Class 9 Geography 6. The Properties of Sea Water:Questions and Answers

Class 9 Geography

6. The Properties of Sea Water

Q1. Tick the correct box according to the salinity of the ocean water 3 

(a) Slanting sunrays, melting snow

Ans: Low 

(b) Cloudy sky, rainfall throughout the year

Ans: Low 

(c) Clear skies for the most part, perpendicular sunrays

Ans: High 

(d) Less supply of freshwater, desert area around

Ans: High 

(e) Low temperatures, ample supply of river water

Ans: Low 

(f) Continental location, desert around, low rainfall

Ans: High 

Q2. Give reasons. 

(a) Salinity is low in the land-locked Baltic Sea.

Ans: The Baltic Sea lies in the temperate region.

In temperate regions, the sunrays are slanting and therefore, the temperatures are lower.

The supply of fresh water is also more, as numerous rivers empty their waters into the Baltic sea.

Therefore, in spite of being landlocked, due to low rate of evaporation and ample supply of fresh water, the salinity of the Baltic Sea is low.

(b) There is higher salinity in the northern 

Red Sea while lower in the southern.

Ans:The Red Sea is one of the saltiest bodies of water in the world, owing to high evaporation.

The salinity is 36%o in the southern part because of the effect of the Gulf of Aden.

It reaches 41 %o in the northern part, owing mainly to the Gulf of Suez and the high evaporation as well as very little precipitation.

Hence, there is more salinity in the northern Red Sea while lesser in the south.

(c) Oceans located at the same latitude 

do not have same salinity.

Ans: The salinity of the oceans depends on factors like rate of evaporation and supply of fresh water.

In Oceans where rate of evaporation is more than the supply of fresh water, the salinity is higher.

In Oceans where supply of freshwater exceeds the rate of evaporation, salinity is low.

Salinity is not affected much in areas where supply of freshwater and the evaporation of water is low.

Thus, oceans located on the same latitude do not have the same salinity.

(d) With increasing depth, the temperature 

of sea water decreases to a certain limit.

Ans:With increasing depth, the temperature of sea water decreases to a certain limit because –

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 the increasing depth.

The temperature decreases up to 2000m depth.

After 2000m, the temperature of the seawater is uniform everywhere.

(e) There are more salt-pans on the 

Western coast of India than its eastern 

coast.

Ans: The Arabian Sea lies lying to the west and Bay of Bengal lies to the east of India.

Many large peninsular rivers drain their waters in the Bay of Bengal and on the contrary, only small seasonal coastal rivers drain in the Arabian Sea.

Hence, the salinity of the eastern coast is 34%, while it is 35% in the Arabian Sea.

Thus, there are more salt-pans on the Western coast of India than its Eastern coast.

(f) Salinity increases in the mid- latitudinal zones.

Ans: Mid-latitudinal zones lie between 25° to 35° north and south of the equator.

In this zone, the rainfall is less and the supply of fresh water from rivers is also low.

This region experiences high-temperature conditions which are marked by the presence of hot deserts of the world. These high-temperature conditions lead to a high rate of evaporation.

Thus, the salinity of the seas is found to be higher in mid-latitudinal zones.

Q3. Answer the following questions. 

(a) What are the factors affecting the salinity of the sea water?

Ans: The uneven distribution of temperature on earth and uneven supply of freshwater affects the salinity of seawater.

In the tropical zone, the temperature is higher. The rate of evaporation is also higher and therefore, the salinity is higher.

Around 5° N and S of the equator, in the equatorial calm belt, the sky is cloudy for a long period of time and convectional rainfall occurs every day.

Large rivers like Congo and Amazon in the equatorial regions meet the sea. Therefore, the supply of freshwater is abundant, too. But because of higher temperatures, rate of evaporation is more and therefore, the seas in these areas are more saline.

In mid-latitudinal zones (25° to 35° N and S), rainfall is lesser and the supply of freshwater from rivers is also low. This zone has the hot deserts of the world. Thus, the salinity of the seas is found to be higher here.

In temperate regions, the sunrays are slanting and therefore, the temperatures are lower. Because of the melting of the snow, the supply of water is also more, and therefore, in this zone, salinity decreases with increasing latitudes.

In the polar areas, temperatures are very low. Evaporation is also very less in polar areas. So, salinity is low.

Landlocked seas have higher salinity than open seas as the rate of evaporation is more. There is a lack of supply of fresh water from large rivers. Thus, there is a difference in the salinities of open and closed seas.

(b) Explain the distribution of salinity 

around the Tropic of Cancer and 

tropic of Capricorn.

Ans:The uneven distribution of temperature on earth, and uneven supply of freshwater affects the salinity of sea water.

Region, between the Tropic of Cancer and Tropic of Capricorn is called as the Tropical zone.

In the tropical zone, temperature is higher.

Hence, the rate of evaporation is also higher, and therefore, salinity is higher.

(c) What are the factors affecting the 

temperature of the sea water?

Ans:Temperature is a major property of the sea water.

Sea water up to the depth of 500m is called surface water. The surface temperature of the sea water is not uniform everywhere. This is dependent on different factors. .

Latitudinally, the surface temperature of the seawater decreases from the equatorial areas towards the poles.

The average temperature in equatorial areas is around 25° C, while it is about 2° C near the poles.

Besides this, cyclones, rainfall, sea waves, ocean currents, salinity, pollution, convectional currents, and seasons also affect the surface temperature.

Regions where cold ocean currents flow, the surface temperature of ocean water is less, while the regions where the warm currents move, the temperature increase.

(d) Explain the changes occurring in the 

temperature of sea water according to 

the depth.

Ans: 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 the increasing depth.

The 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. In low latitudes, because the salinity of the landlocked seas is more, the temperature of the landlocked seas is higher than the open seas.

(e) Name the factors affecting salinity.

Ans:Due to the sun’s heat, evaporation happens at a faster rate. Evaporated water turns into water vapour and reduces in quantity. But the amount of salt remains the same in the remaining water and therefore the salinity of water increases.

In seas where the rate of evaporation is higher than the supply of fresh water, salinity is high.

In seas where the supply of freshwater exceeds the rate of evaporation, salinity is low.

Salinity is not affected much in seas where both the supply of freshwater and evaporation of water is low.

Q4. Explain how temperature affects the 

following. 

(a) the density of sea water

Ans:If temperature reduces, density of water increases.

Hence, cold water is denser than hot water.

(b) the salinity of sea water

Answer:

If the salinity of water is high, the density of water increases.