Ch 2: Acids, Bases and Salts (NCERT Solutions)

NCERT Solutions for Class 10 Science Ch 2: Acids, Bases and Salts, is given below.

You will get solutions for;

• In-text questions as well as,
• Exercises questions.

So let’s dive into it!

In-text Questions Set-1

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?

Solution:
We can use red litmus paper to identify the contents of each test tube by observing the color change:

– Pour the three solutions onto separate pieces of red litmus paper.
– The solution that turns red litmus paper blue is basic.
– Divide the resulting blue litmus paper into two parts.
– The solution that turns blue litmus paper back to red is acidic.
– The solution that does not change the color of either red or blue litmus paper is water.

In-text Questions Set-2

1) Why should curd and sour substances not be kept in brass and copper vessels?

Solution:
Curd and sour foods contain acids that can react with metals, turning the food toxic and harmful to health.

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?

Solution:
When an acid reacts with a metal, it produces salt and hydrogen gas.
Metal + Acid → Salt + Hydrogen gas

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.

Solution:
The evolved gas is CO₂ because the metal compound released is calcium chloride. This means metal A is likely calcium carbonate. The reaction between calcium carbonate and hydrochloric acid is:

CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + CO₂(g) + H₂O(l)

In-text Questions Set-3

1) Why do HCl, HNO3, etc., show acidic characters in aqueous solutions while solutions of compounds like alcohol and glucose do not show an acidic character?

Solution:
The release of H⁺ ions in water determines whether a compound is acidic or not. Acids are substances that dissociate in water to produce hydrogen ions. For example, compounds like HCl and HNO₃ show acidic properties because they release hydrogen ions when dissolved in water.

On the other hand, compounds like glucose and alcohol contain hydrogen but do not behave as acids. This is because their hydrogen atoms do not dissociate to form hydrogen ions in water.

2) Why does an aqueous solution of acid conduct electricity?

Solution:
Charged particles, known as ions, are responsible for the conductance of electricity in an acid. They enable the flow of electricity by moving within the solution.

3) Why does dry HCl gas not change the color of the dry litmus paper?

Solution:
HCl gas does not release hydrogen ions, so it doesn’t behave like an acid. As a result, the color of litmus paper doesn’t change when it comes into contact with HCl gas.

4) While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?

Solution:
When diluting an acid, always add the acid to water, not the other way around. Adding water to a concentrated acid releases a lot of heat, which can cause an explosion and result in acid burns. To stay safe, add the acid to water.

5) How is the concentration of hydronium ions (H₃O)⁺ affected when a solution of an acid is diluted?

Solution:
When acid is added to water, hydronium ions form in the solution. If the solution is diluted, the number of hydronium ions per unit volume decreases, which lowers the hydronium concentration.

6) How is the concentration of hydroxide ions (OH^–) affected when excess base is dissolved in a solution of sodium hydroxide?

Solution:
When a base is dissolved in sodium hydroxide solution, the concentration of hydroxide ions increases. However, once the solution reaches saturation, adding more base doesn’t increase the concentration of hydroxide ions any further.

In-text Questions Set-4

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 of these is acidic, and which one is basic?

Solution:
To find the hydrogen ion concentration, use this rule: “The pH of a solution is inversely proportional to its hydrogen ion concentration.” This means a solution with a lower pH has a higher hydrogen ion concentration.

So, solution A will have a higher hydrogen ion concentration. Additionally, solution B will be basic, while solution A will be acidic.

2) What effect does the concentration of H⁺(aq) ions have on the nature of the solution?

Solution:
The concentration of hydrogen ions determines the nature of a solution. If the hydrogen ion concentration increases, the solution becomes acidic. If it decreases, the solution becomes basic.

3) Do basic solutions also have H+(aq) ions? If yes, then why are these basic?

Solution:
Basic solutions have more hydroxide ions (OH⁻) compared to hydrogen ions (H⁺). The high concentration of hydroxide ions makes the solution basic.

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)?

Solution:
If the soil is acidic (pH below 7), treat it with quick lime (calcium oxide), slaked lime (calcium hydroxide), or chalk (calcium carbonate).

In-text Questions Set-5

1) What is the common name of the compound CaOCl₂?

Solution:
Bleaching powder

2) Name the substance, which on treatment with chlorine, yields bleaching powder.

Solution:
Calcium hydroxide

3) Name the sodium compound which is used for softening hard water.

Solution:
Sodium carbonate

4) What will happen if a solution of sodium hydrocarbonate is heated? Give the equation of the reaction involved.

Solution:
When sodium bicarbonate is heated, it produces sodium carbonate and releases carbon dioxide gas.

2NaHCO₃ + heat ⟶  Na₂CO₃ + H₂O + CO₂

5) Write an equation to show the reaction between Plaster of Paris and water.

Solution:
The chemical equation for the reaction between Plaster of Paris and water is:

CaSO₄ . 1/2 H₂O + 3/2 H₂O → CaSO₄ . 2H₂O

This shows how Plaster of Paris (CaSO₄ · 1/2 H₂O) reacts with water to form gypsum (CaSO₄ · 2H₂O).

Exercise Questions

1) A solution turns red litmus blue, its pH is likely to be
a) 1 (b) 4 (c) 5 (d) 10

Solution:
Answer is 10, because litmus paper turns blue in a basic solution (pH greater than 7).

2) A solution reacts with crushed eggshells to give a gas that turns lime-water milky. The solution contains
a) NaCl (b) HCl (c) LiCl (d) KCl

Solution:
The answer is HCl. Eggshells have calcium carbonate, which reacts with HCl to produce CO2 gas. This gas turns lime water milky. The reaction is:

CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂

3) 10 mL of a solution of NaOH is found to be completely neutralized by 8 mL of a given solution of HCl. If we take 20 mL of the same solution of NaOH, the amount of HCl solution (the same solution as before) required to neutralize it will be
(a) 4 mL (b) 8 mL (c) 12 mL (d) 16 mL

Solution:
Since 10 mL of NaOH needs 8 mL of HCl, 20 mL of NaOH will need 16 mL of HCl (8 mL × 2). So, the correct answer is option (d) 16 mL.

4) Which one of the following types of medicines is used for treating indigestion? (a) Antibiotic (b) Analgesic (c) Antacid (d) Antiseptic

Solution:
Answer is “Antacid”. Because indigestion happens when the stomach produces too much acid. Antacids are medicines used to treat indigestion.

5) 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 aluminum powder.
(d) Dilute hydrochloric acid reacts with iron filings.

Solution:
(a) Reaction of Dilute Sulphuric Acid with Zinc:
  – Dilute sulphuric acid reacts with zinc granules to produce zinc sulfate and hydrogen gas.
  – Chemical equation: H₂SO₄(aq) + Zn → ZnSO₄(aq) + H₂(g)

(b) Reaction of Dilute Hydrochloric Acid with Magnesium:
  – Dilute hydrochloric acid reacts with magnesium ribbon to produce magnesium chloride and hydrogen gas.
  – Chemical equation: 2HCl(aq) + Mg → MgCl₂(aq) + H₂(g)

(c) Reaction of Dilute Sulphuric Acid with Aluminium:
  – Dilute sulphuric acid reacts with aluminum powder to produce aluminum sulfate and hydrogen gas.
  – Chemical equation: 3H₂SO₄(aq) + 2Al(s) → Al₂(SO₄)₃(aq) + 3H₂(g)

(d) Reaction of Dilute Hydrochloric Acid with Iron:
  – Dilute hydrochloric acid reacts with iron filings to produce ferrous chloride and hydrogen gas.
  – Chemical equation: 6HCl(aq) + 3Fe(s) → 3FeCl₂(aq) + 3H₂(g)

7) Why does distilled water not conduct electricity, whereas rain water does?

Solution:
Distilled water does not contain any ionic compounds, while rainwater does. Rainwater absorbs acidic gases like carbon dioxide from the air, forming carbonic acid. This results in the presence of hydrogen and carbonate ions, which allow rainwater to conduct electricity.

8) Why do acids not show acidic behavior in the absence of water?

Solution:
Acids behave as acids because they produce hydrogen ions. These ions can only form in the presence of water, so water is essential for acids to exhibit their acidic properties.

9) Five solutions, A, B, C, D and E, when tested with a 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?

Solution:
In order of increasing hydrogen ion concentration:  pH 11 (C) < pH 9 (E) < pH 7 (D) < pH 4 (A) < pH 1 (B)
pH 11: Strongly alkaline
pH 9: Weakly alkaline
pH 7: Neutral
pH 4: Weakly acidic
pH 1: Strongly acidic

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 the same. In which test tube will the fizzing occur more vigorously and why?

Solution:
HCl is a strong acid, while acetic acid is weaker. Fizzing happens because hydrogen gas is produced when the acids react with the magnesium ribbon. Since HCl is a stronger acid, it produces more hydrogen gas, leading to more fizzing in test tube A.

11) Fresh milk has a pH of 6. How do you think the pH will change as it turns into curd? Explain your answer.

Solution:
Fresh milk turns into curd because of the production of lactic acid, which lowers the milk’s pH.

12) 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?

Solution:
(a) He made the fresh milk slightly alkaline, raising its pH from 6, to prevent it from turning sour due to lactic acid production.

(b) This milk takes longer to turn into curd because the lactic acid first neutralizes the alkalinity before lowering the pH enough to set the milk into curd.

13) Plaster of Paris should be stored in a moisture-proof container. Explain why.

Solution:
Plaster of Paris should be stored in a moisture-proof container because moisture causes it to hydrate, slowing down the setting process and making it unusable.

14) Plaster of Paris should be stored in a moisture-proof container. Explain why.

Solution:
When an acid reacts with a base, they produce salt and water. This process is called a neutralization reaction. For example:

NaOH + HCl → NaCl + H₂O
Mg(OH)₂ + H₂CO₃ → MgCO₃ + 2H₂O

15) Give two important uses of washing soda and baking soda.

Solution:
Washing Soda:
– It is used as an electrolyte.
– It can be used at home as a water softener for laundry.

Baking Soda:
– It can test garden soil for acidity. If bubbles form, the soil is too acidic.
– It can be used to wash the car, removing dead bugs without harming the color or paint.