Class 7 Social Science Notes Chapter 5 (Water) – Our Environment Book

Alright class, let's focus on Chapter 5, 'Water'. This is a fundamental chapter not just for understanding our planet's environment but also carries significant weight for various government exams. Pay close attention as we break down the key concepts.

Chapter 5: Water - Detailed Notes for Exam Preparation

1. The Importance of Water:

• Water is essential for all forms of life – plants, animals, and humans.
• Our bodies are largely composed of water.
• It plays a crucial role in regulating climate.

2. The Water Cycle (Hydrological Cycle):

• Definition: The continuous movement of water on, above, and below the surface of the Earth. It's a closed system with no beginning or end.
• Driving Force: The Sun's heat is the primary engine driving the water cycle.
• Key Processes:
  • Evaporation: The process by which water changes from a liquid to a gas (water vapor). Primarily occurs from oceans, seas, lakes, and rivers due to solar heat. Transpiration is evaporation from plants.
  • Condensation: The process by which water vapor in the air changes back into liquid water, forming clouds. This happens when warm, moist air rises and cools. Water vapor condenses around tiny dust particles.
  • Precipitation: Water released from clouds in the form of rain, snow, sleet, or hail. When water droplets in clouds become too heavy, they fall to Earth due to gravity.
  • Collection/Runoff: Water that falls as precipitation flows over the land surface (runoff) into rivers, lakes, and eventually oceans, or seeps into the ground (infiltration) to become groundwater. This collected water then evaporates again, continuing the cycle.
• Terrarium Analogy: The book uses a terrarium to illustrate a small-scale water cycle.

3. Distribution of Water:

• About three-fourths (71%) of the Earth's surface is covered by water.
• Saline Water (Oceans & Seas): Approximately 97.3% of the Earth's total water is saline (salty) and found in oceans and seas. It's unfit for direct human consumption or most agricultural uses.
• Freshwater: Only about 2.7% of the Earth's total water is freshwater.
• Distribution of Freshwater:
  • Ice-caps and Glaciers: Roughly 68.7% of freshwater is locked up in ice caps (Antarctica, Greenland) and glaciers. It's largely inaccessible. (NCERT figure might slightly vary, older editions used ~2.0% of total water, newer calculations refine the % of freshwater). Focus on the relative proportions.
  • Groundwater: About 30.1% of freshwater is stored underground.
  • Surface Water (Lakes, Rivers): A very small fraction (<1% of freshwater) is readily accessible surface water in lakes, rivers, and swamps. (Atmosphere and soil moisture also hold tiny amounts).
• Key Takeaway: Potable (drinkable) freshwater is extremely scarce and precious.

4. Ocean Circulation:

• Ocean water is never still; it moves continuously. The movements are categorized into waves, tides, and currents.
• a) Waves:
  • Formation: Caused primarily by winds blowing across the ocean surface. The stronger the wind, the bigger the waves.
  • Mechanism: Water particles move mostly up and down in a circular motion; the wave form travels across the water.
  • Tsunami (Harbour Wave):
    • Cause: Not caused by wind, but by underwater earthquakes, volcanic eruptions, or landslides displacing large amounts of ocean water.
    • Characteristics: Can travel at immense speeds (over 700 km/h) in the deep ocean with long wavelengths and low height. As they approach shallow coastal waters, their speed decreases, but their height increases dramatically, causing massive destruction.
    • Example: The 2004 Indian Ocean Tsunami.
• b) Tides:
  • Definition: The rhythmic, twice-daily rise and fall of ocean water levels.
  • Cause: Primarily due to the gravitational pull exerted by the Moon and, to a lesser extent, the Sun on the Earth's surface waters.
  • Types:
    • Spring Tides: Occur during the full moon and new moon days when the Sun, Moon, and Earth are aligned. The combined gravitational pull creates unusually high high tides and unusually low low tides.
    • Neap Tides: Occur during the first and last quarter moon phases when the Sun and Moon are at right angles to each other relative to the Earth. Their gravitational forces partially counteract each other, resulting in lower high tides and higher low tides (i.e., a smaller tidal range).
  • Importance: High tides help in navigation (raising water levels near shores), fishing (bringing more fish closer to shore), and generating tidal energy.
• c) Ocean Currents:
  • Definition: Streams of water flowing constantly on the ocean surface in definite directions.
  • Types:
    • Warm Currents: Originate near the Equator and flow towards the Poles. They bring warmer temperatures to the land surfaces they flow past. Example: Gulf Stream (North Atlantic).
    • Cold Currents: Originate near the Poles or higher latitudes and flow towards the Equator or lower latitudes. They bring cooler temperatures to the land surfaces they flow past. Example: Labrador Current (North Atlantic), Oyashio Current (North Pacific).
  • Effects:
    • Climate: Influence the temperature and rainfall of coastal areas.
    • Navigation: Ships can save fuel and time by following currents.
    • Fishing: Areas where warm and cold currents meet (e.g., off the coast of Japan, eastern North America) are excellent fishing grounds because the mixing brings nutrients to the surface, supporting plankton growth, which attracts fish. However, these areas are also prone to fog, making navigation difficult.

5. Salinity:

• Definition: The amount of salt (in grams) dissolved in 1000 grams (1 kg) of water.
• Average Ocean Salinity: Approximately 35 parts per thousand (ppt) or 35 grams per kg.
• Dead Sea: Located between Israel and Jordan, it has extremely high salinity (around 340 grams per litre or 340 ppt). The high salt concentration increases water density, allowing people to float easily.
• Factors Affecting Salinity: Evaporation rates (higher evaporation increases salinity) and freshwater inflow from rivers or melting ice (decreases salinity).

Multiple Choice Questions (MCQs):

1. What percentage of the Earth's total water is found in oceans as saline water?
   a) 2.7%
   b) 71%
   c) 97.3%
   d) 68.7%

2. The process by which water changes from liquid to gas is called:
   a) Condensation
   b) Precipitation
   c) Evaporation
   d) Infiltration

3. Which of the following is the primary cause of ocean waves?
   a) Gravitational pull of the Moon
   b) Underwater earthquakes
   c) Wind blowing across the water surface
   d) Differences in water temperature

4. Spring Tides occur when:
   a) The Sun and Moon are at right angles to the Earth.
   b) The Sun, Moon, and Earth are in a straight line.
   c) Only during the summer season.
   d) Only the Moon's gravity affects the Earth.

5. Which of the following is an example of a warm ocean current?
   a) Labrador Current
   b) Oyashio Current
   c) Gulf Stream
   d) Canary Current

6. The rhythmic rise and fall of ocean water twice a day is known as:
   a) Wave
   b) Current
   c) Tide
   d) Tsunami

7. What causes a Tsunami?
   a) Strong winds during a storm
   b) The meeting of warm and cold currents
   c) Gravitational pull of the sun and moon
   d) Underwater earthquakes or volcanic eruptions

8. The average salinity of ocean water is approximately:
   a) 10 parts per thousand
   b) 35 parts per thousand
   c) 100 parts per thousand
   d) 340 parts per thousand

9. Where is the majority of Earth's freshwater located?
   a) Rivers and Lakes
   b) Groundwater
   c) Ice-caps and Glaciers
   d) Atmosphere

10. Areas where warm and cold ocean currents meet are known for:
    a) Being the calmest parts of the ocean
    b) Having extremely high salinity
    c) Being excellent fishing grounds but often foggy
    d) Experiencing very high Tsunami waves

Answers to MCQs:

1. c) 97.3%
2. c) Evaporation
3. c) Wind blowing across the water surface
4. b) The Sun, Moon, and Earth are in a straight line.
5. c) Gulf Stream
6. c) Tide
7. d) Underwater earthquakes or volcanic eruptions
8. b) 35 parts per thousand
9. c) Ice-caps and Glaciers
10. c) Being excellent fishing grounds but often foggy

Make sure you revise these points thoroughly. Understanding the water cycle, the distribution of water, and the dynamics of ocean circulation (waves, tides, currents) is crucial. Remember the specific examples given, like the Gulf Stream and Labrador Current, and the causes and characteristics of phenomena like Tides and Tsunamis. Good luck with your preparation!