NCERT CLASS 10 ECIENCE CHAPTER 2 ACID,BASE AND SALTS
NCERT CLASS 10 SCIENCE
CHAPTER 2 – ACIDS, BASES AND SALTS
COMPLETE QUESTIONS AND ANSWERS
PART A: IN-TEXT QUESTIONS AND ANSWERS
1. Understanding the Chemical Properties of Acids and Bases
Q1. You are given three test tubes. One contains distilled water, while the other two contain an acidic solution and a basic solution. If you are given only red litmus paper, how will you identify the three solutions?
Answer:
First, dip the red litmus paper into each test tube.
The solution that turns red litmus blue is the basic solution.
Now use the blue litmus paper obtained from the basic solution to test the remaining two solutions.
The solution that turns blue litmus red is acidic.
The solution that shows no colour change is distilled water.
Q2. Why should curd and sour substances not be kept in brass and copper vessels?
Answer:
Curd and sour substances contain acids. These acids may react with metals such as copper and brass and form harmful substances. Therefore, they should not be stored in such vessels.
Q3. Which gas is usually liberated when an acid reacts with a metal? Give an example. How will you test for the presence of this gas?
Answer:
Hydrogen gas is usually liberated when an acid reacts with a metal.
Example:
Zn + H₂SO₄ → ZnSO₄ + H₂
Hydrogen gas can be tested by bringing a burning splint near the gas. It burns with a characteristic pop sound.
Q4. A metal compound A reacts with dilute hydrochloric acid to produce effervescence. The gas evolved extinguishes a burning candle. Write a balanced chemical equation if one of the compounds formed is calcium chloride.
Answer:
The gas that extinguishes a burning candle is carbon dioxide. The metal compound is calcium carbonate.
Balanced equation:
CaCO₃ + 2HCl → CaCl₂ + CO₂ + H₂O
2. What Do All Acids and Bases Have in Common?
Q5. Why do HCl, HNO₃, etc., show acidic character in aqueous solutions, while solutions of compounds like alcohol and glucose do not show acidic character?
Answer:
HCl and HNO₃ produce hydrogen ions in the presence of water. These hydrogen ions combine with water molecules to form hydronium ions (H₃O⁺), which are responsible for acidic properties.
Alcohol and glucose contain hydrogen but do not release hydrogen ions in water. Therefore, they do not show acidic character.
Q6. Why does an aqueous solution of an acid conduct electricity?
Answer:
An aqueous solution of an acid conducts electricity because acids produce ions in water. These ions are responsible for conducting electricity.
Q7. Why does dry HCl gas not change the colour of dry litmus paper?
Answer:
Dry HCl gas does not produce hydrogen ions in the absence of water. Since hydrogen ions are responsible for changing the colour of litmus paper, dry HCl gas does not change the colour of dry litmus paper.
Q8. While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?
Answer:
Dilution of an acid is a highly exothermic process. A large amount of heat is produced. If water is added to concentrated acid, the heat produced may cause the mixture to splash and cause injury. Therefore, acid should always be added slowly to water with continuous stirring.
Q9. How is the concentration of hydronium ions (H₃O⁺) affected when a solution of an acid is diluted?
Answer:
When an acid is diluted with water, the concentration of hydronium ions per unit volume decreases. Therefore, the solution becomes less acidic.
Q10. How is the concentration of hydroxide ions (OH⁻) affected when excess base is dissolved in a solution of sodium hydroxide?
Answer:
When excess sodium hydroxide is dissolved in its solution, the concentration of hydroxide ions increases. As a result, the solution becomes more basic.
3. How Strong Are Acid or Base Solutions?
Q11. You have two solutions, A and B. The pH of solution A is 6 and the pH of solution B is 8. Which solution has more hydrogen ion concentration? Which solution is acidic and which is basic?
Answer:
Solution A has a pH of 6, while solution B has a pH of 8.
Solution A has a higher hydrogen-ion concentration and is acidic.
Solution B has a lower hydrogen-ion concentration and is basic.
Q12. What effect does the concentration of H⁺(aq) ions have on the nature of a solution?
Answer:
The concentration of hydrogen ions determines the acidic nature of a solution.
Higher concentration of H⁺ ions makes the solution more acidic.
Lower concentration of H⁺ ions makes the solution less acidic.
Q13. Do basic solutions also have H⁺(aq) ions? If yes, then why are these basic?
Answer:
Yes, basic solutions also contain hydrogen ions. However, the concentration of hydroxide ions (OH⁻) is greater than the concentration of hydrogen ions. Therefore, the solution shows basic properties.
4. Importance of pH in Everyday Life
Q14. Why does the pH of rainwater become lower than 5.6 in certain situations?
Answer:
Rainwater becomes more acidic when gases such as sulphur dioxide and nitrogen oxides dissolve in it and form acids. When its pH falls below 5.6, it is called acid rain.
Q15. Why is the pH of the stomach maintained at a low value?
Answer:
The stomach produces hydrochloric acid, which helps in digestion and provides a suitable acidic medium for digestive enzymes. Excess acid can cause indigestion and discomfort.
Q16. Why should the pH of soil be checked before growing crops?
Answer:
Different crops grow well in different soil pH conditions. If the soil is too acidic or too basic, plant growth may be affected. Farmers may add suitable materials to adjust the soil pH.
5. Salts and Their Properties
Q17. What is the chemical name of baking soda? Write its chemical formula.
Answer:
The chemical name of baking soda is sodium hydrogencarbonate. Its chemical formula is NaHCO₃.
Q18. What is the chemical name of washing soda? Write its chemical formula.
Answer:
The chemical name of washing soda is sodium carbonate decahydrate. Its chemical formula is Na₂CO₃·10H₂O.
Q19. What is water of crystallisation? Give an example.
Answer:
Water of crystallisation is the fixed number of water molecules present in one formula unit of a crystalline salt.
Example: Copper sulphate crystals contain five molecules of water of crystallisation.
Formula: CuSO₄·5H₂O.
PART B: EXERCISE QUESTIONS AND ANSWERS
Q1. A solution turns red litmus blue. Its pH is likely to be:
(a) 1
(b) 4
(c) 5
(d) 10
Answer: Option (d) 10.
Explanation: A basic solution turns red litmus blue and has a pH greater than 7.
Q2. A solution reacts with crushed eggshells to give a gas that turns limewater milky. The solution contains:
(a) NaCl
(b) HCl
(c) LiCl
(d) KCl
Answer: Option (b) HCl.
Explanation: Eggshells mainly contain calcium carbonate. It reacts with hydrochloric acid to produce carbon dioxide, which turns limewater milky.
Q3. 10 mL of a solution of NaOH is completely neutralised by 8 mL of a given HCl solution. If we take 20 mL of the same NaOH solution, how much HCl solution will be required?
(a) 4 mL
(b) 8 mL
(c) 12 mL
(d) 16 mL
Answer: Option (d) 16 mL.
Explanation:
10 mL NaOH requires 8 mL HCl.
20 mL NaOH requires:
= (20 × 8) / 10
= 16 mL HCl.
Q4. Which one of the following types of medicines is used for treating indigestion?
(a) Antibiotic
(b) Analgesic
(c) Antacid
(d) Antiseptic
Answer: Option (c) Antacid.
Explanation: Antacids are basic substances that neutralise excess acid in the stomach.
Q5. Write word equations and then balanced chemical equations for the reactions taking place 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.
Answer:
(a) Sulphuric acid + Zinc → Zinc sulphate + Hydrogen
Zn + H₂SO₄ → ZnSO₄ + H₂
(b) Hydrochloric acid + Magnesium → Magnesium chloride + Hydrogen
Mg + 2HCl → MgCl₂ + H₂
(c) Sulphuric acid + Aluminium → Aluminium sulphate + Hydrogen
2Al + 3H₂SO₄ → Al₂(SO₄)₃ + 3H₂
(d) Hydrochloric acid + Iron → Iron chloride + Hydrogen
Fe + 2HCl → FeCl₂ + H₂
Q6. Compounds such as alcohols and glucose also contain hydrogen but are not categorised as acids. Describe an activity to prove this.
Answer:
Take glucose and alcohol solutions separately in two beakers.
Connect each solution to an electric circuit containing a bulb and electrodes.
Place the electrodes in the solutions without allowing them to touch each other.
Observe whether the bulb glows.
The bulb does not glow because glucose and alcohol do not produce hydrogen ions in water. Therefore, they do not show acidic properties.
Q7. Why does distilled water not conduct electricity, whereas rainwater does?
Answer:
Distilled water is almost free from dissolved salts and ions, so it does not conduct electricity effectively.
Rainwater dissolves gases and other substances from the atmosphere, producing ions. These ions help rainwater conduct electricity.
Q8. Why do acids not show acidic behaviour in the absence of water?
Answer:
Acids show acidic behaviour because they produce hydrogen ions, which combine with water to form hydronium ions (H₃O⁺). In the absence of water, these ions are not produced in the same way. Therefore, acids do not show their usual acidic behaviour without water.
Q9. Five solutions A, B, C, D and E have pH values 4, 1, 11, 7 and 9 respectively. Identify the following:
(a) Neutral solution
(b) Strongly alkaline solution
(c) Strongly acidic solution
(d) Weakly acidic solution
(e) Weakly alkaline solution
Also 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:
11, 9, 7, 4, 1.
Note: As pH increases, hydrogen-ion concentration decreases.
Q10. Equal lengths of magnesium ribbons are taken in test tubes A and B. Hydrochloric acid is added to test tube A, while acetic acid is added to test tube B. Both acids have the same concentration and volume. In which test tube will fizzing occur more vigorously and why?
Answer:
Fizzing will occur more vigorously in test tube A containing hydrochloric acid.
This is because hydrochloric acid is a strong acid and produces more hydrogen ions in solution than acetic acid of the same concentration. Therefore, it reacts more vigorously with magnesium.
Q11. Fresh milk has a pH of 6. How do you think the pH will change as it turns into curd? Explain your answer.
Answer:
The pH of milk decreases as it turns into curd.
During the formation of curd, bacteria convert lactose into lactic acid. The formation of lactic acid increases acidity and lowers the pH.
Q12. 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?
Answer:
(a) Baking soda makes the milk slightly alkaline. This delays the development of acidity and helps prevent the milk from turning sour quickly.
(b) The acid produced by bacteria during curd formation is partly neutralised by baking soda. Therefore, it takes longer for the milk to become sufficiently acidic and set into curd.
Q13. Plaster of Paris should be stored in a moisture-proof container. Explain why.
Answer:
Plaster of Paris reacts with water and changes into gypsum, which is a hard substance. If it comes in contact with moisture during storage, it may become hard and lose its usefulness.
Chemical equation:
2(CaSO₄·½H₂O) + 3H₂O → 2(CaSO₄·2H₂O)
Q14. What is a neutralisation reaction? Give two examples.
Answer:
The reaction between an acid and a base to form salt and water is called a neutralisation reaction.
Example 1:
HCl + NaOH → NaCl + H₂O
Example 2:
H₂SO₄ + 2KOH → K₂SO₄ + 2H₂O
Q15. Give two important uses of washing soda and baking soda.
Answer:
Uses of Washing Soda (Na₂CO₃·10H₂O):
It is used for removing permanent hardness of water.
It is used in the manufacture of glass, soap and paper.
Uses of Baking Soda (NaHCO₃):
It is used in baking powder to make bread and cakes soft and fluffy.
It is used in some antacids to neutralise excess stomach acid.
PART C: ADDITIONAL IMPORTANT QUESTIONS AND ANSWERS
Q1. What are acids? Give two examples.
Answer:
Acids are substances that produce hydrogen ions (H⁺) or hydronium ions (H₃O⁺) when dissolved in water.
Examples:
Hydrochloric acid (HCl)
Sulphuric acid (H₂SO₄)
Q2. What are bases? Give two examples.
Answer:
Bases are substances that neutralise acids. Many bases produce hydroxide ions (OH⁻) in aqueous solutions.
Examples:
Sodium hydroxide (NaOH)
Calcium hydroxide [Ca(OH)₂]
Q3. What are indicators? Give examples.
Answer:
Indicators are substances that show different colours in acidic and basic solutions.
Examples:
Litmus
Turmeric
Phenolphthalein
Methyl orange
Q4. What is an olfactory indicator?
Answer:
An olfactory indicator is a substance whose smell changes in acidic or basic solutions.
Examples include onion and vanilla essence.
Q5. What happens when an acid reacts with a metal?
Answer:
When an acid reacts with a metal, it generally produces a salt and hydrogen gas.
General equation:
Metal + Acid → Salt + Hydrogen
Example:
Zn + 2HCl → ZnCl₂ + H₂
Q6. What happens when an acid reacts with a metal carbonate?
Answer:
An acid reacts with a metal carbonate to form salt, carbon dioxide and water.
Example:
CaCO₃ + 2HCl → CaCl₂ + CO₂ + H₂O
Q7. What happens when an acid reacts with a metal hydrogencarbonate?
Answer:
An acid reacts with a metal hydrogencarbonate to produce salt, carbon dioxide and water.
Example:
NaHCO₃ + HCl → NaCl + CO₂ + H₂O
Q8. What is the pH scale?
Answer:
The pH scale is used to measure how acidic or basic a solution is. It generally ranges from 0 to 14.
pH less than 7: Acidic
pH equal to 7: Neutral
pH greater than 7: Basic
Q9. What is the pH of pure water at room temperature?
Answer:
The pH of pure water at room temperature is approximately 7. It is neutral.
Q10. Why is sodium hydroxide called a strong base?
Answer:
Sodium hydroxide is called a strong base because it dissociates almost completely into sodium ions and hydroxide ions in water.
NaOH → Na⁺ + OH⁻
Q11. What is bleaching powder? Write its chemical formula and one use.
Answer:
Bleaching powder is commonly represented by the formula CaOCl₂. It is used for disinfecting drinking water and bleaching cotton and linen.
Q12. What is baking powder?
Answer:
Baking powder is a mixture of baking soda and a mild edible acid, such as tartaric acid. It releases carbon dioxide when heated or mixed with moisture, making cakes and bread soft and fluffy.
Q13. What is gypsum? Write its chemical formula.
Answer:
Gypsum is a naturally occurring hydrated salt of calcium sulphate.
Chemical formula: CaSO₄·2H₂O.
Q14. What is the chemical formula of Plaster of Paris?
Answer:
The chemical formula of Plaster of Paris is CaSO₄·½H₂O.
Q15. Why is Plaster of Paris used for making casts for fractured bones?
Answer:
Plaster of Paris forms a hard structure when mixed with water. It can hold the fractured part of a bone in a fixed position while it heals.
PART D: DIFFERENCES BETWEEN IMPORTANT TERMS
Q1. Differentiate between acids and bases.
Answer:
Acids:
Acids produce H⁺ or H₃O⁺ ions in aqueous solutions.
They turn blue litmus red.
Their pH is less than 7.
Examples: HCl and H₂SO₄.
Bases:
Bases neutralise acids; soluble bases produce OH⁻ ions in water.
They turn red litmus blue.
Their pH is greater than 7.
Examples: NaOH and Ca(OH)₂.
Q2. Differentiate between strong acids and weak acids.
Answer:
Strong Acids:
They ionise almost completely in water.
They produce a relatively high concentration of hydrogen ions at the same concentration.
Examples: HCl and HNO₃.
Weak Acids:
They ionise only partially in water.
They produce a relatively lower concentration of hydrogen ions at the same concentration.
Example: Acetic acid (CH₃COOH).
Q3. Differentiate between strong bases and weak bases.
Answer:
Strong Bases:
They dissociate almost completely in water.
They produce a relatively high concentration of hydroxide ions at the same concentration.
Example: Sodium hydroxide (NaOH).
Weak Bases:
They ionise only partially in water.
They produce a relatively lower concentration of hydroxide ions at the same concentration.
Example: Ammonium hydroxide (NH₄OH).
Q4. Differentiate between baking soda and washing soda.
Answer:
Baking Soda:
Chemical name: Sodium hydrogencarbonate.
Formula: NaHCO₃.
It is used in baking powder and some antacids.
It is a mild basic salt.
Washing Soda:
Chemical name: Sodium carbonate decahydrate.
Formula: Na₂CO₃·10H₂O.
It is used for washing clothes and removing permanent hardness of water.
It is an alkaline salt.
Q5. Differentiate between Plaster of Paris and gypsum.
Answer:
Plaster of Paris:
Formula: CaSO₄·½H₂O.
It is prepared by heating gypsum at about 373 K.
It becomes hard when mixed with water.
Gypsum:
Formula: CaSO₄·2H₂O.
It is a naturally occurring hydrated salt.
It is used to prepare Plaster of Paris.
PART E: IMPORTANT CHEMICAL EQUATIONS
Zinc + Hydrochloric acid:
Zn + 2HCl → ZnCl₂ + H₂
Magnesium + Hydrochloric acid:
Mg + 2HCl → MgCl₂ + H₂
Sodium hydroxide + Hydrochloric acid:
NaOH + HCl → NaCl + H₂O
Calcium carbonate + Hydrochloric acid:
CaCO₃ + 2HCl → CaCl₂ + CO₂ + H₂O
Sodium hydrogencarbonate + Hydrochloric acid:
NaHCO₃ + HCl → NaCl + CO₂ + H₂O
Heating of baking soda:
2NaHCO₃ → Na₂CO₃ + CO₂ + H₂O
Formation of Plaster of Paris:
2(CaSO₄·2H₂O) → 2(CaSO₄·½H₂O) + 3H₂O
Setting of Plaster of Paris:
2(CaSO₄·½H₂O) + 3H₂O → 2(CaSO₄·2H₂O)
PART F: IMPORTANT ONE-MARK QUESTIONS
Q1. What is the pH of a neutral solution?
Answer: 7.
Q2. Which acid is present in lemon juice?
Answer: Citric acid.
Q3. Which acid is present in vinegar?
Answer: Acetic acid.
Q4. Which acid is present in curd?
Answer: Lactic acid.
Q5. What is the chemical formula of baking soda?
Answer: NaHCO₃.
Q6. What is the chemical formula of washing soda?
Answer: Na₂CO₃·10H₂O.
Q7. What is the chemical formula of bleaching powder?
Answer: CaOCl₂.
Q8. What is the chemical formula of Plaster of Paris?
Answer: CaSO₄·½H₂O.
Q9. Which gas is released when an acid reacts with a metal?
Answer: Hydrogen gas.
Q10. Which gas is released when an acid reacts with a metal carbonate?
Answer: Carbon dioxide.
Q11. Which indicator is obtained from lichens?
Answer: Litmus.
Q12. What is the colour of phenolphthalein in a basic solution?
Answer: Pink.
Q13. What is the colour of methyl orange in an acidic solution?
Answer: Red.
Q14. Which substance is used to remove permanent hardness of water?
Answer: Washing soda.
Q15. Which acid is present in an ant's sting?
Answer: Formic acid.
Q16. What is the chemical name of common salt?
Answer: Sodium chloride (NaCl).
Q17. Which ion is responsible for the acidic nature of a solution?
Answer: Hydrogen ion (H⁺), present as hydronium ion (H₃O⁺) in water.
Q18. Which ion is responsible for the basic nature of a solution?
Answer: Hydroxide ion (OH⁻).
Q19. What is the pH value of pure water at room temperature?
Answer: Approximately 7.
Q20. What is water of crystallisation?
Answer: It is the fixed number of water molecules present in one formula unit of a crystalline salt.
PART G: IMPORTANT FORMULAE AND FACTS FOR REVISION
Acid + Metal → Salt + Hydrogen
Acid + Metal carbonate → Salt + Carbon dioxide + Water
Acid + Metal hydrogencarbonate → Salt + Carbon dioxide + Water
Acid + Base → Salt + Water
Metal oxide + Acid → Salt + Water
Non-metal oxide + Base → Salt + Water
Important pH facts:
pH below 7 indicates an acidic solution.
pH equal to 7 indicates a neutral solution.
pH above 7 indicates a basic solution.
Lower pH means higher hydrogen-ion concentration.
Higher pH means lower hydrogen-ion concentration.
Important chemical formulae:
Hydrochloric acid – HCl
Sulphuric acid – H₂SO₄
Nitric acid – HNO₃
Acetic acid – CH₃COOH
Sodium hydroxide – NaOH
Calcium hydroxide – Ca(OH)₂
Sodium chloride – NaCl
Baking soda – NaHCO₃
Washing soda – Na₂CO₃·10H₂O
Bleaching powder – CaOCl₂
Plaster of Paris – CaSO₄·½H₂O
Gypsum – CaSO₄·2H₂O
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