Science Class 10 Chapter 2 NCERT Solutions provide clear and step-by-step answers to all the textbook questions from the chapter Acids, Bases and Salts. This chapter explains important concepts like properties of acids and bases, pH scale, neutralization reactions, uses of common salts, and their role in daily life.
Science Class 10 Chapter 2 NCERT Solutions
1. 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?
Answer: We can find out the content in each of the test tubes using red litmus paper.
- Put the red litmus paper in the first tube. If the litmus paper changes colour from red to blue, it means that the tube has a basic solution.
- If the red litmus paper remains red, then it means that it contains either distilled water or an acidic solution.
- To distinguish between distilled water and acid, use blue litmus paper. If the litmus paper turns blue, then it is an acidic solution, and if the colour of the blue litmus paper does not change, then it is distilled water.
2. Why should curd and sour substances not be kept in brass and copper vessels?
Answer: The curd and sour cream contain acids like lactic acid and citric acid. When these acids come in contact with brass or copper vessels, then they react and form toxic compounds such as copper salts. These compounds can be harmful to health if they are consumed.
3. 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: When an acid reacts with a metal, then it releases hydrogen (H2) gas. For example, Zn + 2HCl -> ZnCl2 + H2, zinc reacts with hydrochloric acid to produce zinc chloride and hydrogen gas. To test the hydrogen gas, you can bring a burning matchstick near the test tube; if hydrogen gas is present, it will burn with a “pop” sound, which confirmsthe presence of hydrogen gas.
4. 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 metal compound A is calcium carbonate (CaCO3). It reacts with dilute hydrochloric acid (HCl) to produce calcium chloride (CaCl2), carbon dioxide (CO2) and water (H2O). The balanced chemical equation are:
CaCO3 + 2HCl -> CaCl2 + CO2↑ + H2O
Note: Effervescence is due to the release of carbon dioxide gas, which extinguishes a burning candle.
5. 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 HCl (hydrochloric acid) and HNO3 (nitric acid) show acidic character when you dissolve them in water because they release hydrogen ions (H+) when dissolved in water.
- HCl -> H+ + Cl–
- HNO3 -> H+ + NO3–
On the other hand, the compounds like alcohol (C2H5OH) and glucose (C6H12O6) do not release H+ ions in water; because of this, they do not show acidic character.
6. Why does an aqueous solution of an acid conduct electricity?
Answer: An aqueous solution of an acid conducts electricity. because acids release hydrogen ions (H+) when dissolved in water. These ions are charged particles and can move freely in the solution and carry electric current.
7. Why does dry HCl gas not change the colour of the dry litmus paper?
Answer: The dry HCl gas does not change the colour of dry litmus paper because it does not release hydrogen ions (H+) in the absence of water.
8. While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?
Answer: The process of dissolving an acid or a base in water is a highly exothermic one. Care must be taken while mixing concentrated nitric acid or sulphuric acid with water. The acid must always be added slowly to water with constant stirring. If water is added to a concentrated acid, the heat generated may cause the mixture to splash out and cause burns. The glass container may also break due to excessive local heating.
9. How is the concentration of hydronium ions (H3O+) affected when a solution of an acid is diluted?
Answer: Mixing an acid or base with water results in a decrease in the concentration of ions (H3O+/OH–) per unit volume. Such a process is called dilution, and the acid or the base is said to be diluted.
10. How is the concentration of hydroxide ions (OH-) affected when excess base is dissolved in a solution of sodium hydroxide?
Answer: Sodium hydroxide (NaOH) releases hydroxide ions (OH–) in solution. When more base is added to the solution, then the concentration of (OH–) ions increases because of this, and then the additional base also releases more (OH–) ions. As a result, the solution becomes more basic (higher pH) due to an increase in (OH–) ions.
11. 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?
Answer: Solution A (pH 6) has a higher hydrogen ion concentration than solution B (pH 8), because a lower pH means a higher H+ concentration.
12. What effect does the concentration of H+(aq) ions have on the nature of the solution?
Answer: The higher the concentration of H+ ions in solution, the more acidic it is. The lower the concentration of H+ ions, the less acidic the solution is.
- More H+ ions → stronger acid → lower pH
- Fewer H+ ions → weaker acid or base → higher pH
13. Do basic solutions also have H+(aq) ions? If yes, then why are these basic?
Answer: Yes, basic solutions have H+(aq) ions, but it is less in amount. If the presence of hydroxide ions (OH–) is more than hydrogen ions (H+) then it is a basic solution.
14. 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: A farmer would treat the soil with quicklime, slaked lime or chalk when the soil is too acidic (low pH). This substance is present in nature and helps to neutralise the excess acidity in the soil and make it suitable for crop growth. This process is known as liming.
15. What is the common name of the compound Ca(ClO)2?
Answer: The common name of the compound Ca(ClO)2 is bleaching powder, and the chemical name is calcium hypochlorite.
16. Name the substance which on treatment with chlorine yields bleaching powder.
Answer: The substance which, on treatment with chlorine, yields bleaching powder is dry slaked lime, which is also known as calcium hydroxide (Ca(OH)2).
17. Name the sodium compound which is used for softening hard water.
Answer: The sodium compound which is used for softening hard water is known as washing soda. It helps to remove calcium (Ca2+) and magnesium (Mg2+) ions from the hard water by forming insoluble precipitates and making the water soft.
18. What will happen if a solution of sodium hydrocarbonate is heated? Give the equation of the reaction involved.
Answer: If the sodium hydrocarbonate (NaHCO3) is heated, then it will decompose in the form of sodium carbonate (Na2CO3), carbon dioxide (CO2) gas and water (H2O).
The balanced chemical equation are:
19. Write an equation to show the reaction between Plaster of Paris and water.
Answer:
20. A solution turns red litmus blue, its pH is likely to be
(a) 1
(b) 4
(c) 5
(d) 10
Answer: (d) 10
21. 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
22. 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
23. Which one of the following types of medicines is used for treating indigestion?
(a) Antibiotic
(b) Analgesic
(c) Antacid
(d) Antiseptic
Answer: (c) Antacid
24. Write word equations and then balanced equations for the reaction 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) dilute sulphuric acid reacts with zinc granules.
25. Compounds such as alcohols and glucose also contain hydrogen but are not categorised as acids. Describe an Activity to prove it.
Answer: To observe that all hydrogen-containing compounds are acids, by testing whether they conduct electricity. Materials needed One beaker, two iron nails, one wooden or rubber cork, connecting wires, a 6-volt battery, a small bulb, one switch, alcohol or glucose solution and hydrochloric acid (HCl) solution.
- Fix the two iron nails into the cork so they can stand upright.
- Place the cork on the beaker.
- Connect the nails to a bulb, battery and switch using wire.
Observation:
Pour alcohol or glucose into the beaker so the nails are submerged. Now turn on the switch; now you can see the bulb not glowing. Now empty the beaker and add the HCl solution. This time the bulb glows. This shows that acid can conduct electricity, but alcohol and glucose do not conduct electricity.
26. Why does distilled water not conduct electricity, whereas rain water does?
Answer:
- The distilled water does not contain ionic compounds.
- Whereas the rainwater has a lot of ionic compounds
- Rainwater contains dissolved acidic gases like carbon dioxide, carbonic acid, etc. It means that it has hydrogen ions and carbonate ions. Due to this ions in rainwater can conduct electricity.
27. Why do acids not show acidic behaviour in the absence of water?
Answer: The acidic behaviour of acids is due to the presence of hydrogen ions [H+ (aq) ions]. The acid produces hydrogen ions only in the presence of water. So, water is required if acids want to show their acidic behaviour.
28. 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 in increasing order of hydrogen-ion concentration.
Answer:
- pH Range (0 – 2): Strong acid
- pH Range (3 – 6): Weak acid
- pH Range (7): Neutral (like pure water)
- pH Range (8 – 10): Weak base (alkaline)
- pH Range (11 – 14): Strong base (alkaline)
Match of each solution:
- Solution A: pH 4
- Solution B: pH 1
- Solution C: pH 11
- Solution D: pH 7
- Solution E: pH 9
Lower pH → more hydrogen ions (H⁺) Higher pH → fewer hydrogen ions
| Solution | pH | Type H⁺ Concentration Rank |
|---|---|---|
| D | 7 | Neutral 3rd |
| C | 11 | Strong base 1st (least H⁺) |
| B | 1 | Strong acid 5th (most H⁺) |
| A | 4 | Weak acid 4th |
| E | 9 | Weak base 2nd |
So arrange pH values from high to low:
11 < 9 < 7 < 4 < 1
Which means:
C < E < D < A < B
→ C has the least H⁺, B has the most.
29. 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?
Answer: HCl is a strong acid, whereas acetic is a weaker acid. Test tube A shows more vigorous fizzing because HCl is a strong acid and produces more hydrogen ions, leading to a faster reaction with magnesium and greater hydrogen gas release.
Note:
- HCl is a strong acid → gives lots of hydrogen ions.
- Acetic acid is weak → gives fewer hydrogen ions.
- More hydrogen ions → faster reaction → more gas → more fizzing
30. 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 the fresh milk turned to curd due to the production of lactic acid. Lactic acid reduces the pH of the milk.
31. 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) The fresh milk has a pH around 6, which is slightly acidic. The milkman adds soda (sodium bicarbonate) to make the milk slightly alkaline, where the pH level of the milk will go to 7; the alkaline helps it stay fresh for a longer time.
- (b) The lactic acid bacteria make the milk more acidic, but if the milk is already alkaline, then it will take a longer time to build acid in the milk, so the milk will take more time to turn into curd.
32. Plaster of Paris should be stored in a moisture-proof container. Explain why?
Answer: The plaster of Paris (POP) is a white powder made from calcium sulphate hemihydrate. If you mix it with water quickly, POP can absorb water vapour from the air, and it will react and become harder, so that is why moisture-proof containers are used.
33. What is a neutralisation reaction? Give two examples.
Answer: The neutralisation reaction happens when an acid reacts with a base to form salt and water.
General formula: Acid + Base → Salt + Water
The reaction cancels out the acidic and basic properties; that’s why it’s called neutralisation.
The two examples are:
- NaOH + HCl → NaCl + H2O
- Mg(OH)2 + H2CO3 → MgCO3 + 2H2O
34. Give two important uses of washing soda and baking soda.
Answer: The two important use of washing soda and baking soda are:
Use of washing soda
- Used in cleaning clothes
- Used in making glass and soaps
- Used in water treatment
- Used in paper and textile industries
Use of Baking Soda
- Used in baking
- Used as antacid
- Used in fire extinguishers
- Used in removing stains
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