Questions & Answers - From NCERT PDF (Class 10)
60 Q&A based on questions extracted from the NCERT PDF. Categorized as 20 Short (1 Mark), 20 Medium (4 Marks), 20 Long (8 Marks). Answers tailored to mark allocation, based on PDF content. Answers are displayed directly below each question.
Short Questions (1 Mark Each) - 20 Total
1. A solution turns red litmus blue, its pH is likely to be
1 Mark Answer: (d) 10. (Basic solution, pH >7)
2. A solution reacts with crushed egg-shells to give a gas that turns lime-water milky. The solution contains
1 Mark Answer: (b) HCl. (Produces CO2)
3. Which one of the following types of medicines is used for treating indigestion?
1 Mark Answer: (c) Antacid. (Neutralizes acid)
4. Which gas is usually liberated when an acid reacts with a metal?
1 Mark Answer: Hydrogen gas.
5. What is the common name of the compound Ca(ClO)2?
1 Mark Answer: Bleaching powder.
6. What effect does the concentration of H+(aq) ions have on the nature of the solution?
1 Mark Answer: Makes it acidic.
7. Do basic solutions also have H+(aq) ions? If yes, then why are these basic?
1 Mark Answer: Yes, but low concentration.
8. Why does dry HCl gas not change the colour of the dry litmus paper?
1 Mark Answer: No H+ ions.
9. Why does an aqueous solution of an acid conduct electricity?
1 Mark Answer: Due to ions.
10. Why do HCl, HNO3, etc., show acidic characters in aqueous solutions while solutions of compounds like alcohol and glucose do not show acidic character?
1 Mark Answer: Produce H+ in water.
11. Name the substance which on treatment with chlorine yields bleaching powder.
1 Mark Answer: Ca(OH)2.
12. Name the sodium compound which is used for softening hard water.
1 Mark Answer: Na2CO3·10H2O.
13. You have been provided with three test tubes. One of them 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?
1 Mark Answer: Basic turns blue.
14. Why does distilled water not conduct electricity, whereas rain water does?
1 Mark Answer: No ions.
15. Why do acids not show acidic behaviour in the absence of water?
1 Mark Answer: No ionization.
16. Give two important uses of washing soda.
1 Mark Answer: Glass, cleaning.
17. Give two important uses of baking soda.
1 Mark Answer: Baking, antacid.
18. What is a neutralisation reaction?
1 Mark Answer: Acid + base → salt + water.
19. Fresh milk has a pH of 6. How do you think the pH will change as it turns into curd?
1 Mark Answer: Decreases.
20. Plaster of Paris should be stored in a moisture-proof container. Explain why?
1 Mark Answer: Sets with water.
Medium Questions (4 Marks Each) - 20 Total
1. Why should curd and sour substances not be kept in brass and copper vessels?
4 Marks Answer: Acids in curd react with metals to form poisonous compounds, causing food poisoning. Use non-reactive vessels like glass or ceramic. The reaction is acid + metal → salt + hydrogen, but salts are toxic. This prevents health hazards from metal leaching.
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?
4 Marks Answer: Hydrogen gas. Example: Zn + 2HCl → ZnCl2 + H2. Test: Bring burning candle near gas; pops. The reaction is displacement. Safety: Gas is flammable; use in ventilated area.
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.
4 Marks Answer: A is CaCO3. Equation: CaCO3 + 2HCl → CaCl2 + H2O + CO2. Gas is CO2, extinguishes candle. Test: Lime water turns milky. Reaction is acid + carbonate → salt + water + CO2.
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?
4 Marks Answer: HCl, HNO3 ionize to H3O+ in water. Alcohol, glucose do not ionize. Example: HCl → H+ + Cl-. Test: Conductivity - acids conduct, others don't. Acids produce H+ for acidity.
5. Why does an aqueous solution of an acid conduct electricity?
4 Marks Answer: Dissociates into ions (H3O+, anions) that carry current. Example: H2SO4 in water. Test: Bulb glows in circuit. Strong acids fully ionize for better conductivity; weak partially.
6. Why does dry HCl gas not change the colour of the dry litmus paper?
4 Marks Answer: Dry HCl is covalent, no H+ ions. Water needed for H3O+. Test: Dry no change, wet red litmus. Importance: Acidity requires aqueous medium.
7. While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?
4 Marks Answer: Exothermic reaction; water to acid causes splashing. Acid to water dissipates heat. Example: H2SO4. Safety: Wear goggles, stir.
8. How is the concentration of hydronium ions (H3O+) affected when a solution of an acid is diluted?
4 Marks Answer: Decreases as volume increases. pH rises. Example: 1M HCl to 0.1M. Test: pH meter shows increase.
9. How is the concentration of hydroxide ions (OH–) affected when excess base is dissolved in a solution of sodium hydroxide?
4 Marks Answer: Increases, solution more basic. pH rises. Example: Adding NaOH to NaOH solution. Test: pH paper shows higher pH.
10. 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?
4 Marks Answer: A has more H+ (10^-6 M vs 10^-8 M). A acidic, B basic. Test: Litmus - A red, B blue.
11. What effect does the concentration of H+(aq) ions have on the nature of the solution?
4 Marks Answer: High [H+] makes acidic, low basic. pH = -log[H+]. Example: High [H+] low pH. Test: Universal indicator.
12. Do basic solutions also have H+(aq) ions? If yes, then why are these basic?
4 Marks Answer: Yes, but [H+] < [OH-]. Basic due to high [OH-]. Kw = 10^-14. Example: NaOH [H+] low.
13. 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)?
4 Marks Answer: Acidic soil (pH <7). Lime neutralizes acid. Example: CaO + 2H+ → Ca2+ + H2O. Test: pH paper.
14. What will happen if a solution of sodium hydrocarbonate is heated? Give the equation of the reaction involved.
4 Marks Answer: Decomposes to Na2CO3 + H2O + CO2. Equation: 2NaHCO3 → Na2CO3 + H2O + CO2. Test: CO2 with lime water.
15. Write an equation to show the reaction between Plaster of Paris and water.
4 Marks Answer: CaSO4·1/2H2O + 3/2H2O → CaSO4·2H2O. Sets to gypsum. Exothermic. Use: Casts.
16. Compounds such as alcohols and glucose also contain hydrogen but are not categorised as acids. Describe an Activity to prove it.
4 Marks Answer: Conductivity test: Acids glow bulb; glucose/alcohol don't. No H+ ions. Setup: Electrodes, battery.
17. Why does distilled water not conduct electricity, whereas rain water does?
4 Marks Answer: Distilled no ions; rain has CO2, acids ions. Test: Bulb no glow for distilled. Example: Acid rain pH <5.6.
18. Why do acids not show acidic behaviour in the absence of water?
4 Marks Answer: No H3O+ without water. Test: Dry HCl no litmus change. Ionization in aqueous medium.
19. Fresh milk has a pH of 6. How do you think the pH will change as it turns into curd? Explain your answer.
4 Marks Answer: pH decreases. Lactic acid formed by bacteria. Fermentation lowers pH to ~4-5. Test: pH paper.
20. A milkman adds a very small amount of baking soda to fresh milk. (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?
4 Marks Answer: (a) Prevents souring by neutralizing acid. (b) Bacteria need time to produce acid against base. pH shift delays.
Long Questions (8 Marks Each) - 20 Total
1. Why should curd and sour substances not be kept in brass and copper vessels?
8 Marks Answer: Curd and sour substances contain acids like lactic acid, which react with brass (Cu+Zn) and copper to form toxic salts and hydrogen gas. The reaction is acid + metal → salt + H2. These salts can cause food poisoning. To prevent this, use non-reactive materials like glass or stainless steel. Activity: Place curd in copper vessel and observe green deposit (copper lactate). Safety: Avoid consuming from metal vessels. From PDF: Acids react with metals. Net: Similar reactions lead to corrosion and toxicity. Importance: Food safety in kitchens.
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?
8 Marks Answer: Hydrogen gas is liberated. Example: Zinc granules + dilute H2SO4 → ZnSO4 + H2 (bubbles formed). Test: Pass gas through soap solution to form bubbles; bring burning candle - pops. From PDF: Activity 2.3 - Zn + H2SO4, bubbles, pop sound. Net: Reactivity series determines if reaction occurs. Safety: Hydrogen is flammable; use small quantities. Importance: Used in hydrogen production. Not all metals react (e.g., Cu no with dilute acids). Equation: 2HCl + Mg → MgCl2 + H2.
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.
8 Marks Answer: A is CaCO3 (limestone/chalk). Equation: CaCO3 + 2HCl → CaCl2 + H2O + CO2. Gas CO2 extinguishes candle (non-supporter). Test: Pass through lime water - milky (CaCO3 precipitate); excess clears (Ca(HCO3)2). From PDF: Activity 2.5 - Na2CO3 + HCl → CO2, lime water milky. Net: General acid + carbonate reaction. Safety: Handle acid carefully. Importance: In antacids, acid rain effects on statues. Applications: Fire extinguishers use this principle.
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?
8 Marks Answer: HCl, HNO3 dissociate to H+ (H3O+) in water, showing acidity. Alcohol, glucose have H but don't dissociate to H+. Equation: HCl + H2O → H3O+ + Cl-. Activity: Conductivity - acids glow bulb, others don't. From PDF: Activity 2.8 - acids conduct. Net: Strong acids fully ionize, weak partially. Importance: Defines Arrhenius acids. Safety: Dilute acids used. Applications: In batteries, cleaning. Contrast: Bases produce OH-.
5. Why does an aqueous solution of an acid conduct electricity?
8 Marks Answer: Acids ionize to H3O+ and anions in water, ions carry current. Example: H2SO4 → 2H3O+ + SO4^2-. Activity: Setup with electrodes, battery, bulb - glows for acids. From PDF: Activity 2.8. Net: Electrolytes vs non-electrolytes. Strong acids better conductors. Safety: Avoid high voltage. Importance: Used in electrolysis, batteries. Contrast: Pure water poor conductor. Applications: Lead-acid batteries.
6. Why does dry HCl gas not change the colour of the dry litmus paper?
8 Marks Answer: Dry HCl is covalent, no H+ ions; acidity needs water for H3O+. Equation: HCl(g) no ions; HCl(aq) → H3O+ + Cl-. Activity: Dry HCl from NaCl + H2SO4 - no change; wet changes. From PDF: Activity 2.9. Net: Ionization theory. Safety: Use guard tube for dry gas. Importance: Shows role of water in acidity. Applications: Gas vs solution properties. Contrast: Aqueous HCl changes litmus red.
7. While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?
8 Marks Answer: Dilution is exothermic; water to acid causes violent splashing from localized heat. Acid to water spreads heat. Equation: Heat released in hydration. Activity: Thermometer shows temperature rise. From PDF: Activity 2.10. Net: Safety protocol. Safety: Wear goggles, add slowly, stir. Importance: Prevents burns in labs. Applications: Preparing solutions. Contrast: Reverse can break glass.
8. How is the concentration of hydronium ions (H3O+) affected when a solution of an acid is diluted?
8 Marks Answer: [H3O+] decreases as volume increases, acidity weakens, pH rises. Example: 1M HCl ([H3O+] =1) to 0.1M ([H3O+] =0.1). Activity: pH paper shows change. From PDF: Intext. Net: Dilution formula. Safety: Dilute carefully. Importance: Controls strength. Applications: Titrations. Contrast: Concentrated high [H3O+].
9. How is the concentration of hydroxide ions (OH–) affected when excess base is dissolved in a solution of sodium hydroxide?
8 Marks Answer: [OH-] increases, basicity strengthens, pH rises. Example: 1M NaOH + excess NaOH → higher [OH-]. Activity: pH meter shows increase. From PDF: Intext. Net: Saturation limit. Safety: Bases corrosive. Importance: Adjust pH. Applications: Soap making. Contrast: Adds more OH- from strong base.
10. 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?
8 Marks Answer: A has more [H+] (10^-6 vs 10^-8), A acidic, B basic. pH = -log[H+]. Activity: Universal indicator - A yellow, B blue. From PDF: Intext. Net: Log scale, 100 times difference. Safety: Handle solutions. Importance: pH classification. Applications: Soil testing. Contrast: A weakly acidic, B weakly basic.
11. What effect does the concentration of H+(aq) ions have on the nature of the solution?
8 Marks Answer: High [H+] makes solution acidic, low basic. Neutral at 10^-7. Equation: pH = -log[H+]. Activity: Test different concentrations with pH paper. From PDF: Intext. Net: Kw = [H+][OH-] = 10^-14. Safety: Dilute acids. Importance: Determines reactivity. Applications: Digestion (stomach low pH). Contrast: Bases have high [OH-].
12. Do basic solutions also have H+(aq) ions? If yes, then why are these basic?
8 Marks Answer: Yes, but [H+] < [OH-]. Basic due to excess OH-. Equation: Kw = 10^-14. Activity: pH of NaOH shows high pH. From PDF: Intext. Net: Water auto-ionization. Safety: Bases slippery. Importance: Neutralization. Applications: Antacids. Contrast: Acids have [H+] > [OH-].
13. 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)?
8 Marks Answer: Acidic soil (pH <7). Lime bases neutralize acids. Equation: CaO + 2H+ → Ca2+ + H2O. Activity: Test soil pH, add lime. From PDF: Intext. Net: Optimal pH 6.5-7.5 for crops. Safety: Lime caustic. Importance: Improves nutrient uptake. Applications: Agriculture. Contrast: Alkaline soil needs acids.
14. What will happen if a solution of sodium hydrocarbonate is heated? Give the equation of the reaction involved.
8 Marks Answer: Decomposes to sodium carbonate, water, CO2. Equation: 2NaHCO3 (heat) → Na2CO3 + H2O + CO2. Activity: Heat, pass CO2 through lime water - milky. From PDF: Intext. Net: Thermal decomposition. Safety: Use test tube holder. Importance: Produces washing soda. Applications: Baking powder. Contrast: Reversible with CO2.
15. Write an equation to show the reaction between Plaster of Paris and water.
8 Marks Answer: CaSO4·1/2H2O + 3/2H2O → CaSO4·2H2O. Sets to hard gypsum, exothermic. Activity: Mix, observe hardening. From PDF: Intext. Net: Hemihydrate to dihydrate. Safety: Avoid skin contact. Importance: Medical casts. Applications: Decoration, molds. Contrast: Gypsum heating reverses.
16. Compounds such as alcohols and glucose also contain hydrogen but are not categorised as acids. Describe an Activity to prove it.
8 Marks Answer: Conductivity test: Dissolve in water, set circuit with bulb - acids glow, alcohol/glucose don't. No H+ ions. Equation: Glucose C6H12O6 no ionization. Activity: Use electrodes, battery. From PDF: Intext. Net: Non-electrolytes. Safety: Use dilute. Importance: Distinguishes ionic/covalent. Applications: Electrolyte solutions.
17. Why does distilled water not conduct electricity, whereas rain water does?
8 Marks Answer: Distilled pure, no ions; rain dissolves CO2 to H2CO3, ions present. Equation: CO2 + H2O → H2CO3 → H+ + HCO3-. Activity: Test conductivity - rain glows slightly. From PDF: Intext. Net: Acid rain has lower pH. Safety: Use pure samples. Importance: Water purity. Applications: Batteries use distilled.
18. Why do acids not show acidic behaviour in the absence of water?
8 Marks Answer: Acids need water to produce H3O+. Dry acids covalent. Equation: HCl(g) no H+; HCl(aq) → H3O+ + Cl-. Activity: Dry HCl no litmus change. From PDF: Activity 2.9. Net: Arrhenius theory. Safety: Handle gases. Importance: Acidity definition. Applications: Anhydrous acids in industry.
19. Fresh milk has a pH of 6. How do you think the pH will change as it turns into curd? Explain your answer.
8 Marks Answer: pH decreases to <6. Bacteria convert lactose to lactic acid. Equation: C12H22O11 → 4C3H6O3. Activity: Test pH of milk and curd. From PDF: Intext. Net: Fermentation process. Safety: Use fresh milk. Importance: Food preservation. Applications: Yogurt making. Contrast: Alkaline addition delays.
20. A milkman adds a very small amount of baking soda to fresh milk. (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?
8 Marks Answer: (a) To prevent souring by neutralizing acid formed. Baking soda (NaHCO3) makes alkaline. (b) Bacteria take longer to produce enough acid to curdle. Equation: NaHCO3 + acid → salt + H2O + CO2. Activity: Compare curdling time. From PDF: Intext. Net: pH control in dairy. Safety: Small amount. Importance: Extends shelf life. Applications: Milk preservation.
Practice Tip: 1M: 2min; 4M: 5min; 8M: 12min; use equations/figures.
