answer: An aqueous solution of an acid conducts electricity because acids dissociate into free, mobile ions in water:
- For example, hydrochloric acid ionizes into hydronium cations (H₃O⁺) and chloride anions (Cl⁻).
- These mobile charge-carrying ions move freely toward oppositely charged electrodes when an electric potential is applied, allowing electric current to flow through the solution.
Question 3 Why does dry HCl gas not change the colour of the dry litmus paper? [BOARD EXAM FAVORITE]
Answer: Dry HCl gas does not change the color of dry blue litmus paper because acids produce H⁺ ions only in the presence of water.
In the total absence of moisture, HCl exists as a neutral covalent gas and cannot dissociate into hydrogen or hydronium ions. Since free H⁺(aq) ions are required to alter indicator dyes, dry HCl shows no acidic properties.
Question 4 While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid? [BOARD EXAM FAVORITE]
Answer: The process of dissolving a concentrated mineral acid in water is extremely exothermic, releasing a massive amount of localized heat energy:
- If water is added to concentrated acid, the sudden generation of intense heat instantly converts the first drops of water into steam, which can cause the concentrated acid to splash out violently onto the face and hands, causing severe chemical burns.
- The glass container may also break due to excessive localized thermal shock.
- Therefore, acid must always be added slowly to water with constant stirring, allowing the large volume of water to safely absorb and dissipate the heat.
Question 5 How is the concentration of hydronium ions (H₃O⁺) affected when a solution of an acid is diluted?
Answer: When an acid solution is diluted by adding water, the concentration of hydronium ions (H₃O⁺) per unit volume decreases. Consequently, the acidic strength of the solution decreases, and its pH value increases toward 7.
Question 6 How is the concentration of hydroxide ions (OH⁻) affected when excess base is dissolved in a solution of sodium hydroxide?
Answer: When an excess amount of base is dissolved in an existing sodium hydroxide solution, the concentration of hydroxide ions (OH⁻) per unit volume increases. Consequently, the basic strength of the solution increases, and its pH value rises closer to 14.
NCERT In-Text Questions: Set 4 (Page No. 28)
Question 1 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 one is basic? [BOARD EXAM FAVORITE]
Answer:
- Hydrogen Ion Concentration: Solution A has a higher hydrogen ion concentration. (On the pH scale, a lower pH value represents a higher H⁺ concentration).
- Acidic Solution: Solution A (pH = 6, which is < 7) is acidic.
- Basic Solution: Solution B (pH = 8, which is > 7) is basic.
Question 2 What effect does the concentration of H⁺(aq) ions have on the nature of the solution?
Answer: The concentration of H⁺(aq) ions directly determines the acidity of a solution:
- A higher concentration of H⁺(aq) ions makes the solution more acidic (resulting in a lower pH value from 0 to 6).
- A lower concentration of H⁺(aq) ions makes the solution less acidic / more basic (resulting in a higher pH value from 8 to 14).
Question 3 Do basic solutions also have H⁺(aq) ions? If yes, then why are these basic? [BOARD EXAM FAVORITE]
Answer: Yes, basic solutions do contain H⁺(aq) ions formed from the auto-ionization of water molecules.
However, they are basic because the concentration of hydroxide ions (OH⁻) is far greater than the concentration of hydrogen ions (H⁺): [OH⁻] is much greater than [H⁺] ([OH⁻] >> [H⁺])
The dominant excess of OH⁻ ions gives the solution its basic characteristics.
Question 4 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)? [BOARD EXAM FAVORITE]
Answer: A farmer treats agricultural soil with quick lime (CaO), slaked lime (Ca(OH)₂), or chalk (CaCO₃) when the soil has become excessively acidic (pH < 6.0) due to acid rain or overuse of chemical fertilizers.
Since all three calcium compounds are basic in nature, they react with and neutralize the excess soil acidity, restoring the optimal pH range (6.5 – 7.5) necessary for healthy crop growth.
NCERT In-Text Questions: Set 5 (Page No. 33)
Question 1 What is the common name of the compound CaOCl₂?
Answer: The common name of the compound CaOCl₂ (Calcium Oxychloride) is Bleaching Powder.
Question 2 Name the substance which on treatment with chlorine yields bleaching powder. [BOARD EXAM FAVORITE]
Answer: Dry Slaked Lime [Calcium Hydroxide, Ca(OH)₂] yields bleaching powder when treated with chlorine gas: Ca(OH)₂(s) + Cl₂(g) → CaOCl₂(s) + H₂O(l)
Question 3 Name the sodium compound which is used for softening hard water. [BOARD EXAM FAVORITE]
Answer: Washing Soda [Sodium Carbonate Decahydrate, Na₂CO₃·10H₂O] is used for removing permanent hardness of water.
Question 4 What will happen if a solution of sodium hydrogencarbonate is heated? Give the equation of the reaction involved. [BOARD EXAM FAVORITE]
Answer: When a solution of sodium hydrogencarbonate (baking soda) is heated, it decomposes thermally into Sodium Carbonate (Na₂CO₃), Water (H₂O), and releases Carbon Dioxide gas (CO₂) with effervescence.
================================================================================ THERMAL DECOMPOSITION OF BAKING SODA: -------------------------------------------------------------------------------- Sodium Hydrogen Carbonate —(Heat)→ Sodium Carbonate + Water + Carbon Dioxide 2NaHCO₃(s) —(Δ)→ Na₂CO₃(s) + H₂O(l) + CO₂(g)↑ ================================================================================
Question 5 Write an equation to show the reaction between Plaster of Paris and water. [BOARD EXAM FAVORITE]
Answer: When Plaster of Paris (CaSO₄·½H₂O) is mixed with water, it rehydrates and sets within 10–15 minutes into a hard crystalline solid mass called Gypsum:
================================================================================ HYDRATION REACTION OF PLASTER OF PARIS: -------------------------------------------------------------------------------- Plaster of Paris + Water → Gypsum (Hard Solid Mass) CaSO₄·½H₂O(s) + 1½H₂O(l) → CaSO₄·2H₂O(s) ================================================================================
NCERT Chapter-End Exercises (Page No. 34-35)
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 Explanation: Basic solutions turn red litmus blue and have a pH value greater than 7. Among the given options, only 10 represents a basic pH.
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 Explanation: Crushed egg-shells are composed of Calcium Carbonate (CaCO₃). When CaCO₃ reacts with hydrochloric acid (HCl), it liberates carbon dioxide gas (CO₂), which turns lime water milky.
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 Explanation: Since 10 mL of NaOH requires 8 mL of HCl, doubling the volume of NaOH to 20 mL will require double the volume of HCl (8 mL × 2 = 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 Explanation: Indigestion is caused by excess production of hydrochloric acid in the stomach. Antacids contain mild bases (like Mg(OH)₂ or NaHCO₃) that neutralize excess acid.
Question 5 Write word equations and then balanced equations for the reaction taking place when: [BOARD EXAM FAVORITE] (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.
Answer:
================================================================================
WORD AND BALANCED CHEMICAL EQUATIONS:
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(a) Word Equation: Sulphuric acid + Zinc → Zinc sulphate + Hydrogen
Balanced Eq: H₂SO₄(aq) + Zn(s) → ZnSO₄(aq) + H₂(g)↑
(b) Word Equation: Hydrochloric acid + Magnesium → Magnesium chloride + Hydrogen
Balanced Eq: 2HCl(aq) + Mg(s) → MgCl₂(aq) + H₂(g)↑
(c) Word Equation: Sulphuric acid + Aluminium → Aluminium sulphate + Hydrogen
Balanced Eq: 3H₂SO₄(aq) + 2Al(s) → Al₂(SO₄)₃(aq) + 3H₂(g)↑
(d) Word Equation: Hydrochloric acid + Iron → Iron(II) chloride + Hydrogen
Balanced Eq: 2HCl(aq) + Fe(s) → FeCl₂(aq) + H₂(g)↑
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Question 6 Compounds such as alcohols and glucose also contain hydrogen but are not categorised as acids. Describe an Activity to prove it. [BOARD EXAM FAVORITE]
Answer: To prove that glucose and alcohol are not acids, we conduct an electrical conductivity activity:
================================================================================ ACTIVITY PROTOCOL TO TEST IONIZATION OF GLUCOSE AND ALCOHOL: -------------------------------------------------------------------------------- 1. Apparatus Setup: Take a 100 mL beaker and place a rubber cork fitted with two iron nails at the bottom. 2. Circuit Connection: Connect the nails to a 6 V battery through an electric bulb and a plug key. 3. Testing with Acid: Pour dilute HCl into the beaker and switch on current. The bulb glows brightly because HCl dissociates into free H⁺ and Cl⁻ ions. 4. Testing with Glucose & Alcohol: Empty, clean, and refill the beaker with glucose solution (C₆H₁₂O₆), then repeat separately with ethyl alcohol (C₂H₅OH). 5. Observation: In both glucose and alcohol solutions, the bulb does NOT glow. 6. Conclusion: Although glucose and alcohol contain hydrogen, they do not ionize in water to release free H⁺(aq) ions. Hence, they are not categorized as acids. ================================================================================
Question 7 Why does distilled water not conduct electricity, whereas rain water does? [BOARD EXAM FAVORITE]
Answer:
- Distilled Water: It is pure water completely devoid of dissolved salts or minerals. It does not undergo significant self-ionization and lacks free, mobile ions to carry an electric current, making it an electrical insulator.
- Rain Water: As rain falls through the atmosphere, it dissolves atmospheric acidic gases like Carbon Dioxide (CO₂), Sulphur Dioxide (SO₂), and Nitrogen Oxides (NO₂), forming trace mineral acids like Carbonic Acid (H₂CO₃). These acids dissociate into free mobile ions (H⁺ and anions), allowing rainwater to conduct electricity.
Question 8 Why does dry HCl gas not show acidic behaviour in the absence of water?
Answer: Acidic properties are exhibited solely by free hydronium ions (H₃O⁺).
In the total absence of water, HCl exists as a non-ionized covalent gas. Separation of H⁺ from the Cl⁻ ion cannot occur without the polar pull of water molecules. Thus, dry HCl gas cannot produce H⁺ ions and cannot show acidic behavior.
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: [BOARD EXAM FAVORITE] (a) neutral? (b) strongly alkaline? (c) strongly acidic? (d) weakly acidic? (e) weakly alkaline? Arrange the pH 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)
Arrangement in Increasing Order of Hydrogen-ion [H⁺] Concentration: Solution C (pH 11) < Solution E (pH 9) < Solution D (pH 7) < Solution A (pH 4) < Solution B (pH 1)
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 the acids are same. In which test tube will the fizzing occur more vigorously and why? [BOARD EXAM FAVORITE]
Answer: Fizzing will occur much more vigorously in Test Tube A (containing Hydrochloric Acid, HCl).
- Scientific Reason: HCl is a strong acid that dissociates completely in aqueous solution, providing a very high concentration of H⁺ ions per unit volume. This causes a rapid chemical reaction with magnesium, liberating hydrogen gas rapidly.
- In contrast, Acetic Acid (CH₃COOH) is a weak acid that only partially ionizes, producing a low concentration of H⁺ ions and resulting in a much slower, gentle reaction.
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. [BOARD EXAM FAVORITE]
Answer: As fresh milk turns into curd, its pH will decrease below 6 (becoming more acidic, around pH 4.5 – 5.0).
- Explanation: Bacteria called Lactobacillus ferment the lactose sugar present in milk into Lactic Acid. The continuous accumulation of lactic acid increases the concentration of H⁺ ions in the medium, which causes the pH value to drop.
Question 12 A milkman adds a very small amount of baking soda to fresh milk. [BOARD EXAM FAVORITE] (a) Why does he shift the pH of the fresh milk from 6 to slightly alkaline? (b) Why does this milk take a long time to set as curd?
Answer:
- (a) Reason for shifting pH: Baking soda (NaHCO₃) is a mild alkaline salt. Adding it makes the milk slightly alkaline (pH 7.5 – 8.0), which prevents the milk from turning sour quickly in warm weather by neutralizing the lactic acid produced by bacteria.
- (b) Reason for delayed setting: When milk turns into curd, it requires an acidic medium. Since the milk has been made alkaline with baking soda, the lactic acid formed must first neutralize the alkaline base before the pH drops low enough to coagulate milk proteins into curd.
Question 13 Plaster of Paris should be stored in a moisture-proof container. Explain why? [BOARD EXAM FAVORITE]
Answer: Plaster of Paris (CaSO₄·½H₂O) is highly hygroscopic and readily absorbs moisture from the atmospheric air.
If stored in an open or damp container, it reacts with atmospheric water vapor and sets into a hard, rigid crystalline solid mass of Gypsum (CaSO₄·2H₂O), rendering it permanently useless for casting or medical plastering.
Question 14 What is a neutralisation reaction? Give two examples. [BOARD EXAM FAVORITE]
Answer: A neutralization reaction is a chemical reaction in which an acid reacts quantitatively with a base to form salt and water, with the release of thermal energy. Acid + Base → Salt + Water
================================================================================ TWO BALANCED EXAMPLES OF NEUTRALIZATION: -------------------------------------------------------------------------------- Example 1: Strong Acid + Strong Base Hydrochloric Acid + Sodium Hydroxide → Sodium Chloride + Water HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l) Example 2: Strong Acid + Weak Base Sulphuric Acid + Potassium Hydroxide → Potassium Sulphate + Water H₂SO₄(aq) + 2KOH(aq) → K₂SO₄(aq) + 2H₂O(l) ================================================================================
Question 15 Give two important uses of washing soda and baking soda. [BOARD EXAM FAVORITE]
Answer:
Two Important Uses of Washing Soda (Na₂CO₃·10H₂O):
- It is widely used in the manufacturing of glass, soap, paper, and chemical compounds like Borax.
- It is used as a laboratory and industrial reagent for removing permanent hardness of water.
Two Important Uses of Baking Soda (NaHCO₃):
- It is used as an active ingredient in baking powder to make cakes, breads, and pakoras soft, light, and spongy.
- It is used in medicine as an antacid to neutralize excess stomach acidity and in soda-acid fire extinguishers.
Frequently Asked Questions (FAQs) – Class 10 Chemistry Chapter 2
Question 1: What is an alkali? Are all bases alkalis? Answer: An alkali is a base that is completely soluble in water (such as NaOH, KOH). No, all bases are not alkalis; only water-soluble bases are categorized as alkalis.
Question 2: What is the chemical name and formula of Bleaching Powder? Answer: The chemical name is Calcium Oxychloride, and its chemical formula is CaOCl₂.
Question 3: What is the Chlor-Alkali process? Answer: It is the industrial process of passing electricity through an aqueous solution of sodium chloride (brine) to produce Sodium Hydroxide (NaOH), Chlorine gas (Cl₂), and Hydrogen gas (H₂).
Question 4: What is the pH range of the human body? Answer: The human body carries out all vital metabolic and biological functions within a narrow pH range of 7.0 to 7.8.
Question 5: What is acid rain, and what is its pH value? Answer: Rainwater having a pH value less than 5.6 due to dissolved industrial pollutants like SO₂ and NO₂ is termed acid rain.
Question 6: Why does tooth decay start when mouth pH falls below 5.5? Answer: At pH < 5.5, mouth acids corrode tooth enamel (made of Calcium Hydroxyapatite, the hardest substance in the human body).
Question 7: Which acid is present in an ant sting, and what is its chemical remedy? Answer: Ant stings inject Methanoic Acid (Formic Acid, HCOOH). Applying a paste of a mild base like baking soda (NaHCO₃) neutralizes the acid and relieves pain.
Question 8: What is water of crystallization? Give an example. Answer: It is the fixed number of water molecules chemically attached to one formula unit of a crystalline salt. For example, hydrated copper sulphate (CuSO₄·5H₂O) contains 5 water molecules.
Question 9: What is the difference between Baking Soda and Baking Powder? Answer: Baking Soda is pure Sodium Hydrogen Carbonate (NaHCO₃). Baking Powder is a mixture of baking soda and a mild edible acid like Tartaric Acid, which neutralizes the bitter taste of sodium carbonate formed on heating.
Question 10: What is the chemical formula of Gypsum? Answer: The chemical formula of Gypsum is CaSO₄·2H₂O (Calcium Sulphate Dihydrate).
Question 11: Why is Plaster of Paris represented with half a water molecule (½H₂O)? Answer: It is written as CaSO₄·½H₂O because two formula units of CaSO₄ share one molecule of water of crystallization.
Question 12: What is the color of phenolphthalein in sodium hydroxide solution? Answer: Phenolphthalein turns deep pink in sodium hydroxide solution (basic medium).
Question 13: What happens when carbon dioxide is passed through lime water for a short time and then in excess? Answer: Lime water turns milky due to the formation of insoluble CaCO₃ precipitate. On passing excess CO₂, the milkiness clears due to the formation of soluble Ca(HCO₃)₂.
Question 14: Name an olfactory indicator and its behavior with sodium hydroxide. Answer: Vanilla essence or clove oil; their characteristic pleasant smell is completely destroyed in the presence of NaOH (base).
Question 15: Why are non-metallic oxides called acidic oxides? Answer: Non-metallic oxides (such as CO₂, SO₂) react with bases to form salt and water, exhibiting characteristic acidic chemical behavior.
Mastering the NCERT Solutions for Class 10 Science Chapter 2, “Acids, Bases and Salts”, equips students with the balanced equations, reaction mechanisms, and industrial preparation formulas required for the CBSE Board Examination. Review the indicator color transitions, salt preparation reactions, and the 15 board-level FAQs above to secure a perfect score in your chemistry evaluations.