Acids, Bases and Salts | Class 10 Science Notes

2.3 How Strong Are Acid or Base Solutions?

This section introduces the pH scale, a numerical measure of the acidity or alkalinity of a solution, ranging from 0 to 14. The pH scale quantifies the concentration of hydrogen ions (H+) in a solution: lower pH values indicate higher acidity (more H+ ions), pH 7 is neutral, and higher pH values indicate alkalinity (more OH- ions). Universal indicator, a mixture of several indicators, shows different colors corresponding to different pH values. Testing various common substances with universal indicator paper reveals their pH and nature (acidic, neutral, or basic). Strong acids produce more H+ ions and have lower pH values, while weak acids produce fewer H+ ions and have higher pH values. Similarly, strong bases produce more OH- ions and have higher pH values, while weak bases produce fewer OH- ions. The section also discusses the importance of pH in everyday life: the human body maintains a narrow pH range for proper functioning; acid rain with pH below 5.6 harms aquatic life; soil pH affects plant growth; stomach acid aids digestion; antacids neutralize excess stomach acid; low pH in the mouth causes tooth decay; and some animals and plants use acids or bases for defense.

📊 Diagram: Figure 2.6 Variation of pH with the change in concentration of H+(aq) and OH-(aq) ions; Figure 2.7 pH of some common substances shown on a pH paper (colours are only a rough guide).

🧪 Activity: Activity 2.11: Testing pH of various substances using universal indicator paper; Activity 2.12: Testing soil pH for plant growth suitability.

🔗 Connection: Leads to detailed study of salts, their preparation, properties, and uses.

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