DAV Board Class 7 Social Science (We and Our World) | 100% Authentic Textbook Solutions

DAV Class 7 SST Chapter 5 Water Surrounding the Earth Solutions

Get complete, 100% authentic textbook solutions for DAV Class 7 Social Science (We and Our World – Geography) Chapter 5: Water Surrounding the Earth. This comprehensive study guide features detailed chapter summaries and core concepts (Hydrosphere distribution, Water Cycle stages, Movements of Ocean Water: Waves, Tsunamis, Tides: Spring and Neap Tides, Ocean Currents: Warm and Cold currents, impact on climate and global fishing grounds), key scientific definitions, complete textbook exercise solutions (Section A: MCQs, Section B: Fill in the Blanks, Section C: Technical Terms, Section D: Short Answer Questions, Section E: Detailed Explanations with Water Cycle Diagram), Value Based Questions (VBQ), Higher Order Thinking Skills (HOTS), and project activities strictly adhering to the DAV Education Board curriculum.

1. Detailed Chapter Summary and Core Concepts

A. The Hydrosphere and Global Water Distribution

• The Water Planet: Earth is called the Water Planet or the Blue Planet because approximately 71 percent of its total surface area is covered with water.
• Volumetric Distribution of Water:
  • 97 percent: Saline water stored in vast oceans and inland seas (unfit for direct human consumption or agriculture).
  • 2 percent: Fresh water locked in massive polar ice sheets, ice caps, and high mountain glaciers.
  • Less than 1 percent: Accessible liquid fresh water present in rivers, freshwater lakes, ponds, and subterranean groundwater aquifers.
• Hemispheric Distribution: Water area is unequally partitioned across the globe: 43 percent lies in the Northern Hemisphere, while 57 percent is concentrated in the Southern (Water) Hemisphere.
• Four Great Oceans: The Pacific Ocean (largest and deepest), Atlantic Ocean (busiest commercial ocean), Indian Ocean (named after India), and Arctic Ocean (surrounding the North Pole).

B. The Hydrological Cycle (Water Cycle)

Water is in a perpetual state of dynamic closed-loop circulation across the lithosphere, hydrosphere, and atmosphere without beginning or end:

• 1. Evaporation: Solar thermal radiation heats ocean and surface water, converting liquid water into buoyant water vapour.
• 2. Transpiration: Living terrestrial vegetation releases additional water vapor into the air through microscopic stomatal pores in plant leaves.
• 3. Condensation: Rising water vapour ascends into cooler upper tropospheric zones, condensing around suspended dust particles (hygroscopic nuclei) to form clouds.
• 4. Precipitation: Cloud droplets coalesce into heavier drops and fall back onto the Earth’s surface as rain, snow, sleet, or hail.
• 5. Surface Runoff and Groundwater Infiltration: Rainwater drains through surface runoff into streams, rivers, and oceans, while a fraction percolates deep into soil layers to replenish the groundwater table.

C. Continuous Movements of Ocean Water

• 1. Ocean Waves:
  • Rhythmic undulations of the water surface where water particles move primarily in an up-and-down orbital motion.
  • Key Principle: Water particles themselves do not travel horizontally across distances; only energy propagates forward across the ocean surface.
  • The height and magnitude of a wave depend upon the speed of the blowing wind and the continuous duration of wind travel in a single direction.
• 2. Tsunamis (Harbour Waves):
  • A Japanese word meaning “harbour wave”. These are colossal, high-velocity seismic sea waves generated by underwater submarine earthquakes, volcanic caldera collapses, or underwater landslides.
  • December 2004 Tsunami Disaster: A catastrophic undersea earthquake off Sumatra (measuring 9.1 magnitude) generated massive tsunami walls of water that battered coastal regions of India, Sri Lanka, Indonesia, and Thailand, killing over 230,000 people.
• 3. Ocean Tides:
  • The periodic, rhythmic rise and fall of ocean water levels occurring twice daily, caused by the joint gravitational attraction of the Moon and the Sun on Earth.
  • High Tide: The highest water elevation cresting along the shore; Low Tide: The lowest water trough receding from the shore.
  • The exact time interval between a high tide and the next low tide is 6 hours and 13 minutes.
  • Spring Tides: Exceptionally high tides that occur on New Moon and Full Moon days when the Sun, Moon, and Earth align in a straight line (syzygy), compounding gravitational pull.
  • Neap Tides: Below-average high tides that occur during the Moon’s first and third quarters when the Sun and Moon pull at right angles (quadrature) to each other.
  • Significance: Allows deep-draft ocean cargo vessels to navigate shallow riverine ports (such as Kolkata Port on River Hooghly), clears harbour silt deposits, and provides clean tidal electric energy.
• 4. Ocean Currents:
  • Continuous, directed streams of vast ocean water masses moving along distinct geographic paths over thousands of kilometers.
  • Driven by equatorial-polar temperature differentials, water density disparities, prevailing planetary winds, and Earth’s Coriolis rotation.
  • Warm Currents: Originate near the warm Equator and flow poleward along the eastern margins of continents (e.g., Gulf Stream in the Atlantic, Kuroshio in the Pacific). They raise coastal temperatures and bring humid winds with rainfall.
  • Cold Currents: Originate in chilly polar latitudes and flow equatorward along the western margins of continents (e.g., Labrador Current, Oyashio, Canary Current, Peru/Humboldt Current). They chill coastal air, suppress evaporation, and foster arid desert belts.
  • World Fishing Grounds: The oceanic convergence zones where warm and cold currents meet (such as the Grand Banks of Newfoundland where the Gulf Stream meets the Labrador Current, and Dogger Bank in the North Sea) create ideal nutrient conditions for massive blooms of microscopic plankton, supporting the world’s most productive commercial fisheries.

2. Key Terms and Definitions

3. Complete Textbook Exercise Solutions

Section A: Tick (✓) the Correct Option

1. The water cycle is:
(a) the continuous movement of water [✓]
(b) passage of water through a house
(c) the life of an organism in water
(d) a static system
Answer: (a) the continuous movement of water

2. The three states of water are:
(a) cloud, rain, river
(b) solid, liquid, gas [✓]
(c) salt water, fresh water, rain water
(d) waves, tides, currents
Answer: (b) solid, liquid, gas

3. What turns water on the earth into vapours in the water cycle?
(a) the core of the earth
(b) global warming
(c) the sun [✓]
(d) nuclear energy
Answer: (c) the sun

4. When clouds get too heavy to hold water:
(a) the water runs off
(b) the water falls on the earth [✓]
(c) more clouds are formed
(d) water gets collected
Answer: (b) the water falls on the earth

5. Which is the only warm current from among the following?
(a) Oyashio
(b) Canary
(c) Kuroshio [✓]
(d) Labrador
Answer: (c) Kuroshio

Section B: Fill in the Blanks

• 1. The largest natural source of water is oceans.
• 2. When the level of ocean water rises, it is a high tide.
• 3. Kolkata port in India is an example of tidal port.
• 4. The Gulf stream takes its name from the Gulf of Mexico.
• 5. Peru (or Humboldt) is a cool current on the west coast of South America.

Section C: Give a Technical Term for Each Statement

1. The process in which water is converted to water vapours through heat of the sun.
Answer: Evaporation

2. The continuous circulation of water in the three spheres of the earth.
Answer: Water Cycle (Hydrological Cycle)

3. A movement in the ocean in which water particles move up and down.
Answer: Waves

4. It is the purest form of water.
Answer: Rainwater

5. Periodic rise and fall in the level of the ocean water.
Answer: Tides

Section D: Answer the Following Questions in Brief

Question 1. On what factors does the height of a wave depend?
Answer: The vertical height of an ocean wave depends primarily on two meteorological factors:
1. The speed and velocity of the wind blowing over the ocean surface.
2. The duration of time for which the wind blows continuously across an unobstructed open water expanse (fetch).

Question 2. Define an ocean current.
Answer: An ocean current is the regular, continuous, and predictable stream-like movement of a massive body of ocean water flowing in a definite geographic direction across thousands of kilometers.

Question 3. How are tides caused?
Answer: Tides are caused by the combined gravitational attraction exerted by the Moon and the Sun on the Earth’s oceanic water mass, causing periodic gravitational bulges as the Earth rotates.

Question 4. Name the important components of the water cycle.
Answer: The five key stages of the global water cycle are:
1. Evaporation
2. Transpiration
3. Condensation
4. Precipitation
5. Surface Runoff and Groundwater Infiltration.

Question 5. Name three valuable resources which are found in abundance in the ocean.
Answer: Three valuable resources abundant in the oceans are:
1. Fossil Fuels: Petroleum (crude oil) and natural gas extracted from continental shelves.
2. Metallic & Non-Metallic Minerals: Common salt (sodium chloride), magnesium, and deep-sea polymetallic manganese nodules.
3. Marine Biological Resources: Abundant edible fish, crustaceans, and plankton supporting human nutrition and pharmaceuticals.

Section E: Answer the Following Questions in Detail

Question 1. How are tides useful to us?
Answer: Tides offer multiple strategic, economic, and practical advantages:

• 1. Harbour Navigation: During high tides, water depth at river mouths and shallow coastal inlets increases significantly, allowing heavy international merchant vessels to enter or leave ports safely (e.g., Kolkata Port situated on the Hooghly River).
• 2. Tidal Energy Production: The immense kinetic surge of rising and falling tides is channeled through turbines in coastal barrages to generate clean, renewable, and pollution-free tidal electricity.
• 3. Desilting of Harbours: The forceful backwash of receding low tides scours and carries away fine silt and sediments deposited at river estuaries, keeping commercial shipping navigation channels clear.
• 4. Commercial Inshore Fishing: High tides sweep vast shoals of marine fish closer to coastlines, enabling local fishermen to secure substantial catches without venturing into dangerous deep seas.

Question 2. How are oceans beneficial to us in improving the quality of life?
Answer: Oceans serve as the planetary lifeline supporting human life and civilization in the following ways:

• 1. Invaluable Global Maritime Trade: Ocean sea-routes provide the most economical, cost-effective mode for bulk transportation of heavy industrial goods, raw minerals, and agricultural commodities across continents.
• 2. Global Climate Moderation: Massive ocean currents distribute equatorial solar heat toward higher latitudes, moderating extreme regional temperatures and keeping northern coastal ports ice-free in winter.
• 3. Nutritional Security: Marine fisheries supply a primary source of animal protein, omega oils, and essential micronutrients for billions of people worldwide.
• 4. Abundant Energy & Mineral Reserves: Seabeds hold offshore reserves of petroleum, natural gas, sulfur, and deep-sea manganese nodules essential for global industries.

Question 3. Explain the water cycle with the help of a diagram.
Answer: The water cycle is the unbroken, closed loop in which water constantly transforms between solid, liquid, and gaseous phases:

Question 4. Explain three effects of ocean currents.
Answer:

• 1. Influence on Coastal Temperature and Rainfall:
  • Warm currents (like the Gulf Stream) warm chilly coastal margins, elevate atmospheric moisture, and bring rainfall to adjacent landmasses.
  • Cold currents (like the Canary or Peru Current) cool coastal air, suppress evaporation, and create desiccating conditions that lead to adjacent coastal desert formations.
• 2. Creation of World-Class Fishing Grounds: Where warm and cold currents converge (such as the meeting of the warm Gulf Stream and cold Labrador Current off Newfoundland), the mixture creates ideal temperature and nutrient conditions for dense blooms of plankton, supporting world-famous fishing grounds like the Grand Banks and Dogger Bank.
• 3. Maritime Navigation & Fuel Efficiency: Navigators sailing in the direction of moving ocean currents experience enhanced ship velocity and significant fuel savings, optimizing commercial global sea lanes.

Question 5. Suggest four ways to help people affected by devastation due to Tsunami.
Answer: Four critical disaster response and rehabilitation measures include:

• 1. Emergency Relief and Safe Shelter: Establish well-equipped temporary shelter relief camps and distribute clean packaged drinking water, emergency food rations, dry clothes, and blankets to displaced families.
• 2. Emergency Medical Care & Disease Prevention: Mobilize specialized emergency medical trauma teams to treat physical fractures and administer essential vaccinations to forestall waterborne cholera and typhoid epidemics.
• 3. Economic Livelihood Rehabilitation: Provide financial ex-gratia relief grants, interest-free microcredit, and replacement fishing boats/nets to fishermen and small coastal vendors whose livelihoods were wiped out.
• 4. Early Warning Infrastructure & Bio-Shields: Deploy deep-ocean automated tsunami detection buoys connected to coastal satellite alert networks, and plant dense coastal mangrove forests as natural bio-shields against future storm surges.

4. Value Based Questions (VBQ) and HOTS

Value Based Question (VBQ)

Context: ‘We should respect and protect our oceans and seas. Oceans made Earth a habitable planet by giving birth to life. But we are turning our oceans into dumping grounds of waste material.’

1. What steps should be taken to prevent loss of precious marine life?
Answer: To conserve fragile marine biodiversity, governments must enforce a total legal ban on discharging untreated industrial toxic chemical effluents and city sewage into rivers and ocean basins. Additionally, strict seasonal bans on commercial mechanized trawling during fish breeding seasons and extensive community coastal cleanups to ban single-use plastics from entering marine food chains are imperative.

2. Give two suggestions to ban the disposal of waste material into oceans and seas.
Answer:
1. Enact Heavy Penal Legislation: Implement stringent environmental maritime laws prescribing heavy financial penalties and criminal prosecution for commercial shipping vessels, coastal factories, and municipalities caught dumping garbage, plastics, or chemical waste into the sea.
2. Mandate Modern Waste Treatment Plants: Mandate high-capacity advanced Sewage Treatment Plants (STPs) and industrial effluent recycling units in all coastal towns, ensuring that only zero-pollutant treated recycled water is released into watercourses.

HOTS (Higher Order Thinking Skills)

Q1. Why are the world’s major hot deserts located along the western margins of continents in the tropical regions?
Answer: The western margins of tropical continents are continuously washed by cold ocean currents (e.g., the Canary Current off the Sahara Desert, the Peru/Humboldt Current off the Atacama Desert, and the Benguela Current off the Kalahari Desert). Cold currents chill the lower layer of coastal air, creating thermal stability that inhibits vertical convection, condensation, and cloud formation. Coupled with offshore Trade Winds that lose their moisture over eastern continental interiors, this lack of precipitation fosters hyper-arid tropical deserts.

Q2. What would happen to global trade and shipping if ocean tides ceased to occur?
Answer: If ocean tides ceased completely, shallow estuarine river ports (such as the Port of Kolkata on the Hooghly River) would become inaccessible to large deep-draft container ships, crippling global trade networks. Additionally, the lack of tidal scour would cause river channels to rapidly silt up, necessitating constant and expensive artificial dredging. Furthermore, coastal nations would lose a predictable source of clean tidal hydroelectric energy.

5. Something To Do / Project Activities

A. Working Model: Global Ocean Currents

• Objective: Construct an educational visual display map tracing primary warm and cold global ocean current circuits.
• Materials: World outline board, red insulated wire (warm currents), blue insulated wire (cold currents), pins, and labels.
• Execution:
  1. Use Red Wire to trace equatorial warm currents moving poleward (Gulf Stream, Kuroshio Current, North Atlantic Drift).
  2. Use Blue Wire to trace polar cold currents moving equatorward (Labrador Current, Oyashio, Peru Current, Canary Current).
  3. Place miniature fish icons at confluence points (Grand Banks and Dogger Bank) to highlight world-famous commercial fishing banks.

B. Scrapbook Project: Marine Resources & Tsunami Recovery

• Marine Wealth Portfolio: Compile photographs and infographics illustrating offshore oil rigs (e.g., Mumbai High), deep-sea manganese nodules, and marine aquaculture.
• Disaster Management Album: Document photographic contrasts between 2004 tsunami damage and contemporary bio-shield mangrove plantations planted along vulnerable coastlines.

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