Chapter: 05. Water
OCCURRENCE OF WATER
A. Tick (✓) the correct options.
of the earth's surface is covered with water.
How much water present is saline? Which of the following is true about water? Dissolved salts can be removed from sea water by a process called What is/are the possible reason(s) for the shortage of drinking water, even when two-thirds of our earth’s surface is covered with water? B. State whether the following statements are true or false. Rewrite the false statements correctly.
6. Water is an element.
* False. Water is a compound.
7. Water is made up of hydrogen and oxygen in a fixed ratio of 1: 2.
* False. Water is made up of hydrogen and oxygen in a fixed ratio of 2: 1.
8. Surface water is the purest form of water on earth.
* False. Rainwater is the purest form of water on earth.
9. Sea water has a high concentration of common salt.
* True.
10. Underground water is the largest source of water on earth.
* False. Oceans and seas are the largest source of water on earth.
11. Groundwater is obtained by digging deep wells, tube wells and hand pumps.
* True.
STATES OF WATER
A. Fill in the blanks.
is a natural process showing the interconversion of the states of water.
acts as a medium for transporting minerals and other nutrients from one part of the plant to other.
Water regulates the temperature of our body by . Water is used in hydroelectric power plants to produce . B. Tick (✓) the correct options.
7. Which of the following is an incorrect pair?
* (b) liquid-water vapour
8. The tiny droplets of water formed after the condensation of water vapour on cold objects is called
* (b) dew
9. When water is heated to 100° C, it changes into and when cooled to 0° C, it changes into .
* (d) steam, ice
10. A boy digging the ground near a water body finds that the soil is moist. As he keeps digging deeper and deeper, he reaches a level where all the spaces between particles of soil and gaps between rocks are filled with water. The upper limit of this layer is called
* (b) water table
POTABLE WATER
A. Tick (✓) the correct options.
Acute diarrhoea is a symptom of Which of the following methods is the easiest way to purify water on a small scale? Over the past decade, the population of a small village has tripled. As a result, a larger number of people are now sharing the same resources. Which one of the following would be a direct consequence on the natural resources of such a population boom, in the village? (d) increase in consumption of drinking water Which of the following is not a characteristic of potable water? (d) It lacks essential minerals required by the body. Which of the following is not a water-borne disease? B. Assertion-Reason based Question.
Assertion: Bleaching powder is used in the purification of water.
Reason: Bleaching powder releases chlorine, that can kill germs.
* (a) Both A and R are true and R is the correct explanation of the assertion.
WATER—A UNIVERSAL SOLVENT
A. Fill in the blanks.
Salt in water is a solvent. A solution in which no more solute can be dissolved at a given temperature is called a/an solution. The maximum amount of a solute that can be dissolved in 100 grams of a solvent to form a saturated solution at a given temperature is called its . B. Tick (✓) the correct options.
4. A substance that is dissolved in another substance to form a solution is called a .
* (a) solute
5. A substance in which a solute is dissolved to form a solution is called a .
* (b) solvent
6. A homogeneous mixture of a solute and solvent is called a/an .
* © solution
7. Which of the following methods does not help in the dissolving of a substance faster in water?
* © freezing
8. The solubility of a substance in water generally decreases with a/an in temperature.
* (a) decrease
WATER POLLUTION
A. Fill in the blanks.
The contamination of water with unwanted substances that makes it unfit for drinking is called . Excess rain may cause and no rainfall may cause . B. Tick (✓) the correct options.
3. Which of the following is not a water pollutant?
* (d) oxygen
4. The major cause of water pollution in seas is
* © oil spills
5. Which of the following ways cannot help prevent/control water pollution?
* © Use of non-biodegradable detergents
C. Assertion-Reason based Question.
Assertion: It is good to drink boiled and cooled water in rainy season.
Reason: Water contains dissolved minerals in it.
* (b) Both A and R are true but R is not the correct explanation of the assertion.
Complete the flowchart for Revision
Dissolves many substances to form SOLUTION Solution comprises: Solute, Solvent EXERCISE
Short Answer Questions:
What is the importance of water for humans? Water is essential for fulfilling bodily needs, dissolving nutrients for absorption, dissolving waste for removal (sweat and urine), and regulating body temperature through sweating. What is potable water? List the characteristics of potable water. Potable water is water that is fit and safe for drinking. It should be colourless, transparent, and odourless; free from germs; free from suspended impurities; and contain a small amount of essential minerals and salts. What are water-borne diseases? Name any four. Water-borne diseases are diseases caused by disease-causing microorganisms present in unclean water. Four examples are: Cholera, Diarrhoea, Gastroenteritis, Hepatitis. What is meant by solubility of a substance? How is it affected by stirring and heating? Solubility is the maximum amount of a solute that can be dissolved in 100 grams of a solvent to form the saturated solution at a given temperature. Both stirring and heating help in faster dissolving of a substance. Differentiate between saturated solutions and unsaturated solutions. Give examples. Saturated Solution: A solution in which no more solute can be dissolved at a given temperature. (Example: A solution where excess sugar has settled at the bottom because no more can dissolve at that temperature). Unsaturated Solution: A solution in which more solute can be dissolved in a solvent at a particular temperature. (Example: A solution where a small amount of sugar has dissolved completely, and more could still be added). Long Answer Questions:
Describe the steps of large scale purification of water. Large scale purification of water involves: (1) Sedimentation tank where large impurities settle, (2) Loading tank where alum is added to settle suspended impurities faster, (3) Filtration tank where water passes through layers of sand, gravel, and charcoal to filter out impurities, (4) Chlorination tank where bleaching powder is added to kill germs, and (5) Storage tank from which purified water is supplied to homes. How will you recover a solute and a solvent from a solution? Describe the process with the help of a diagram. A solute and a solvent can be recovered from a solution by distillation. In this process, the solution is heated in a distillation flask, causing the solvent (e.g., water) to evaporate. The vapour then passes into a condenser, where it is cooled and condensed back into liquid solvent, which is collected in a separate flask. The solute (e.g., salt) remains as a residue in the distillation flask. (Refer to Fig. 5.11 for a diagram). How can we conserve water? Water can be conserved by: closing leaking taps, preventing overhead tank overflow, reusing water (e.g., vegetable washing water for plants), collecting rainwater (including rooftop harvesting), planting more trees, using drip and sprinkle irrigation, using treated wastewater for industries/agriculture, and constructing reservoirs and dams. What happens when it rains heavily or does not rain? What are its effects? Heavy rains cause floods, leading to rising water levels in water bodies, soil erosion, submerging of low-lying areas, and extensive damage to crops, animals, and human life. No rainfall for a long duration causes drought, resulting in animal deaths due to water shortage, damage to crops and plants, decreased water levels in water bodies, and lowered soil quality. 
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