Textbook Solutions: The Hydrosphere | Window on the Word Class 7: Solutions, Notes & Worksheets PDF Download

A. Mark the sentences as True or False. Correct the statements that are false.

1: 
Ans: False. Oceans and seas contain about 97% of the total water in the hydrosphere.

2:
Ans: False. Water of rivers is fresh and is suitable for use in the cultivation of crops.

3:
Ans: True.

4:
Ans: False. Spring tides occur during Full Moon and New Moon, when the Sun and the Moon are on the same or opposite sides of the Earth.

5:
Ans: False. Ocean currents flow from one place to another in particular directions.

B. Fill in the blanks

1. 
Ans: Oceans and seas are saline, meaning they contain a large amount of salt.

2.
Ans: A giant wave triggered off by underwater earthquakes is called a tsunami.

3.
Ans: When water recedes and sea level is low, it is low tide.

4. 
Ans: Warm currents originate near the equator and flow towards the poles.

5. 
Ans: Ocean currents play a role in regulating temperature around the world.

C. Choose the correct answer

1.
Ans: (d)

2.
Ans: (b)

3.
Ans: (d)

4.
Ans: (a)

D. Answer in one or two sentences

1.
Ans: Ocean water is unsuitable for drinking because it is highly saline, meaning it contains a large amount of salt.

2.
Ans: Waves are mainly caused by strong winds that blow across the surface of the oceans. The stronger the force of the wind, the greater is its thrust on the water, producing bigger waves.

3.
Ans: Tides refer to the periodic rise and fall of ocean waters caused by the gravitational pull exercised by the Moon and the Sun upon the Earth, and the rotation of the Earth.

4.
Ans: Ocean currents are influenced by varying temperatures in different parts of the ocean, the rotation of the Earth, prevailing winds, density and salinity of ocean water, and the shape of the adjacent coastlines.

5.
Ans: Places where warm and cold currents meet are excellent fishing grounds because the turbulence generated when the two currents meet leads to an upsurge of nutrients from deep within the oceans, which fuels the rich growth of phytoplankton, attracting fish and marine mammals.

E. Answer in a paragraph.

1.
Ans: The water cycle is a continuous process where water changes its form, starting with evaporation, where heat from the sun transforms water from oceans, rivers, lakes, ponds, and even plants into water vapour. Over time, this water vapour cools down and returns to a liquid form through the process of condensation, forming clouds. When these clouds become saturated, precipitation occurs, and water falls back to the ground as rain, snow, hail, or sleet, replenishing the water in oceans, rivers, lakes, and ponds, and underground. This cycle maintains the balance of water on the surface of the Earth, under the ground, and in the air.

2.
Ans: A tsunami is a series of large waves caused by a disturbance beneath the ocean surface or the seafloor. The 2004 Indian Ocean tsunami, one of the most devastating tsunamis in recorded history, was triggered by one of the largest earthquakes ever recorded. The waves travelled rapidly across the Indian Ocean, reaching over 9 metres in height by the time they reached the shoreline, flooding coastal regions of South Asia and East Africa. The incident claimed about 2,50,000 lives and devastated land and property in fourteen countries, including Indonesia, India, Sri Lanka, Myanmar, Somalia and the Maldives.

3.
Ans: Tides are the periodic rise and fall of ocean waters, influenced by the gravitational pull of the Moon and the Sun on the Earth and the Earth's rotation. There are two types of tides: spring and neap tides. Spring tides occur during the Full Moon and New Moon phases when the Sun and the Moon's combined gravitational pull makes the oceans bulge more than usual, causing higher and lower tides. Neap tides occur when the Sun and the Moon are at right angles to each other, partially cancelling out each other's pull and producing lower high tides and higher low tides.

4.
Ans: The quality and distribution of water in the world are facing various problems. Human activities like waste dumping into and near water bodies, urban expansion and construction blocking natural drainage channels, deforestation, and overuse of freshwater for domestic and industrial activities have impacted water distribution and quality, causing scarcity. There is an urgent need for water conservation measures like rainwater harvesting, preventing water pollution, reusing water wherever possible, and repairing leaking taps and pipes.