Class 10 Science Chapter 2 – Acids, Bases, and Salts

Exercise Questions and Answers:

Q 1:

Define the following:
(a) Acid
(b) Base
(c) Salt

Answer:
(a) Acid: An acid is a substance that releases hydrogen ions (H⁺) when dissolved in water. Example: Hydrochloric acid (HCl).

(b) Base: A base is a substance that releases hydroxide ions (OH⁻) when dissolved in water. Example: Sodium hydroxide (NaOH).

(c) Salt: A salt is a compound formed when an acid reacts with a base. Example: Sodium chloride (NaCl).

Q 2:

What is the pH of distilled water? Why is it neutral?

Answer:
The pH of distilled water is 7, which is considered neutral. This is because it contains equal concentrations of hydrogen ions (H⁺) and hydroxide ions (OH⁻), leading to no acidic or basic properties.

Q 3:

Write the chemical equations for the following reactions:
(a) Reaction of hydrochloric acid with sodium hydroxide.
(b) Reaction of zinc with hydrochloric acid.
(c) Reaction of sodium hydroxide with acetic acid.

Answer:
(a) Reaction of hydrochloric acid with sodium hydroxide:

 

$$\text{HCl (aq)} + \text{NaOH (aq)} → \text{NaCl (aq)} + \text{H}_2\text{O (l)}$$

 

(b) Reaction of zinc with hydrochloric acid:

 

$$\text{Zn (s)} + 2\text{HCl (aq)} → \text{ZnCl}_2\text{ (aq)} + \text{H}_2\text{ (g)}$$

 

(c) Reaction of sodium hydroxide with acetic acid:

 

$$\text{CH}_3\text{COOH (aq)} + \text{NaOH (aq)} → \text{CH}_3\text{COONa (aq)} + \text{H}_2\text{O (l)}$$

 

Q 4:

What is an indicator? Give an example.

Answer:
An indicator is a substance that changes color when it comes into contact with an acid or base.
Example: Litmus paper is an indicator. It turns red in acidic solutions and blue in basic solutions.

Q 5:

What is the use of litmus paper? How is it used to test the pH of a solution?

Answer:
Litmus paper is used to determine whether a solution is acidic or basic.

• Red litmus paper turns blue in basic solutions and stays red in acidic solutions.
• Blue litmus paper turns red in acidic solutions and stays blue in basic solutions.

Q 6:

How do you prepare a solution of hydrochloric acid from its concentrated form?

Answer:
To prepare a dilute solution of hydrochloric acid from concentrated hydrochloric acid:

1. Measure a small amount of concentrated hydrochloric acid using a pipette.
2. Add it carefully to a beaker containing distilled water.
3. Stir the solution well.
4. Ensure the final concentration is safe for use by checking the desired pH or concentration.

Q 7:

Explain how washing soda is prepared. Write its chemical formula.

Answer:
Washing soda is prepared by heating sodium carbonate decahydrate (Na₂CO₃·10H₂O) at high temperature, which causes it to lose water molecules.

Chemical formula for washing soda:

 

$$\text{Na}_2\text{CO}_3 \cdot 10\text{H}_2\text{O}$$

 

Q 8:

What happens when a small amount of phenolphthalein is added to a solution of sodium hydroxide?

Answer:
When phenolphthalein is added to a solution of sodium hydroxide, it turns pink. This indicates that the solution is basic, as phenolphthalein is colorless in acidic solutions and turns pink in basic solutions.

Q 9:

What is the effect of acid rain on buildings, trees, and soil?

Answer:
Acid rain is harmful to the environment and has the following effects:

1. Buildings: Acid rain corrodes the surfaces of buildings and statues, especially those made of limestone and marble.
2. Trees: It damages leaves and stems, weakening the trees. It also leaches nutrients from the soil, affecting plant growth.
3. Soil: It lowers the pH of the soil, which makes it acidic, leading to the depletion of essential minerals and harming crops.

Q 10:

What is the role of bases in neutralization reactions?

Answer:
In a neutralization reaction, a base reacts with an acid to produce a salt and water, thereby neutralizing the properties of the acid and base.
Example:

 

$$\text{HCl (aq)} + \text{NaOH (aq)} → \text{NaCl (aq)} + \text{H}_2\text{O (l)}$$

 

 

Additional important questions and answers

Q 1:
What is the role of acids in our daily life? Give examples.

Answer:
Acids play a vital role in various aspects of our daily life:

1. In the human body:
   • Hydrochloric acid (HCl) is present in our stomach and helps in digestion by breaking down food.
2. In food:
   • Citric acid is found in citrus fruits (like oranges and lemons) and is used as a preservative.
   • Acetic acid is found in vinegar and is used in cooking.
3. In cleaning:
   • Acids like oxalic acid and hydrochloric acid are used in cleaning products to remove stains and scale.

Q 2:
What is the difference between strong acids and weak acids? Give examples.

Answer:

• Strong acids are acids that completely dissociate in water to release hydrogen ions. They have a low pH (close to 0).
  Examples: Hydrochloric acid (HCl), Sulfuric acid (H₂SO₄), Nitric acid (HNO₃).
• Weak acids are acids that only partially dissociate in water. They have a higher pH compared to strong acids (closer to 7).
  Examples: Acetic acid (CH₃COOH), Carbonic acid (H₂CO₃).

Q 3:
How does the pH scale work? What does it measure?

Answer:
The pH scale measures the acidity or basicity of a solution. It ranges from 0 to 14:

• pH 7 indicates a neutral solution (e.g., distilled water).
• pH less than 7 indicates an acidic solution. The lower the pH, the stronger the acid.
• pH greater than 7 indicates a basic (alkaline) solution. The higher the pH, the stronger the base.

Q 4:
What is an antacid? How does it work?

Answer:
An antacid is a substance that neutralizes excess stomach acid to relieve discomfort caused by acid reflux or indigestion.

• How it works: Antacids, typically made of weak bases like magnesium hydroxide (Mg(OH)₂) or calcium carbonate (CaCO₃), react with the excess hydrochloric acid (HCl) in the stomach to form water and salts, thus neutralizing the acid.

Q 5:
What is the chemical formula of plaster of Paris? How is it prepared?

Answer:
The chemical formula for plaster of Paris is CaSO₄·½H₂O (calcium sulfate hemihydrate).

Preparation:
Plaster of Paris is prepared by heating gypsum (CaSO₄·2H₂O) at 150°C:

 

$$\text{CaSO}_4·2\text{H}_2\text{O} → \text{CaSO}_4·\frac{1}{2}\text{H}_2\text{O} + \text{H}_2\text{O}$$

 

Q 6:
What is the role of acids and bases in natural water bodies like rivers and lakes?

Answer:

1. Acids:
   • When acid rain falls into rivers or lakes, it lowers the pH of the water, making it more acidic. This can harm aquatic life by disrupting the natural balance.
2. Bases:
   • Bases in water bodies help to neutralize the acidic components, maintaining the pH within a range suitable for aquatic life. However, excessive alkalinity can also be harmful.

Q 7:
Explain the process of neutralization with an example.

Answer:
Neutralization is the process where an acid reacts with a base to form a salt and water. The acidic and basic properties are neutralized.
Example:
When hydrochloric acid (HCl) reacts with sodium hydroxide (NaOH), it produces sodium chloride (NaCl) and water (H₂O).

 

$$\text{HCl (aq)} + \text{NaOH (aq)} → \text{NaCl (aq)} + \text{H}_2\text{O (l)}$$

 

This reaction results in a neutral solution.

Q 8:
What are the different types of salts? Give one example of each.

Answer:
There are several types of salts, including:

1. Normal Salts: Formed by the complete neutralization of an acid by a base.
   Example: Sodium chloride (NaCl) formed from the neutralization of hydrochloric acid by sodium hydroxide.
2. Acidic Salts: Formed when a strong acid reacts with a weak base.
   Example: Ammonium chloride (NH₄Cl), formed from ammonia and hydrochloric acid.
3. Basic Salts: Formed when a weak acid reacts with a strong base.
   Example: Sodium carbonate (Na₂CO₃), formed from carbonic acid and sodium hydroxide.
4. Double Salts: Formed by the combination of two salts.
   Example: Potassium alum (KAl(SO₄)₂·12H₂O), formed from potassium sulfate and aluminum sulfate.

Q 9:
What is the importance of salts in our body?

Answer:
Salts play a crucial role in maintaining various bodily functions, including:

1. Electrolyte balance: Sodium, potassium, and chloride ions help maintain the osmotic balance and proper hydration.
2. Nerve function: Sodium and potassium ions are essential for nerve impulse transmission.
3. Muscle function: Calcium and magnesium ions help in muscle contraction and relaxation.
4. pH regulation: Sodium bicarbonate (NaHCO₃) helps regulate the pH levels in the blood.

Q 10:
How do we test for the presence of an acid or a base in a solution?

Answer:
To test for the presence of an acid or base in a solution:

1. Use an indicator like litmus paper:
   • Red litmus paper turns blue in a basic solution and stays red in an acidic solution.
   • Blue litmus paper turns red in an acidic solution and stays blue in a basic solution.
2. Use a pH meter or pH paper: The pH scale helps determine whether the solution is acidic (pH < 7) or basic (pH > 7).

 

Related: Class 10 Science Chapter 1 – Chemical Reactions and Equations