Chapter 2 · Question 9

Explain how pH affects plant growth and aquatic life. What is acid rain, and what happens when soil becomes too acidic? Mention how a farmer can treat acidic soil.

Answer Revealed

Direct Answer:

Plants require a specific pH range for healthy growth and can only survive within a narrow range. Soil pH can be tested (Activity 2.12) by mixing a soil sample with water, filtering, and testing the filtrate with universal indicator paper. Acid rain — rainwater with a pH below 5.6 — flows into rivers, lowering the pH of river water and making survival difficult for aquatic life. If soil is too acidic, a farmer can treat it by adding quicklime (

\text{CaO}

), slaked lime (

\text{Ca(OH)}_2

), or chalk (

\text{CaCO}_3

) — all basic substances that neutralise the excess acid in the soil.

Simple Explanation

Plants need the soil pH to stay in a suitable range to grow well. Acid rain is rainwater with pH below 5.6; when it flows into rivers, it lowers the river-water pH and makes life difficult for aquatic organisms. If soil is too acidic, a farmer can add quicklime, slaked lime, or chalk. These basic substances neutralise the excess acidity and bring the soil closer to a plant-friendly range.

Exam-Ready Structure

The pH sensitivity of the natural environment — soil, water bodies, and rainfall — is a critical application of acid-base chemistry in ecology and agriculture, covered in NCERT Class 10 Chapter 2. 1. Living organisms and pH: The human body works within the pH range 7.0 to 7.8, and living organisms can survive only within a narrow range of pH change. 2. Acid rain: (a) When the pH of rainwater falls below 5.6, it is termed acid rain. (b) Impact: When acid rain flows into rivers, it lowers the pH of river water. This makes survival difficult for aquatic life. 3. Soil pH and plants: (a) Plants require a specific pH range for healthy growth. (b) To find the pH required for healthy plant growth in a region, one can collect soil from different places, note the plants growing there, mix about 2 g of soil with 5 mL of water in a test tube, shake, filter, and test the filtrate with universal indicator paper (Activity 2.12). 4. Treatment of acidic soil: If soil is too acidic, farmers use basic substances to neutralise soil acidity: quicklime (calcium oxide,

\text{CaO}

), slaked lime (calcium hydroxide,

\text{Ca(OH)}_2

), or chalk (calcium carbonate,

\text{CaCO}_3

Key Points

Living beings survive within a narrow pH range, typically 7.0 to 7.8
Acid rain: rainwater with pH below 5.6 (normal rain ~5.6 due to dissolved CO₂)
Acid rain lowers river pH, harming aquatic life
Soil pH test (Activity 2.12): mix soil with water, filter, test filtrate with universal indicator
Treatment for acidic soil: add quicklime ( $\text{CaO}$ ), slaked lime ( $\text{Ca(OH)}_2$ ), or chalk ( $\text{CaCO}_3$ )

Explain how pH affects plant growth and aquatic life. What is acid rain, and what happens when soil becomes too acidic? Mention how a farmer can treat acidic soil.