Posts
VIII SCIENCE WATER EVALUATION QUESTION ANSWERS
WATER
TEXTBOOK EXERCISES
I. Choose the correct answer.
1. Water changes to ice at __________________
a) 00C
b) 1000C
c) 1020C
d) 980C
Ans: a) 00C
2. Solubility of carbon dioxide in water is high when the ____________
a) pressure is low
b) pressure is high
c) temperature is high
d) None of the above
Ans: b) pressure is high
3. The gas collected at the cathode on electrolysis of water is
a) oxygen
b) hydrogen
c) nitrogen
d) carbon dioxide
Ans: b) hydrogen
4. Which of the following is a water pollutant?
a) Lead
b) Alum
c) Oxygen
d) Chlorine
Ans: a) Lead
5. Permanent hardness of water is due to the presence of __________
a) sulphates and chlorides
b) dust particles
c) carbonates and bicarbonates
d) other soluble particles
Ans: a) sulphates and chlorides
II. Fill in the blanks.
1. Water is colourless, odourless and __________.
Ans: tasteless
2. The boiling point of water is __________
Ans: 100°C
3. Temporary hardness of water can be removed by __________ of water.
Ans: boiling
4. The density of water is maximum at __________.
Ans: 4°C
5. Loading speeds up the process of __________.
Ans: sedimentaion
III. State true or false. If false, correct the statement.
1. Sewage should be treated well before being discharged into water bodies.
Ans: True
2. Sea water is suitable for irrigation as it contains dissolved salts.
Ans: False.
Sea water is not suitable for irrigation as it contains dissolved salts.
3. Excessive use of chemical fertilizers depletes the soil and causes water pollution.
Ans: True
4. The density of water will not change at all temperature?
Ans: False.
Density of water varies at different temperatures.
5. Soap lathers well in hard water.
Ans: False.
Soap lathers well in soft water.
IV. Match the following.
Ans:
V. Give reasons for the following.
1. Alum is added to water in sedimentation tanks.
Ans: Water from lakes or rivers is collected in large sedimentation tanks.To speed up the process of sedimentation,potash alum is added to water.This process is called loading.
2. Water is a universal solvent.
Ans: Water has a unique property to dissolve more substances than any other solvents. It can dissolve solids, liquids and gases. Therefore, it is called as universal solvent.
3. Ice floats on water.
Ans: Because ice is lighter than water. The density of ice is lower than that of water, so ice floats on water.
4. Aquatic animals can breathe in water.
Ans: Ice is a bad conductor of heat; it does not allow heat to pass through it. So, the water below the ice remains in liquid form, this enables the aquatic animals and plants to survive even in extreme cold conditions.
5. Sea water is unfit for drinking.
Ans: Every litre of sea water contains 35 grams of sodium chloride (NaCl). Such water is called saline water. It is not suitable for drinking and is said to be non potable water.
6. Hard water is not good for washing utensils.
Ans: It damages the utensils and containers and it will leave a white scale on the utensils over a period of time, which may lead to health issues.
VI. Define the following.
1. Freezing point
The temperature at which a material changes from a liquid state to a solid state is called as freezing point.
2. Boiling point
The temperature at which a material changes from a liquid state to a gaseous state is called as boiling point.
3. Specific heat capacity
The amount of heat that is needed to raise the temperature of a unit mass of a substance by 1°C is called the specific heat capacity of that substance.
4. Latent heat of fusion
The amount of heat energy required by ice to change into water is called latent heat of fusion of ice. Ice has the highest latent heat of fusion, i.e., 80 calories/g. or 336 J/g.
5. Potable water
The water suitable for drinking is called potable water. Every litre of potable water contains 1- 2 grams of dissolved salts. Potable water also contains dissolved gases.
VII. Answer in brief.
1. Name the gas evolved at cathode and anode when water is electrolysed. State their ratio by volume.
Electrolysis of water experiment confirms that water is made up of hydrogen and oxygen. The ratio of hydrogen and oxygen is 2:1. Hence, for every two volumes of hydrogen collected at the cathode, there is one volume of oxygen collected at the anode.
2. State the importance of dissolved oxygen and carbon dioxide in water.
•Air dissolved in water is important for the living organisms to survive.
• Fish extracts the oxygen from the water and expels water through the gills. Fish can survive in water only through the dissolved oxygen present in water.
• Aquatic plants make use of dissolved carbon dioxide for photosynthesis
• Carbon dioxide dissolved in water reacts with limestone to form calcium bicarbonate. Marine organisms such as snails, oysters, etc., extract calcium carbonate from calcium bicarbonate to build their shells.
3. What are the causes of temporary hardness and permanent hardness of water?
Hardness may be temporary or permanent.
• Temporary hardness is due to the presence of carbonate and bicarbonate salts of calcium and magnesium.
•Permanent hardness results due to the presence of chloride and sulphate salts of calcium and magnesium.
4. Explain specific latent heat of vaporization of water.
•When water attains the temperature of 100°C, it starts changing its state from liquid to gaseous state.
•However, the temperature of water does not rise above 100°C.
•It is because the supplied heat energy only changes the state of the boiling water. •This heat energy is stored in steam and is commonly called latent heat of vaporization of steam.
•The steam has the highest latent heat of vaporization and its value is 540 calories/g or 2268 J/g.
5. What are the methods of removing hardness of water?
•Hardness of water is due to the presence of dissolved salts of calcium and magnesium.
•Different methods are followed to remove the hardness from water depending on whether it is temporary hardness or permanent hardness.
•Boiling
•Adding washing soda
•Ion – exchange
•Distillation
VIII. Answer in detail.
1. How is water purified at a water purification plant?
Out of the total fresh water available on the earth, only 1% is present in water bodies such as rivers and lakes and the rest is frozen in glaciers and polar-regions.
•Water from these water bodies is unfit for drinking, cooking, washing or bathing because it contains suspended and dissolved impurities and microorganisms such as bacteria
• If this water is consumed without purifying, it may cause water-borne diseases such as typhoid and cholera.
Therefore, water should be treated and purified before it reaches our home.
In conventional water treatment plants water is subjected to different processes for purification.
The processes are:
Sedimentation:
Water from lakes or rivers is collected in large sedimentation tanks. There, it is allowed to stand undisturbed so that suspended impurities settle down at the bottom of the tank. Sometimes, a chemical substance such as potash alum is added to water, to speed up the process of sedimentation. This process is called loading. The particles of potash alum combine with the suspended impurities and make them settle down at a faster rate.
Filtration:
Water from the sedimentation tanks is then pumped to the filtration tanks. Filtration tanks contain filter beds made up of gravel, sand, pebbles, activated charcoal and concrete. Water passes through these layers and becomes free from any remaining dissolved or suspended impurities completely.
Sterilisation:
The filtered water is treated chemically to remove the remaining germs or bacteria. This process is called sterilisation. The chemicals that are used in this process are chlorine and ozone.
Chlorination:
The water from filtration tanks is pumped into chlorination tanks, where chlorine is added to remove harmful bacteria and other germs. The process of adding chlorine, in adequate amounts, to water is called chlorination.
Ozonisation:
Ozonisation is a process in which water is treated with ozone gas to kill the germs present in it. The sterilisation of water can also be done by exposing it to air and sunlight. Oxygen from the air and sunlight destroy the germs present in water.
Aeration:
Aeration is the process in which air under pressure is blown into filtered water. This also helps to kill the germs.
2. What is permanent hardness of water? How can it be removed?
Different methods are followed to remove the permanent hardness of water.
Adding washing soda:
Washing soda is used to remove permanent hardness of water. Adding washing soda converts chlorides and sulphates into insoluble carbonates. These insoluble carbonates are removed by filtration.
Ion – exchange:
Another method used to remove the hardness of water is to pass it through a column of ion-exchange resins where calcium and magnesium ions get replaced by sodium ions. This converts hard water into soft water.
Distillation:
Permanent hardness can be removed by the method of distillation. The water obtained after distillation is called distilled water. It is the purest form of water.
3. What is Electrolysis? Explain the electrolysis of water.
Electrolysis:
Breaking down of substances by the passage of electric current is called electrolysis.
Electrolysis of Water:
•Electrolysis of water experiment set up consists of a glass beaker fixed with two carbon electrodes filled with water upto one third of its volume.
•The positive carbon electrode acts as anode and the negative carbon electrode acts as cathode.
•Two test tubes are placed on the electrodes as shown in Figure.
•The electrodes are connected to a battery and current is passed until the test tubes are filled with a particular gas.
•If the gas collected is tested using a burning splint we can notice that the gas in the cathode side burns with a popping sound when the extinguish splint is brought near the mouth of the test tube.
•This property is usually shown by hydrogen gas and so it is confirmed that the gas inside the test tube is hydrogen.
•The burning splint placed near the anode side burns more brightly confirming that it is oxygen gas.
•This experiment shows that water is made up of hydrogen and oxygen. The ratio of hydrogen and oxygen is 2:1.
•Hence, for every two volumes of hydrogen collected at the cathode, there is one volume of oxygen collected at the anode
4. Explain the different ways by which water gets polluted.
Water gets polluted in various ways.
Some of the sources of water pollution are explained below
a. Household detergents:
-Household and cleaning detergents are a major cause of water pollution.
-Synthetic (non biodegradable) detergents have chemicals that do not break down and can end up polluting both surface and groundwater.
-Some shampoo, face wash, shower gel and toothpaste have small round pieces of plastic added to them. These are called microbeads. They are added for different reasons like scrubing and cleaning your skin, polishing your teeth etc.
-When we use products with microbeads, they go down our drain and pollute water bodies. Fish and other animals feed them by accident and get sick.
b. Domestic Sewage:
-Wastewater that is disposed of from households is known as domestic sewage. -Domestic sewage should be treated before being disposed of into water bodies like rivers, lakes, etc.
-Untreated sewage contains impurities such as organic matter from food waste, toxic chemicals from household products and it may also contain disease-causing microbes.
c. Domestic waste and plastics:
-Solid waste including plastics are disposed of or end up in water bodies such as lakes, rivers and the ocean.
-Plastics block drains spreading vector borne diseases such as malaria and dengue. Waste in water bodies, negatively impact aquatic life.
d. Agricultural activities:
-Fertilizers, pesticides and insecticides used in agriculture can dissolve in rainwater and flow into water bodies such as rivers and lakes.
-This causes an excess of nutrients such as nitrates and phosphates as well as toxic chemicals in water bodies. It is called Eutrophication.
-These substances can also be harmful to aquatic life.
e. Industrial waste:
-Many industries release toxic wastes such as lead, mercury, cyanides, cadmium, etc. -If this waste is unregulated and is released into water bodies it has a huge impact on humans, plants, animals and aquatic life.
