Ncert Solutions for Class 7 Social Science Chapter 5 – Water
Page No 37:
Question 1:
Answer the following questions.
(i) What is precipitation?
(ii) What is water cycle?
(iii) What are the factors affecting the height of the waves?
(iv) Which factors affect the movement of ocean water?
(v) What are tides and how are they caused?
(vi) What are ocean currents?
Answer:
(i) Sun’s heat causes water to evaporate. When the water vapour rises, it begins cooling. The condensing water vapour forms droplets of water. Masses of such water droplets form clouds and float in air. When these droplets become too heavy to float in air, they fall upon land or sea in the form of rain, snow or sleet. This is known as precipitation.
(ii) The water present on the Earth’s surface (on land and in water bodies) gets evaporated by the Sun’s heat. The water vapours rise, condense and form clouds. When the clouds become heavy, the condensed water vapours fall back upon land or sea in the form of rain, snow or sleet. The processes of evaporation, condensation and precipitation continue in a cyclical manner. This way, water continually changes its form and circulates between oceans, atmosphere and land. This continuous circulation of water is known as the water cycle.
(iii) Waves are formed when winds scrape across the ocean surface. Winds blowing at very high speed form huge waves. An earthquake, a volcanic eruption and underwater landslides also affect the height of waves. Such phenomena can shift large amounts of ocean water, thereby causing huge waves to be formed.
(iv) The movements that occur in oceans are broadly categorised as waves, tides and currents.
The factors affecting these movements of ocean water are as follows:
(a) Waves are formed as a result of winds, earthquakes, volcanic eruptions and underwater landslides.
(b) Tides are caused by the strong gravitational pull exerted by the Sun and the Moon on the Earth’s surface.
(c) Another factor affecting the movement of ocean water is temperature. Ocean currents are formed because of the temperature differences between the ocean water present at different latitudes.
(v) Tides are the rhythmic rise and fall of ocean water twice in a day. A high tide occurs when ocean water covers much of the shore by rising to its highest level. When ocean water falls to its lowest level and recedes from the shore, it is referred to as a low tide. Tides are caused by the strong gravitational pull exerted by the Sun and the Moon on the Earth’s surface.
Formation of high and low tides:
The water of the Earth closer to the Moon gets pulled under the influence of Moon’s gravitational force, thereby causing a rise of ocean water towards the Moon. At the same time, the ocean water at the opposite side of the Earth bulges away from the moon. These bulges (or rises in the height of ocean water) are called high tides. When ocean water falls back, low tides occur. Say, point I is a point on Earth that faces the Moon and point IV is at the opposite end of the Earth. Under the gravitational pull of the Moon, the ocean water at point I moves towards the pulling force. At the same time, the ocean water at point IV moves away from the pulling force. Both these movements are high tides as the ocean water rises to its highest level at both these points. Due to the Earth’s rotation on its axis and the Moon’s revolution in its orbit, after a certain time period, the ocean water at both points I and IV recedes to its lowest level, thereby causing low tides. When the moon is directly above point IV, the entire process is repeated, with points I and IV first experiencing high tides, and then, low tides. Hence, in a day, both points I and IV experience two high tides and two low tides.
Formation of spring and neap tides:
When the Sun, the Moon and the Earth are in the same line, the combined gravitational pulls of both the Sun and the Moon cause the highest high tides, i.e., high tides are the highest. These tides are called spring tides.
When the gravitational pull of the Moon on the Earth is perpendicular to the gravitational pull of the Sun on the Earth, the gravitational forces counteract, thereby causing the formation of the lowest low tides, i.e., low tides are the lowest. These tides are called neap tides.
(vi) Ocean currents are streams of water flowing constantly on the ocean surface in definite directions. Ocean currents are formed because of the temperature differences between the ocean water present at different latitudes. The further one travels from the Equator, the colder the ocean water generally becomes. Due to convection, the warm currents that originate near the Equator move towards the Poles. The cold currents carry ocean water from the Polar or higher latitudes to the tropical or lower latitudes.
Question 2:
Give reasons.
(i) Ocean water is salty.
(ii) The quality of water is deteriorating.
Answer:
(i) The ocean bodies and the seas contain salty water. The water of oceans is salty or saline as it contains large amount of dissolved salts. Most of this salt is sodium chloride.
(ii)Though three-fourth of the Earth’s surface is covered with water, almost 98% of it is unfit for use. Oceans and seas, which constitute the major portion of the hydrosphere, contain saline water. The fresh water reserves of the Earth are mostly made up of ice-caps, ground water, lakes and rivers. The quantity of water on Earth remains the same. It constantly changes form and circulates between oceans, atmosphere and land. Considering that the fresh water reserves remain constant and that the world population rises day-by-day, it is important that we use these water resources judiciously. However, due to factors like deforestation, industrial and agricultural pollution of rivers and lakes, wastage of water, pollution of ground water, etc., the quality of water that is available to many people in this world today is not suitable for human consumption.
Page No 38:
Question 3:
Tick the correct answer.
(i) The process by which water continually changes its form and circulates between oceans, atmosphere and land
| (a) Water cycle | (b) Tides | (c) Ocean currents |
(ii) Generally the warm ocean currents originate near
| (a) Poles | (b) Equator | (c) None of these |
(iii) The rhythmic rise and fall of ocean water twice in a day is called
| (a) Tide | (b) Ocean current | (c) Wave |
Answer:
(i) The process by which water continually changes its form and circulates between oceans, atmosphere and land
(a) Water cycle | (b) Tides | (c) Ocean currents |
(ii) Generally the warm ocean currents originate near
| (a) Poles | (b) Equator | (c) None of these |
(iii) The rhythmic rise and fall of ocean water twice in a day is called
| (a) Tide | (b) Ocean current | (c) Wave |
Question 4:
Match the following.
| (i) Caspian Sea | (a) Largest lake |
| (ii) Tide | (b) Periodic rise and fall of water |
| (iii) Tsunami | (c) Strong seismic waves |
| (iv) Ocean currents | (d) Streams of water moving along definite paths |
–
| (e) Water cycle |
Answer:
| (i) Caspian Sea | (a) Largest lake |
| (ii) Tide | (b) Periodic rise and fall of water |
| (iii) Tsunami | (c) Strong seismic waves |
| (iv) Ocean currents | (d) Streams of water moving along definite paths |
Question 5:
For fun.
Be a Detective
(i) The name of one river is hidden in each of the sentences below. Spot it.
Example: Mandra, Vijayalakshmi and Surinder are my best friends
Answer: Ravi
(a) The snake charmer’s bustee, stables where horses are housed, and the piles of wood, all caught fire accidentally. (Hint: Another name for River Brahmputra)
(b) The conference manager put pad, material for reading and a pencil for each participant. (Hint: A distributary on the Ganga−Brahmputra delta)
(c) Either jealousy or anger cause a person’s fall. (Hint: Name of a juicy fruit!)
(d) Bhavani germinated the seeds in a pot. (Hint: Look for her in West Africa)
(e) “I am a zonal champion now” declared the excited athlete. (Hint: The river that has the biggest basin in the world)
(f) The tiffin box rolled down and all the food fell in dusty potholes. (Hint: Rises in India and journeys through Pakistan)
(g) Malini leaned against the pole when she felt that she was going to faint. (Hint: Her delta in Egypt is famous)
(h) Samantha mesmerised everybody with her magic tricks. (Hint: London is situated on her estuary)
(i) “In this neighbourhood, please don’t yell! Owners of these houses like to have peace,” warned my father when we moved into our new flat. (Hint: colour!)
(j) ‘Write the following words, Marc!’ “On”, “go”, “in”…….. said the teacher to the little boy in KG Class. (Hint: Rhymes with ‘bongo’)
Answer:
(a) The snake charmer’s bustee, stables where horses are housed, and the piles of wood, all caught fire accidentally.
Answer: Teesta
(b) The conference manager put pad, material for reading and a pencil for each participant.
Answer: Padma
(c) Either jealousy or anger cause a person’s fall.
Answer: Orange
(d) Bhavani germinated the seeds in a pot.
Answer: Niger
(e) “I am a zonal champion now” declared the excited athlete.
Answer: Amazon
(f) The tiffin box rolled down and all the food fell in dusty potholes.
Answer: Indus
(g) Malini leaned against the pole when she felt that she was going to faint.
Answer: Nile
(h) Samantha mesmerised everybody with her magic tricks.
Answer: Thames
(i) “In this neighbourhood, please don’t yell! Owners of these houses like to have peace,” warned my father when we moved into our new flat.
Answer: Yellow
(j) ‘Write the following words, Marc!’ “On”, “go”, “in”…….. said the teacher to the little boy in KG Class.
Answer: Congo
