NCERT Solutions Class 4 EVS Chapter 13: A River's Tale

The Journey of a River: From Mountains to Cities

The story of a river begins high up in the mountains, where melting glaciers and rainwater create crystal-clear streams. At its source, the river is pristine, carrying clear, cool water that supports a massive variety of fish and aquatic plants. However, as the river flows down towards plains and enters human settlements, its journey changes completely. People start using the river water for various daily activities like washing clothes, bathing animals, and cleaning domestic utensils. Factories discharge harmful chemical wastes directly into the flowing stream without treating them. Farmers also use chemical pesticides and fertilizers on crops, which wash into the river during heavy rains. By the time the river reaches big cities, it is filled with plastic bags, household garbage, and toxic industrial chemicals. This changes the color of the water to a dark brown or green, kills the fish, and makes the water completely unsafe for drinking or bathing.

Difference Between Clean Water and Polluted Water

Step-by-Step: How a River Gets Polluted

1. Step 1: Domestic Sewage Discharge — Untreated sewage water from houses, toilets, and kitchens is directly drained into local water bodies, introducing harmful bacteria.
2. Step 2: Industrial Waste Dumping — Factories dump toxic chemicals, acids, heavy metals, and warm water into the river, destroying aquatic ecosystems.
3. Step 3: Agricultural Runoff — Rainwater washes chemical fertilizers and pesticides from agricultural fields into nearby rivers, causing excess algae growth.
4. Step 4: Public Activities — Washing clothes, throwing plastic packages, immersion of idols, and bathing livestock directly contaminate river banks.

How to Clean and Purify Drinking Water at Home

• Boiling: Boiling water for at least 10 to 15 minutes is the simplest way to kill all disease-causing germs and bacteria.
• Using Water Filters: Modern water purifiers (RO and UV systems) filter out physical impurities and harmful chemicals effectively.
• Alum (Fitkari): Adding a small piece of alum to muddy water helps suspended soil particles settle down at the bottom of the container.
• Chlorination: Adding chlorine tablets in correct proportions helps disinfect large amounts of water, especially during rainy seasons.

Exam Tip: Key Terms to Remember

In your exams, you might be asked about Soluble and Insoluble substances.

• Soluble substances are those that dissolve completely in water and cannot be easily seen (like salt or sugar).
• Insoluble substances do not dissolve in water and remain visible (like sand, oil, or plastic sawdust). Remember, even if something dissolves and becomes invisible, it can still make the water polluted and unsafe to drink!

Practice Questions with Solutions

• Q: Why does the color of the river change when it flows near cities?
  A: Step 1: Identify what enters the river near cities. Cities have sewage pipes, factories, and heavy human populations.
  Step 2: Connect this to the physical changes. Sewage, industrial chemical waste, plastics, and household garbage are dumped directly into the river water.
  Step 3: State the final effect. These added pollutants mix with the clean water, turning it muddy brown, black, or green.
  Final answer: The color of the river changes because toxic chemicals, industrial waste, household garbage, and sewage water are dumped into it as it passes through cities.

• Q: What will happen if animals drink polluted river water?
  A: Step 1: Analyze the contents of polluted water. It contains harmful chemicals, pathogens, and bacteria.
  Step 2: Understand the effect of these toxins on an animal's body.
  Step 3: Conclude the health outcome.
  Final answer: If animals drink polluted water, they can fall severely ill with waterborne diseases like cholera, typhoid, or diarrhea, and many of them can even die.

• Q: Mention two things you can do to prevent water pollution in your daily life.
  A: Step 1: Think of small, practical steps at home.
  Step 2: Choose actions that stop waste from entering water drains.
  Final answer: First, we must never throw garbage, plastic bags, or leftover food into rivers or open drains. Second, we should minimize the use of chemical detergents and soaps while washing near natural water sources.

• Q: Is clean-looking water always safe for drinking? Explain your answer.
  A: Step 1: Understand that clean-looking water might still contain microscopic germs or dissolved chemicals.
  Step 2: Explain that some harmful substances are soluble and invisible to the naked eye.
  Final answer: No, clean-looking water is not always safe to drink. It may look clear, but it can still contain harmful, invisible disease-causing germs, bacteria, or dissolved toxic chemicals that can make us sick.

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