Chapter 6 - The Properties of Sea Water
Q1. Tick the correct box according to the salinity of the ocean water ✓
| High Salinity | Medium Salinity | Low Salinity | |
|---|---|---|---|
(a) Slanting sun rays, melting snow | |||
(b) Cloudy sky, rainfall throughout the year | |||
(c) Clear skies for the most part, perpendicular sunrays | |||
(d) Less supply of freshwater, desert area around | |||
(e) Low temperatures, ample supply of river water | |||
(f) Continental location, desert around, low rainfall |
Ans:
| High Salinity | Medium Salinity | Low Salinity | |
|---|---|---|---|
(a) Slanting sun rays, melting snow | ✓ | ||
(b) Cloudy sky, rainfall throughout the year | ✓ | ||
(c) Clear skies for the most part, perpendicular sunrays | ✓ | ||
(d) Less supply of freshwater, desert area around | ✓ | ||
(e) Low temperatures, ample supply of river water | ✓ | ||
(f) Continental location, desert around, low rainfall | ✓ |
Q2. Give reasons.
(a) Salinity is low in the land-locked Baltic Sea.
Ans:
(i) The Baltic Sea lies in a temperate region.
(ii) In temperate regions, the sunrays are slanting, and therefore, the temperatures are lower.
(iii) The supply of fresh water is also greater, as numerous rivers empty their waters into the Baltic Sea.
(iv) Therefore, in spite of being landlocked, due to the 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:
(i) The Red Sea is one of the saltiest bodies of water in the world, owing to high evaporation.
(ii) The salinity is 36‰ in the southern part because of the effect of the Gulf of Aden.
(iii) It reaches 41 ‰ in the northern part, owing mainly to the Gulf of Suez and the high evaporation as well as very little precipitation.
(iv) Hence, there is more salinity in the northern Red Sea and less in the south.
(c) Oceans located at the same latitude do not have the same salinity.
Ans:
(i) The salinity of the oceans depends on factors like the rate of evaporation and the supply of fresh water.
(ii) In oceans where the rate of evaporation is greater than the supply of fresh water, the salinity is higher.
(iii) In oceans where the supply of freshwater exceeds the rate of evaporation, salinity is low.
(iv) Salinity is not affected much in areas where the supply of freshwater and the evaporation of water are low.
(v) 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 –
(i) While most of the sun’s rays radiate back from the surface of the sea, some of them penetrate to certain depths in the water.
(ii) As a result, the intensity of the sun’s rays decreases with increasing depth.
(iii) The temperature decreases up to a depth of 2000 m.
(iv) After 2000 m, 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:
(i) The Arabian Sea lies to the west and the Bay of Bengal lies to the east of India.
(ii) Many large peninsular rivers drain their waters into the Bay of Bengal, whereas, on the contrary, only small seasonal coastal rivers drain into the Arabian Sea.
(iii) Hence, the salinity of the eastern coast is 34%, while it is 35% in the Arabian Sea.
(iv) Thus, there are more salt pans on the western coast of India than on its eastern coast.
(f) Salinity increases in the mid-latitudinal zones.
Ans:
(i) The mid-latitudinal zone lies between 25° and 35° north and south of the equator.
(ii) In this zone, the rainfall is less and the supply of fresh water from rivers is also lower.
(iii) This region experiences high-temperature conditions, which are marked by the presence of hot deserts around the world. These high-temperature conditions lead to a high rate of evaporation.
(iv) 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 factors that affect the salinity of seawater are mentioned below:
(i) The salinity of water in the surface layer of oceans depends mainly on evaporation and precipitation.
(ii) Surface salinity is greatly influenced in coastal regions by the freshwater flow from rivers and in polar regions by the processes of freezing and thawing ice.
(iii) Wind also influences the salinity of an area by transferring water to other areas.
(iv) The ocean currents contribute to the salinity variations. Salinity, temperature, and density of water are interrelated.
(vi) Hence, any change in the temperature or density influences the salinity of water in an area.
(b) Explain the distribution of salinity around the Tropic of Cancer and tropic of Capricorn.
Ans:
(i) The Tropic of Cancer and the Tropic of Capricorn are located at 23°27′ North and South of the equator, respectively. The water in these zones has a high salinity due to various reasons.
(ii) Due to the high temperatures in these regions, the evaporation of water is also high, leading to higher salinity in the waters.
(iii) Besides, these areas receive low precipitation, which implies that the inflow of fresh water is lower.
(iv) One of the more saline water bodies, the Red Sea, is located in these latitudes.
(c) What are the factors affecting the temperature of the sea water?
Ans:
Temperature is an important factor in determining the salinity of seawater. However, the temperature is also affected by many factors, which are as follows:
(i) Latitudes
The distribution of heat is uneven across the latitudes of the Earth. For example, the temperature of the water in the polar areas is low while that of the tropical zones is high.
(ii) Depth of the Water
The temperature of seawater decreases with increasing depth. The surface water is located to a depth of 500 metres, beyond which the sunlight and other factors do not influence the temperature of the water.
(iii) Other factors
Factors such as Ocean currents, the unequal distribution of land and water, and prevailing winds also influence the temperature of the seawater.
(d) Explain the changes occurring in the temperature of sea water according to the depth.
Ans:
(i) The temperature of the seawater is not the same across the latitudes. While this can be called the horizontal distribution of temperature, the vertical distribution of temperature also takes place based on the depth of the seawater.
(ii) The surface water is located up to a depth of 500 m. This water is influenced by various factors such as sunlight, ocean currents, and winds. Therefore, the temperature is comparatively high.
(iii) However, after a depth of 500 m, the influence of these factors starts to reduce.
(iv) Below 1000 m, there is no change in the temperature of the seawater as the sunlight, winds, and ocean currents cannot affect it.
(v) Therefore, the temperature reduces with the increase in depth of the seawater.
(e) Name the factors affecting salinity.
Ans: Salinity is the measure of the number of salts in a given quantity of water. It depends on various factors such as;
(i) Temperature
The increase in temperature results in an increase in salinity. Due to the high temperatures, the evaporation rate is also high, which leads to greater salinity. This is experienced in the seawater in the tropical zones.
(ii) Precipitation
Due to precipitation, freshwater is added to the seawater, which dilutes it and reduces its salinity. In areas with high precipitation, the salinity of seawater is low.
(iii) The influx of river water
River water is freshwater and contains low to negligible levels of salinity. When these rivers submerge into the oceans and seas, they dilute the saltwater and reduce its salinity.
(iv) Winds
Areas that experience strong trade winds have high salinity due to the higher rate of evaporation, and vice versa.
Q4. Explain how temperature affects the following.
(a) the density of seawater
Ans: The density of water is greatly influenced by temperature.
(i) When water is heated, its particles develop spaces between them, and the volume of the water increases. As a result, the density decreases.
(ii) On the other hand, when the water has a lower temperature (or is cold), the particles of water are comparatively closer and the density increases.
(iii) This explains why the density of the water increases with the increase in depth.
(b) the salinity of seawater
Ans: Temperature is an important factor that affects salinity.
(i) The temperature is distributed unevenly on the earth. In the tropical zones, the temperature is comparatively high, while it is cold in the polar regions.
(ii) When the temperature is high, the evaporation rate is also high. This leads to high levels of salinity.
(iii) On the other hand, low temperatures lead to low salinity due to lower evaporation rates.
(iv) This is the reason why the ocean waters in tropical zones have a higher salinity than those of the polar regions.