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Practice Class 10 Science Chapter 2 – Acids, Bases and Salts with 5 NCERT-based MCQs on the pH scale, acids and bases in everyday life, dilution, water, and the importance of pH in biological systems.
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Chapter Information
Subject: Science
Class: 10
Chapter: Acids, Bases and Salts
Question Type: Multiple Choice Questions (MCQs)
Practice Set: 3
Difficulty Level: Moderate
Based On: NCERT Syllabus
Introduction
The pH scale helps us understand the acidic or basic nature of different substances. The pH of substances around us can have important effects on biological processes and everyday activities.
From digestion in our stomach to the effect of acidic substances on tooth enamel, maintaining a suitable pH is important for living organisms.
This Practice Set 3 focuses on pH in everyday life, dilution of acids and bases, the role of pH in the human body and the importance of pH in biological processes.
What You Will Learn
✔ Importance of pH in everyday life
✔ pH of the human stomach
✔ Role of hydrochloric acid in digestion
✔ Effect of pH on tooth enamel
✔ Importance of pH in biological systems
✔ Dilution of acids and bases
✔ Safety while diluting concentrated acids
Why This Topic Is Important
pH is not limited to laboratory experiments. It plays an important role in our digestive system, dental health, soil, aquatic environments and other biological processes.
NCERT-based questions often test whether students can connect the concept of pH with real-life situations.
5 MCQs with Answers and Detailed Solutions
Q1. Which acid is naturally present in the human stomach and helps in digestion?
A) Nitric acid
B) Hydrochloric acid
C) Sulphuric acid
D) Carbonic acid
Answer: B) Hydrochloric acid
Detailed Solution
The human stomach contains hydrochloric acid (HCl).
It creates an acidic medium that is important for the functioning of digestive enzymes and helps in the digestion process.
Therefore, the correct answer is B) Hydrochloric acid.
Exam Tip
Remember:
Stomach → Hydrochloric acid (HCl)
Q2. Why does tooth decay begin when the pH of the mouth falls below about 5.5?
A) Tooth enamel becomes stronger
B) Tooth enamel starts dissolving
C) Saliva becomes completely neutral
D) Calcium cannot exist in the body
Answer: B) Tooth enamel starts dissolving
Detailed Solution
The enamel of our teeth is made mainly of compounds containing calcium.
When the pH in the mouth falls below approximately 5.5, the acidic environment can cause the tooth enamel to dissolve gradually, increasing the risk of tooth decay.
Therefore, maintaining a suitable pH in the mouth is important for dental health.
Hence, the correct answer is B) Tooth enamel starts dissolving.
Exam Tip
Remember:
Mouth pH below about 5.5 → Tooth enamel can start dissolving → Tooth decay risk increases
Q3. What happens to the concentration of hydrogen ions when an acid is diluted with water?
A) It increases
B) It decreases
C) It always becomes zero
D) It remains exactly the same
Answer: B) It decreases
Detailed Solution
When an acid is diluted by adding water, the number of hydrogen ions per unit volume decreases.
As the concentration of hydrogen ions decreases, the solution becomes less acidic and its pH increases.
Therefore, the correct answer is B) It decreases.
Exam Tip
Remember:
Dilution of acid → H⁺ concentration decreases → pH increases
Q4. While diluting a concentrated acid, which method should be followed?
A) Add water rapidly to the acid
B) Add acid slowly to water with continuous stirring
C) Mix acid and water without stirring
D) Heat the acid before adding water
Answer: B) Add acid slowly to water with continuous stirring
Detailed Solution
Dilution of concentrated acids is highly exothermic and releases a considerable amount of heat.
Therefore, concentrated acid should be added slowly to water with continuous stirring.
Adding water directly to concentrated acid can cause rapid heating and splashing, which may be dangerous.
Hence, the correct answer is B.
Exam Tip
A very important laboratory safety rule:
Always add acid to water, not water to acid.
Q5. Which statement correctly describes the relationship between pH and hydrogen ion concentration?
A) Higher H⁺ concentration generally means higher pH
B) Lower H⁺ concentration generally means lower pH
C) Higher H⁺ concentration generally means lower pH
D) pH has no relationship with H⁺ concentration
Answer: C) Higher H⁺ concentration generally means lower pH
Detailed Solution
The pH of a solution is related to its hydrogen ion concentration.
A solution containing a higher concentration of hydrogen ions is more acidic and therefore has a lower pH.
Conversely, a lower hydrogen ion concentration corresponds to a higher pH.
Therefore, the correct answer is C.
Exam Tip
Remember:
H⁺ concentration ↑ → pH ↓
H⁺ concentration ↓ → pH ↑
Important Concepts
1. pH in the Stomach
Our stomach produces hydrochloric acid, which provides an acidic environment required for digestion.
Excess acid can cause discomfort and acidity. Certain medicines called antacids are used to neutralise excess stomach acid.
2. pH and Tooth Decay
Tooth enamel can be damaged when the pH of the mouth falls below approximately 5.5.
Bacteria in the mouth can produce acids from food particles, lowering the pH.
Regular dental care helps reduce the risk of tooth decay.
3. Dilution of Acids
When water is added to an acid, the concentration of hydrogen ions per unit volume decreases.
As a result:
H⁺ concentration decreases → pH increases → acidity decreases
4. Safety During Dilution
Dilution of concentrated acids releases heat.
Therefore, concentrated acid should be added slowly to water with continuous stirring.
This helps prevent excessive heating and splashing.
5. Relationship Between pH and H⁺ Concentration
The pH scale is closely related to hydrogen ion concentration.
In general:
Higher H⁺ concentration → Lower pH
Lower H⁺ concentration → Higher pH
Related Practice Questions
Q1. Why is hydrochloric acid present in the stomach?
Q2. At approximately what pH does tooth enamel start getting damaged?
Q3. What happens to the pH of an acid when it is diluted?
Q4. Why should acid be added to water during dilution?
Q5. How are pH and hydrogen ion concentration related?
Mini Quiz Challenge
Try answering these questions within 60 seconds.
Q1. Which acid is present in the stomach?
A) HCl
B) H₂SO₄
C) HNO₃
D) CH₃COOH
Q2. Tooth enamel can start getting damaged when mouth pH falls below approximately:
A) 9.5
B) 7.5
C) 5.5
D) 2.5
Q3. What happens to H⁺ concentration when an acid is diluted?
A) It increases
B) It decreases
C) It becomes infinite
D) It remains unchanged
Q4. Which should be added slowly to water during dilution?
A) Concentrated acid
B) Salt
C) Sugar
D) Sand
Q5. A lower pH generally indicates:
A) Lower acidity
B) Higher acidity
C) Neutrality only
D) Absence of ions
Exam Tips
✔ Remember the role of HCl in the stomach.
✔ Learn why pH below about 5.5 can damage tooth enamel.
✔ Remember that dilution decreases hydrogen ion concentration.
✔ Learn the correct safety procedure for diluting concentrated acids.
✔ Remember the inverse relationship between pH and H⁺ concentration.
✔ Connect pH concepts with everyday-life examples.
Quick Revision Notes
✔ Stomach acid: Hydrochloric acid (HCl)
✔ Tooth enamel: Can start dissolving when mouth pH falls below about 5.5
✔ Dilution of acid: Decreases H⁺ concentration
✔ Dilution: Generally increases the pH of an acid
✔ Safety rule: Add acid slowly to water
✔ Higher H⁺ concentration: Lower pH
✔ Lower H⁺ concentration: Higher pH
Common Mistakes Students Make
❌ Thinking that dilution makes an acid more concentrated.
❌ Forgetting the safety rule for dilution of concentrated acids.
❌ Confusing stomach acid with the acids used in laboratories.
❌ Thinking that a lower pH means a weaker acid.
❌ Forgetting that tooth enamel can be damaged by excessive acidity.
❌ Assuming pH and hydrogen ion concentration increase together.
Key Takeaways
✔ Hydrochloric acid is naturally present in the stomach.
✔ An acidic stomach environment helps digestion.
✔ Tooth enamel can start dissolving when mouth pH falls below about 5.5.
✔ Dilution decreases the concentration of hydrogen ions in an acid.
✔ Diluting an acid generally increases its pH.
✔ Concentrated acid should be added slowly to water with continuous stirring.
✔ Higher hydrogen ion concentration corresponds to lower pH.
FAQs
Q. Which acid is present in our stomach?
Answer:
The stomach contains hydrochloric acid (HCl), which helps provide the acidic medium required for digestion.
Q. Why does tooth decay occur?
Answer:
When the pH of the mouth falls below approximately 5.5, tooth enamel can start dissolving, increasing the risk of tooth decay.
Q. What happens when an acid is diluted?
Answer:
Dilution decreases the concentration of hydrogen ions per unit volume, so the acid becomes less acidic and its pH increases.
Q. Why should acid be added to water and not water to acid?
Answer:
Dilution of concentrated acid releases heat. Adding acid slowly to water with stirring helps control the heat and reduces the risk of splashing.
Q. What is the relationship between pH and H⁺ concentration?
Answer:
Higher hydrogen ion concentration generally means lower pH, while lower hydrogen ion concentration means higher pH.
Conclusion
Class 10 Science Chapter 2 – Acids, Bases and Salts focuses on the practical importance of pH in everyday life. It covers the role of hydrochloric acid in the stomach, tooth decay, dilution of acids and the relationship between pH and hydrogen ion concentration.
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