Water

This is Chapter 5 of Our Environment (Class 7 Geography): 9 pages (30–38). It covers the water cycle (Fig. 5.1), a world map of seas, lakes and rivers (Fig. 5.2), the distribution of water with a table and a spoons activity, salinity, water conservation (a newspaper collage and World Water Day), and ocean circulation — waves (Fig. 5.4) with a two-page box on the 2004 tsunami, tides (Fig. 5.5) and ocean currents (Fig. 5.6). It has three activities (terrarium, spoons, bucket), four ‘Do you know?’ boxes beside the text plus two on the tsunami page, and exercises ending in a hidden-rivers puzzle. A short chapter, so this page has 7 concept sections.

• Before this: gess204 (Air) explained humidity, condensation, clouds and rainfall — the air half of the water cycle; gess203 (Our Changing Earth) explained earthquakes, which cause tsunamis, and the work of sea waves on coasts. Next: gess206 (Natural Vegetation and Wildlife).
• Goes further in Class 8 and 11: hess402 (Land, Soil, Water… Resources, Class 8 — water as a resource, its scarcity and conservation); kegy212 (Water (Oceans) — the hydrological cycle, ocean relief, temperature and salinity); kegy213 (Movements of Ocean Water — waves, tides, currents and their effects); kegy210 (Water in the Atmosphere); kegy203 (the 2004 tsunami as an earthquake hazard).
• UPSC, honestly: GS1 physical geography and GS3 disaster management. Prelims asked these basics directly — freshwater as 2.7% of all water (1987) and its split (2010, 2013), the water cycle (1992), neap tides (1991, 2001), the causes and uses of tides (2000, 2015), the Labrador as a cold current (1989), fishing grounds and fog where currents meet (2013, 1988), sea water's density (1990) — and Mains asked about tsunamis (2025) and ocean currents (2015). We link 14 (12 Prelims, 2 Mains).

• Water cycle: evaporation → condensation (clouds) → precipitation (rain, snow, sleet) → run-off; the same water for ever.
• Oceans 97.3% (saline); ice-caps 2.0%, ground water 0.68%; lakes and rivers a sliver. Fresh water ≈ 2.7%.
• Salinity = grams of salt per 1,000 g of water; oceans average 35‰; Dead Sea about 340 g per litre.
• Waves: wind on the surface; tsunami (‘harbour wave’): earthquake, volcano or undersea landslide — 26 Dec 2004.
• Tides: twice a day; pull of moon and sun. Spring (in a line: new and full moon) vs neap (right angles: quarters).
• Currents: warm from the equator polewards (Gulf Stream), cold the other way (Labrador); meeting zones = fog + best fishing.

The sun's heat causes evaporation of water into vapour. When the vapour cools, it condenses and forms clouds. From there it may fall on land or sea as rain, snow or sleet. The process by which water continually changes its form and circulates between oceans, atmosphere and land is the water cycle (Fig. 5.1). The earth is like a terrarium: the same water that existed centuries ago still exists today — water irrigating a field in Haryana may have flowed down the Amazon a hundred years ago. The major sources of fresh water are rivers, ponds, springs and glaciers; oceans and seas hold salty water.

Ocean water is salty (saline) because it contains large amounts of dissolved salts — mostly sodium chloride, the common table salt we eat. Salinity is the amount of salt in grams in 1,000 grams of water; the average salinity of the oceans is 35 parts per thousand. The Dead Sea has a salinity of 340 grams per litre, so dense that swimmers float. About three-fourths of the earth's surface is water, yet many countries face scarcity, because most of it is not available to us. The book's table: oceans 97.3%; ice-caps 2.0%; ground water 0.68%; fresh water lakes 0.009%; inland seas and salt lakes 0.009%; atmosphere 0.0019%; rivers 0.0001%.

Water is absolutely essential for survival; only water quenches thirst. The book asks: are we not wasting a precious resource when we use water carelessly? A collage of newspaper cuttings shows the worry — rivers drying up, polluted drinking water, dying rivers, and catching rainwater where it falls — and asks why water is important and how we can conserve it at home and at school. A ‘Do you know?’ box notes that 22 March is World Water Day, when the need to conserve water is stressed.

Unlike the calm water of ponds and lakes, ocean water keeps moving continuously. Its movements are waves, tides and currents. When the water on the surface rises and falls alternately, we call it waves — a ball dropped in the sea at the beach gets washed back to the shore by them. A ‘Do you know?’ box adds: waves form when winds scrape across the ocean surface; the stronger the wind, the bigger the wave. In a storm, very fast winds build huge, destructive waves. An earthquake, a volcanic eruption or an underwater landslide can shift large amounts of water and form a tsunami, which may be as high as 15 m.

A box tells the story. On 26 December 2004 a tsunami struck the Indian Ocean. It came from an earthquake with its epicentre close to the western boundary of Sumatra, of magnitude 9.0 on the Richter scale (the book's figure), as the Indian plate went under the Burma plate. The ocean floor was displaced by about 10–20 m and tilted downwards; water rushed in to fill the gap, so the sea first withdrew from the coasts of South and South-east Asia, then rushed back. The tsunami travelled at about 800 km per hour — like a commercial aircraft — washing away some islands. Indira Point in the Andaman and Nicobar Islands, India's southernmost point, was submerged. Near the Andamans and Sri Lanka the wavelength shortened as the water shallowed, and the speed fell from 700–900 km/h to under 70 km/h. The waves reached up to 3 km inland, killing more than 10,000 people and damaging over a lakh of houses; in India the worst hit were the coasts of Andhra Pradesh, Tamil Nadu, Kerala, Puducherry and the Andaman and Nicobar Islands. Earthquakes cannot be predicted, but a three-hour notice of a tsunami is possible; the book says such warning systems existed in the Pacific but not in the Indian Ocean, where tsunamis are rarer. The damage came from the lack of monitoring, early warning and public knowledge: when the sea withdrew, people gathered on the shore to watch instead of running to high ground, and many were killed.

The rhythmic rise and fall of ocean water twice a day is a tide. It is high tide when water rises to its highest level and covers much of the shore, and low tide when it falls to its lowest and recedes. Tides are caused by the strong gravitational pull of the sun and the moon: the water on the side of the earth nearer the moon is pulled towards it, causing high tide. On full-moon and new-moon days, the sun, the moon and the earth are in a line and the tides are highest — spring tides. When the moon is in its first and last quarter, the sun and the moon pull in different directions and the tides are neap tides (Fig. 5.5). High tides help navigation — they raise the water near the shore so ships can reach the harbour more easily — and fishing, as more fish come close to the shore. The rise and fall of tides is used to generate electricity in some places.

Ocean currents are streams of water flowing constantly on the ocean surface in definite directions. They may be warm or cold (Fig. 5.6). Generally, warm currents originate near the equator and move towards the poles; cold currents carry water from polar or higher latitudes to tropical or lower latitudes. The Labrador current is cold; the Gulf Stream is warm. Currents influence the temperature of the areas they pass — warm currents bring warmth to the land. Where warm and cold currents meet are the best fishing grounds of the world, such as the seas around Japan and the eastern coast of North America; these areas also have foggy weather, which makes navigation difficult. An activity shows how a current forms: heat one side of a bucket of water, cool the other with ice, and a drop of red ink traces the flow — convection.

The questions asked in the text, the collage's questions and the three activities.

Six short answers, two reasons, three ticks, a match and the hidden-rivers puzzle. Try first, then open.

Numbers

One-liners

• The water cycle moves the same water between oceans, atmosphere and land.
• Nearly all the earth's water is salty; most fresh water is frozen.
• Salinity is grams of salt per 1,000 grams of water; the ocean average is 35.
• Waves form when wind scrapes the sea surface.
• Tsunamis are caused by earthquakes, volcanic eruptions or undersea landslides.
• Tides are caused by the pull of the moon and the sun.
• Spring tides come when sun, moon and earth are in line; neap tides when sun and moon are at right angles.
• High tides help navigation and fishing and can make electricity.
• Warm currents flow from the equator towards the poles; cold currents flow the other way.
• Where warm and cold currents meet there is fog and rich fishing.

Traps to avoid

• Fresh water is about 2.7% of all water, and ice holds about three-quarters of it, not 95%.
• After ice, the largest store of fresh water is ground water, not lakes or rivers.
• Inland seas and salt lakes are salty, though the book's brace lists them as fresh.
• Neap tides have the smallest range — high tides lower, low tides higher — not simply 'low tides'.
• Neap tides come at the first and last quarter, not at new moon.
• Spring tides have nothing to do with the season; they come at new and full moon.
• A tsunami is not a 'tidal wave'; tides play no part in it.
• The Labrador and Oyashio currents are cold; the Gulf Stream and Kuroshio are warm.
• The best fishing grounds are where warm and cold ocean currents meet, not atmospheric ones.
• The Teesta is a tributary of the Brahmaputra, not another name for it.

Prelims — what UPSC asks

• From this chapter: freshwater as about 2.7% of all water (1987), its split between ice and ground water (2010, 2013); the water cycle (1992); denser sea water (1990); neap tides at right angles (1991) and their small range (2001); the causes of tides (2015) and their uses (2000); the Labrador as a cold current (1989); fishing grounds (2013) and Newfoundland fog (1988) where warm and cold currents meet.
• Elsewhere: high tides at antipodes (1986), the factors behind currents (2002, 2012) and current–coast matches (1985, 1999) on kegy213; mangroves and the tsunami (2011) on kegy206; ocean salinity (Mains 2017) on kegy212.
• Method: learn the book's table in round shape (97 salt / under 3 fresh, mostly ice, then ground water), the spring–neap geometry, and a short list of currents by warm/cold. Most questions are one-step definitions or classifications.

Mains — where it appears and how to answer

• GS1 2025: ‘What are tsunamis? How and where are they formed? What are their consequences?’ — this chapter's 2004 box gives the cause (plate thrust, sea-floor shift), the behaviour (fast in deep water, tall at the coast, the sea withdrawing first) and the toll; add Pacific ‘Ring of Fire’ examples and India's warning centre †. GS1 2015: ocean currents and their influence on climate, fishing and navigation — the book supplies warm-coast temperatures, fishing grounds and fog; add the forces (wind, density, rotation) from kegy213.
• For GS3 (water management, disasters), use the book's distribution table to show scarcity amid plenty, rainwater harvesting as a solution, and the 2004 tsunami as a case for early warning and community awareness.

Tap an option. NCERT's own exercise questions come first; the rest test the figures and common confusions.

Open any question to attempt it in the Question bank — your attempt is saved to your progress.