Simply CBSE

Water — Class 7 Social Science

Quick revision notes for exam preparation.

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📌 Key Points

  • Water cycle (hydrological cycle): continuous movement of water through evaporation, condensation, precipitation, and collection — driven by the Sun
  • 71% of Earth's surface is covered with water; 97.3% is saltwater in oceans; only 2.7% freshwater; only about 1% usable
  • Freshwater sources: rivers, lakes, groundwater, glaciers; most freshwater (2%) locked in ice caps
  • Salinity: amount of salt in 1,000 g of water; average ocean salinity = 35‰; higher where evaporation is high, lower near river mouths
  • Ocean temperature: warm at surface, cold at depth; warm near equator, cold near poles
  • Three ocean movements: waves, tides, and ocean currents
  • Waves: rise and fall of surface water; caused by wind; crest (highest) and trough (lowest); water moves in circles, energy moves forward
  • Tsunami: huge destructive wave caused by earthquake/volcanic eruption under the sea — NOT caused by wind; 2004 Indian Ocean tsunami
  • Tides: rhythmic rise and fall caused by Moon's and Sun's gravitational pull; two high tides and two low tides daily
  • Spring tides: during full/new moon; Sun-Moon-Earth in a line; very high tides (gravitational pulls combine)
  • Neap tides: during quarter moon; Sun and Moon at right angles; moderate tides (pulls partially cancel)
  • Tides are useful for: navigation, fishing, tidal energy, cleaning shorelines, salt production
  • Warm ocean currents: flow from equator to poles; bring rainfall; Example: Gulf Stream (keeps Europe warm)
  • Cold ocean currents: flow from poles to equator; can cause deserts; Example: Labrador Current, Benguela Current
  • Where warm and cold currents meet = excellent fishing grounds (e.g., near Newfoundland)
  • Water conservation is essential: rainwater harvesting, fixing leaks, drip irrigation, recycling water, protecting water bodies, planting trees

📘 Important Definitions

Water Cycle
The continuous movement of water on, above, and below Earth's surface through evaporation, condensation, precipitation, and collection.
Evaporation
The process by which liquid water turns into water vapour (gas) due to heat from the Sun.
Condensation
The process by which water vapour cools and changes back into tiny water droplets, forming clouds.
Precipitation
Water falling from clouds to Earth in the form of rain, snow, sleet, or hail.
Salinity
The amount of salt dissolved in 1,000 grams of ocean water, measured in parts per thousand (‰). Average = 35‰.
Crest
The highest point of a wave.
Trough
The lowest point of a wave.
Tsunami
A huge, destructive wave caused by an earthquake, volcanic eruption, or landslide on the ocean floor. From Japanese meaning 'harbour wave'.
Tides
The rhythmic rise (high tide) and fall (low tide) of ocean water caused by the gravitational pull of the Moon and the Sun.
Spring Tides
Very high tides during full/new moon when Sun, Moon, and Earth are in a straight line and gravitational pulls combine.
Neap Tides
Moderate tides during quarter moon when Sun and Moon are at right angles and their gravitational pulls partially cancel.
Ocean Currents
Large-scale movements of water in a definite direction through the ocean, like rivers within the ocean.
Warm Currents
Ocean currents that flow from equatorial regions towards the poles, carrying warm water. Example: Gulf Stream.
Cold Currents
Ocean currents that flow from polar regions towards the equator, carrying cold water. Example: Labrador Current.

⚠️ Common Mistakes

✗ Wrong: Thinking most water on Earth is freshwater

✓ Correct: 97.3% of Earth's water is saltwater in oceans. Only 2.7% is freshwater, and only about 1% is usable. Freshwater is extremely scarce.

✗ Wrong: Thinking water in a wave moves forward

✓ Correct: In a wave, water moves in a CIRCULAR motion — only the ENERGY moves forward. The water itself stays roughly in the same place.

✗ Wrong: Confusing tsunamis with wind-caused waves

✓ Correct: Normal waves are caused by WIND. Tsunamis are caused by EARTHQUAKES, volcanic eruptions, or landslides under the sea. They are fundamentally different.

✗ Wrong: Thinking spring tides are related to the spring season

✓ Correct: Spring tides occur during FULL MOON and NEW MOON (not the spring season). The word 'spring' here means 'to spring up' or rise high.

✗ Wrong: Confusing warm and cold current direction

✓ Correct: Warm currents flow FROM the equator TO the poles (carrying warm water). Cold currents flow FROM the poles TO the equator (carrying cold water).

✗ Wrong: Thinking tides are caused by wind

✓ Correct: Tides are caused by the GRAVITATIONAL PULL of the Moon (mainly) and the Sun — NOT by wind. Wind causes waves, not tides.

📝 Exam Focus

These questions are frequently asked in CBSE exams:

Describe the water cycle with its stages
3m
What is the distribution of water on Earth? Why is freshwater scarce?
3m
What is salinity? How does it vary in different oceans?
1m
Distinguish between spring tides and neap tides
3m
What is a tsunami? How is it different from normal waves?
3m
Explain warm and cold ocean currents with examples
3m
What causes tides? Discuss their importance
5m
Describe the three types of ocean movements in detail
5m
Why is water conservation important? List methods of conservation
5m
What is the effect of ocean currents on climate? Give examples
3m

🎯 Last-Minute Recall

Close your eyes and try to recall: Key definitions, formulas, and 3 common mistakes. If you can recall 80% without looking, you're exam-ready!