The Properties of Sea WaterClass 9 Geography Chapter 6 Notes – Maharashtra Board

Summary: The Properties of Sea Water

The surface temperature of the sea falls from the equator (about 25 °C or more) to the mid-latitudes (about 16 °C) and the poles (about 2 °C). Ocean currents, cyclones, rain, waves, salinity, pollution and seasons also affect it; cold currents lower it and warm currents raise it.

Temperature falls with depth because sunlight reaches only the upper layers. Below about 2,000 m the water is roughly 4 °C everywhere, so the deep sea does not freeze. The drop with depth is sharp in the tropics and small near the poles.

Salinity is the weight of dissolved salts in 1,000 g of sea water, written in parts per thousand (‰). The average for the oceans is about 35‰. It rises where evaporation is greater than the supply of fresh water and falls where rivers, rain and melting snow add more fresh water.

Salinity is highest around the tropics (about 25°–35° N and S, near hot deserts), slightly lower at the rainy equator, and low in temperate and polar seas. Enclosed seas in hot, dry areas (Mediterranean 39‰, Red Sea 41‰, Dead Sea about 330‰) are very salty; the Baltic Sea (about 7‰) is very fresh.

Density depends on temperature and salinity: cold or salty water is denser, and temperature matters more. Temperature, salinity and density change in the top 500 m or so and stay almost constant below about 1,000 m. Differences in these properties create ocean currents, which spread heat around the globe.

Key points to remember

• Surface temperature: equator ~25 °C, mid-latitudes ~16 °C, poles ~2 °C
• Cold currents lower and warm currents raise surface temperature
• Temperature falls with depth to about 2,000 m; below that it is about 4 °C everywhere
• The deep sea does not freeze
• Salinity = grams of salt per 1,000 g of sea water, in ‰; ocean average about 35‰
• High evaporation + little fresh water → high salinity
• Much fresh water (rivers, rain, melting snow) → low salinity
• Highest salinity near the tropics; low in polar seas
• Arabian Sea (~35‰) is saltier than the Bay of Bengal (~34‰)
• Dead Sea: extremely salty, people float, its shore is over 400 m below sea level
• Cold and salty water is denser; temperature affects density more than salinity
• Properties change little below about 1,000 m; differences drive ocean currents

The Properties of Sea Water – Notes

1. Temperature of sea water

• Water heats and cools more slowly than land, so the sea's temperature changes less than the land's.
• With latitude: surface water is warmest near the equator (average about 25 °C; the graph shows the tropics near 20 °C or more), about 16 °C in mid-latitudes and about 2 °C near the poles, because the sun's rays become more slanting towards the poles.
• Other factors: ocean currents, cyclones, rainfall, waves, salinity, pollution, convection currents and seasons. Where a cold current flows, the surface water is cooler; where a warm current flows, it is warmer.
• With depth: most sunlight is reflected from the surface and only some penetrates a short way, so water gets colder as depth increases. The fall continues to about 2,000 m; below that the temperature is about 4 °C everywhere, from the equator to the poles.
• Because deep water stays around 4 °C (and its salt and pressure lower the freezing point), the deep ocean never freezes, and sea life survives below ice.
• In the tropics the temperature drops sharply with depth (warm surface, cold depths); near the poles the surface is already cold, so the change with depth is small.
• The book adds that, in low latitudes, landlocked seas are warmer than open seas.

2. Salinity and how it is measured

• Salt is used in food, chemicals and medicines, for preserving things and in ice factories (salt makes ice colder, which is why it was used in the 'make rain' activity). We obtain it from salt pans.
• Salt makes sea water more buoyant, which helps ships float — useful for water transport. But if salinity becomes too high, most life dies.
• Salinity tells us how many grams of salt are dissolved in every 1,000 g of sea water, written in parts per thousand (‰). If 1 kg of sea water contains 40 g of salt, its salinity is 40‰.
• It can also be measured with a hydrometer, refractometer or salinometer.
• The three-container activity shows the rule: water left in the sun evaporates, so the same salt stays in less water and tastes saltier; water topped up with fresh water tastes less salty.
• Rules: evaporation greater than fresh-water supply → high salinity; fresh-water supply greater than evaporation → low salinity; both low → salinity changes little.
• The salts come mainly from rocks on land, dissolved by rivers and carried to the sea over millions of years, and from undersea volcanic activity. The book says the ocean's salts would weigh 120 million tonnes and make a 150 m layer on land; the 150 m layer is right, but the real total is far larger — about 50 million billion tonnes (5 × 10¹⁶ t).

3. Distribution of salinity

• Equator (about 5° N to 5° S): high temperature means strong evaporation, but cloudy skies, daily convectional rain and huge rivers (Amazon, Congo) add a lot of fresh water. So salinity is moderate — about 34–35‰, a little below the tropics. (The book's sentence here ends by calling these seas 'more saline'; the map in Fig. 6.9 shows them lower than the subtropics.)
• Tropics (about 25°–35° N and S): clear skies, high temperature, little rain and few rivers — this is the belt of hot deserts. Evaporation is high, so salinity is the highest in the open ocean (36–37‰; above 37‰ in the North Atlantic).
• Temperate zones: slanting rays and lower temperatures reduce evaporation, and melting snow adds fresh water, so salinity decreases as latitude increases.
• Polar regions: very low temperature and evaporation, plus melting ice — salinity is low (about 32–33‰ or less).
• Enclosed (landlocked) seas in hot, dry regions have higher salinity than open seas — Mediterranean about 39‰, while the Atlantic, the saltiest ocean, averages about 35‰. Enclosed seas fed by many rivers in cold regions (Baltic) are very fresh.
• India: the Arabian Sea (about 35‰) is saltier than the Bay of Bengal (about 34‰) because the Bay receives huge amounts of fresh water from the Ganga, Brahmaputra, Mahanadi, Godavari, Krishna and Kaveri, and more rain.
• Dead Sea (between Israel and Jordan): only the Jordan river feeds it, rain is low and evaporation very high. Its salinity is about 330‰ (the book gives 332‰), nearly ten times the ocean. Fish carried in by the Jordan die; salt pillars form; the dense water lets people float easily. Its shore is one of the lowest places on land, over 400 m below sea level.

4. Density of sea water

• Density depends on temperature and salinity.
• Lower temperature → higher density. Higher salinity → higher density.
• Temperature has the bigger effect. So cold water can be denser than warmer water even if it is a little less salty, and warm, low-salinity water is the least dense.
• Fig. 6.10: with depth, temperature decreases, while density increases. Salinity changes in the upper layers and then stays almost the same. The changes are greatest in the top 500 m and become almost zero below about 1,000 m.
• The top ~500 m is called surface water. It is affected by sunlight, winds and currents. In the deep ocean these do not reach, so the properties stay steady.

5. Ocean currents and climate

• Differences in temperature, salinity and density between water masses make sea water move — dense water sinks and lighter water flows in to replace it.
• These movements, along with winds, form ocean currents.
• Currents carry warm water towards the poles and cold water towards the equator, so they act as the Earth's temperature controllers.
• By spreading heat, ocean currents influence the climate of coastal lands across the world.
• Salinity and temperature therefore matter not only for fishing and salt-making but for weather and climate everywhere.

Salinity of some seas

Key terms

The Properties of Sea Water – Flashcards

Read the question, answer it in your head, then tap the card to check. 20 cards.

The Properties of Sea Water – MCQ Quiz

15 questions. Tap an option — you will see at once if it is right and why.

How to study The Properties of Sea Water

⏱ About 40 minutes

1. Temperature (12 min)
   Learn how sea temperature changes with latitude and depth; read Fig. 6.1.
2. Salinity and density (15 min)
   Learn the evaporation vs fresh-water rule, the belt-wise pattern and the special seas (Red, Baltic, Mediterranean, Dead Sea); then how temperature and salinity control density.
3. Check yourself (13 min)
   Answer the 'Try it yourself' questions, take the quiz and play 🎮 Play.