Class 10 Science
Chapter - 2: Acids, Bases, and Salts
Textual Questions and Answers:
1. You have been provided with three test tubes. One of them contains distilled water, another contains an acidic solution, and the third contains a basic solution. If you are given only red litmus paper, how will you identify the contents of each test tube?
Answer: Using red litmus paper:
- A basic solution will turn red litmus paper blue.
- The acidic solution will turn blue litmus paper red.
- The test tube that does not change the color of either litmus paper contains distilled water.
Textual Questions and Answers:
1. Why should curd and sour substances not be kept in brass and copper vessels?
Answer: Curd and sour substances contain acids that react with metals like brass and copper, leading to the formation of harmful salts and causing corrosion.
2. Which gas is usually liberated when an acid reacts with a metal? Illustrate with an example. How will you test for this gas?
Answer: (i) Hydrogen gas (H₂) is liberated when an acid reacts with a metal.
(ii) Illustration: Set up an apparatus. Take some Zinc granules in the test tube. Add about 5 ml dilute hydrochloric acid slowly. Soon the reaction between Zinc and hydrochloric acid starts and hydrogen gas is evolved.
(iii) Test for H2 gas: H2 gas is not soluble in water. When passed through soap solution, it gets trapped into bubbles which burn with explosion.
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: Since the gas evolved extinguishes a burning candle, it is carbon dioxide (CO₂). The metal compound must be calcium carbonate (CaCO₃). The reaction is:
CaCO₃ + 2HCl → CaCl₂ + CO₂ + H₂O
Textual Questions and Answers:
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?
Answer: HCl and HNO₃ show acidic characters in aqueous solutions because they ionize completely in water to produce H⁺ ions, which are responsible for their acidic properties. However, alcohol and glucose do not ionize in water to produce H⁺ ions; therefore, they do not exhibit acidic behavior.
2. Why does an aqueous solution of an acid conduct electricity?
Answer: Acids ionize in aqueous solutions to produce H⁺ ions, which allow the solution to conduct electricity.
3. Why does dry HCl gas not change the color of dry litmus paper?
Answer: HCl gas does not dissociate into H⁺ ions in the absence of water, and therefore, it does not change the color of litmus paper.
4. While diluting an acid, why should the acid be added to water and not vice versa?
Answer: Adding acid to water ensures that heat is dissipated gradually. If water is added to acid, the large amount of heat generated can cause splashing, leading to burns.
5. How is the concentration of hydronium ions (H₃O⁺) affected when a solution of an acid is diluted?
Answer: The concentration of H₃O⁺ ions per unit volume decreases when an acid is diluted with water.
6. How is the concentration of hydroxide ions (OH⁻) affected when excess base is dissolved in sodium hydroxide solution?
Answer: The concentration of OH⁻ ions increases when a soluble base is added to a sodium hydroxide solution.
Textual Questions and Answers:
1. 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 one is acidic and which one is basic?
Answer: A solution with a pH value less than 7 is acidic, while a solution with a pH value greater than 7 is basic. Therefore, the solution with pH = 6 is acidic and has more hydrogen ion concentration than the solution with pH = 8, which is basic.
2. What effect does the concentration of H⁺ (aq) ions have on the nature of the solution?
Answer: Acids that give rise to more H⁺ ions are strong acids, while acids that give fewer H⁺ ions are weak acids.
3. Do basic solutions also have H⁺ (aq) ions? If yes, then why are these basic?
Answer: Yes, basic solutions also contain H⁺ ions, but they have a much higher concentration of OH⁻ ions, which makes them basic in nature.
4. Under what soil condition do you think a farmer would treat the soil of his fields with quick lime (calcium oxide), slaked lime (calcium hydroxide), or chalk (calcium carbonate)?
Answer: If the soil becomes too acidic, it negatively impacts crop growth. To neutralize the acidity, farmers apply basic substances such as quick lime, slaked lime, or chalk to restore the soil's pH balance.
Textual Questions and Answers:
1. What is the common name of the compound CaOCl₂?
Answer: Bleaching powder.
2. Name the substance which, on treatment with chlorine, yields bleaching powder.
Answer: Calcium hydroxide.
3. Name the sodium compound that is used for softening hard water.
Answer: Sodium carbonate.
4. What will happen if a solution of sodium hydrogen carbonate is heated? Give the equation of the reaction involved.
Answer: When sodium hydrogen carbonate is heated, sodium carbonate and water are formed along with the evolution of carbon dioxide gas.
The reaction involved is:
2NaHCO₃ → Na₂CO₃ + CO₂ + H₂O
5. Write an equation to show the reaction between plaster of Paris and water.
Answer: When plaster of Paris (CaSO₄·½H₂O) is mixed with water, it forms gypsum (CaSO₄·2H₂O) and hardens.
The reaction is:
CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O
Exercises:
1. A solution turns red litmus blue. Its pH is likely to be:
(a) 1
(b) 4
(c) 5
(d) 10
Answer: (d) 10
2. A solution reacts with crushed eggshells to produce a gas that turns lime water milky. The solution contains:
(a) NaCl
(b) HCl
(c) LiCl
(d) KCl
Answer: (b) HCl
3. 10 ml of NaOH solution completely neutralizes 8 ml of a given HCl solution. How much HCl solution is required to neutralize 20 ml of NaOH solution?
(a) 4 ml
(b) 8 ml
(c) 12 ml
(d) 16 ml
Answer: (d) 16 ml
4. Which type of medicine is used for treating indigestion?
(a) Antibiotic
(b) Analgesic
(c) Antacid
(d) Antiseptic
Answer: (c) Antacid
5. Write word equations and balanced equations for the following reactions:
(a) Dilute sulfuric acid reacts with zinc granules.
(b) Dilute hydrochloric acid reacts with magnesium.
(c) Dilute sulfuric acid reacts with aluminum powder.
(d) Dilute hydrochloric acid reacts with iron filings.
Answer:
(a) Zn + H₂SO₄ → ZnSO₄ + H₂
(b) Mg + 2HCl → MgCl₂ + H₂
(c) 2Al + 3H₂SO₄ → Al₂(SO₄)₃ + 3H₂
(d) Fe + 2HCl → FeCl₂ + H₂
6. Compounds such as alcohols and glucose also contain hydrogen beet are not categorised as acids. Describe an activity to prove it.
Answer: Take solutions of glucose and alcohols in a beaker. Fix two nails on a cork and place the cork in the beaker. connect the nails to the two terminals of a 6 volt battery through a bulb and a switch. Switch on the current. The bulb does not glow. That means current does not pass through the circuit. This show that no H+ ions are present in the solution. This experiment shows that alcohol and glucose are not acid.
7. Why does distilled water not conduct electricity, whereas rainwater does?
Answer: Distilled water lacks dissolved ions, whereas rainwater contains dissolved salts and acids, making it conductive.
8. Why do acids not show acidic behavior in the absence of water?
Answer: Acids release H⁺ ions only in aqueous solutions, which are responsible for acidic behavior.
9. Five solutions A, B, C, D and E when tested with universal indicator showed Pá´´ 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 Pá´´ in increasing order of hydrogen ion concentration.
Answer: (a) Neutral solution is D
(b) Strongly alkaline is C
(c) Strongly acidic is B
(d) Weakly acidic is A
(e) Weakly alkaline is E
Pá´´ values in increasing order of hydrogen ion concentration 11<9<7<4<1
10. Equal lengths of magnesium ribbons are placed 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. In which test tube will fizzing occur more vigorously? Why?
Answer: Fizzing occurs more vigorously in test tube A because HCl is a strong acid and reacts faster with magnesium than acetic acid, which is weaker.
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: The pH decreases as milk turns into curd due to the formation of lactic acid.
12. A milkman adds a very small amount of baking soda to fresh milk.
(a) Why does the shift the Pá´´ of the fresh milk from 6 to slightly alkaline?
(b) Why does this milk take a long time to set as curd?
Answer: (a) The milkman adds a little baking soda to fresh milk to make it slightly alkaline so that milk can be preserved for a longer time.
(b) The lactic acid is used for neutralise the base initially and when more lactic acid is formed then the milk sets as curd.
13. Plaster of Paris should be stored in a moisture-proof container. Explain why.
Answer: Plaster of Paris (CaSO₄·½H₂O) absorbs moisture and converts to gypsum (CaSO₄·2H₂O), making it hard and unusable.
14. What is a neutralization reaction? Give two examples.
Answer: A neutralization reaction occurs when an acid reacts with a base to form salt and water.
Example 1: HCl + NaOH → NaCl + H₂O
Example 2: H₂SO₄ + 2KOH → K₂SO₄ + 2H₂O
15. Give two important uses of washing soda and baking soda.
Answer:
- Washing soda (Na₂CO₃·10H₂O): Used in water softening and detergent manufacturing.
- Baking soda (NaHCO₃): Used in baking powder and fire extinguishers.
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