Maharashtra Board Class 9 Geography Chapter 6 The properties of Sea Water PDF Download

The Properties Of Sea Water

Which is the largest water storage of the world?

Why is the seawater salty?

In the earlier classes, we have studied the tides, ocean currents with reference to oceans. In this lesson, we will study the major properties of seawater.

Temperature

What is the difference in the temperatures of the land and the sea?

What would be the difference in the temperature of the seawater from equatorial region to the polar areas?

The changes in the temperature of sea water with reference to depths in different latitudes is shown in fig 6.1.

Observe this graph and answer the following questions:

What is the maximum temperature of seawater in equatorial areas? How much is this temperature at a depth of 500 m?

What is the temperature of seawater at the sea level in the mid-latitudes?

How much has this temperature changed at 1500m depth?

What does the thermal graph for the high latitudes say? What is its temperature at 500, 1000 and 1500m depths?

After what depth does the seawater temperature remain stable everywhere?

Temperature is a major property of the seawater. The surface temperature of the seawater is not uniform everywhere. This is dependent on different factors.

Latitudinally speaking, the surface temperature of the seawater decreases from the equatorial areas towards the poles. The average temperature in equatorial areas is around 25°C, it is 16°C in mid-latitudes while it is about 2°C near the poles.

Besides this, cyclones, rainfall, sea waves, ocean currents, salinity, pollution, convectional currents, seasons, etc. also affect the surface temperature. Ocean currents also have similar effects on temperature of the sea water. In regions where cold ocean currents flow, the surface temperature of ocean water is less while the regions where the warm currents move, the temperatures increase.

Teacher's Note

Sea temperature changes from the equator to the poles, just like the air temperature. Near the equator it is very hot, but near the poles it is very cold.

Exam Trick

Remember: Equator = hottest ocean water. Poles = coldest ocean water. As you go deeper in the sea, temperature always decreases.

Points to Remember

Ocean water temperature is higher near the equator and lower near the poles.
Temperature decreases as we go deeper in the sea.
After 2000 meters depth, temperature stays the same everywhere at about 4°C.
Cold and warm ocean currents change the temperature of sea water.
Ocean water at greater depths does not freeze because it stays at 4°C.

Salinity

Why is the taste of seawater salty?

What are the reasons for high salinity in the oceans?

What is the use of the salts in the ocean to us?

How can we obtain the salts from the ocean water?

Salt is used in the food we eat. Salt is also used for making various chemicals and medicines. Salt is also used to preserve things for longer periods. Salt is also used in ice factories. We obtain salt from the salt-pans.

The buoyancy of the seawater increases because of salinity. This is useful for water transport.

But if the salinity is more than bearable limits, the life in the water gets destroyed.

While most of the sunrays radiate back from the surface of the sea, some of them penetrate to certain depths in the water. As a result, the intensity of sunrays decreases with increasing depth. Temperature decreases up to 2000m depth. After 2000m, the temperature of the seawater is uniform everywhere. It is around 4°C everywhere from the equatorial regions to the polar areas. Temperature reduces only up to 4°C according to depth. And therefore, the water at greater depths does not freeze.

The temperature of the seawater changes rapidly with depth at the equatorial areas. The difference in temperature is lesser in polar areas.

There is also a difference in open seas and landlocked seas. As the salinity of the landlocked seas is more, the temperature of these landlocked seas is higher than the open seas. This is true for low latitudinal areas.

What would have happened if the temperature of the seawater near the seabed would have gone below the freezing point?

Take 1.5 litres of water in a big container. Put 100gm salt in it and stir.

Now take 3 containers of same size. To identify them put three dots of different colours on them.

Put the water containing salt equally in the three containers. Make sure the containers are half-empty after filling the water.

Taste the water out of the three containers, keep one in the sun outside. Cover it with meshed lid.

Keep the remaining two in the classroom. Cover them with net lids too.

Put half a glass of freshwater daily in one of the containers in the classroom.

After 3-4 days, bring all the containers together at one place. Observe the level of water in all the three containers.

Taste the water in all the three containers. Experience the difference in the taste. Write about all the three tastes in one line each.

Answer the following questions:

In which container has the water increased or decreased?

What could be the reasons behind the decrease or increase?

What could be the reason behind the low and high salinity of the water in the container?

You must have realised that because of the sun's heat, evaporation happens at a faster rate. Evaporated water turns into water vapour and reduces in quantity. But amount of salt remains the same in the remaining water and therefore the salinity of the water increases.

Teacher's Note

Salinity means how much salt is in the sea water. When water from a puddle evaporates in the sun, salt is left behind, making it salty.

Exam Trick

Remember: More evaporation = More salt left = Higher salinity. It's like when you leave water in a glass under the sun, it becomes salty as water leaves but salt stays.

Points to Remember

Salinity is the amount of salt dissolved in sea water.
Salinity is measured in parts per thousand (‰).
When water evaporates, salt remains, so salinity increases.
The ocean has salinity of about 35‰ on average.
Salinity is higher where there is less rain and more heat.

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