Water Book Back Questions 8th Science Lesson 13
8th Science Lesson 13
13] Water
Book Back Questions with Answer and Do You Know Box Content
Do You Know?
Henry Cavendish was a British philosopher, scientist, chemist, and physicist. Cavendish is noted for his discovery of hydrogen. He called it inflammable air. He mixed metals with strong acids and created hydrogen. He created carbon dioxide also by combining metals with strong bases.
When the skaters move on ice, they exert pressure on it. This pressure lowers the freezing point. As a result, the ice melts underneath the skate and allows the skaters to glide across the ice with little effort. When the skaters move forward, pressure is decreased and the water re-freezes to ice again.
Pure water has the following physical properties. Pure water boils at 100oC at one atmospheric pressure. Pure water freezes at exactly 0oC at one atmospheric pressure. Pure water has a density of 1 gm/cm3.
The freshness of fish and meat can be maintained by placing them in contact with ice. With its larger latent heat, ice is able to absorb a large quantity of heat from the fish as it melts. Thus, food can be kept at a low temperature for an extended period of time.
Copper does not react with water at any temperature. That is why it is used for making pipes and bodies.
Tap water, river water and well water contain dissolved solids but rainwater and distilled water do not contain dissolved solids. Hence concentric rings are not formed in the rain water and distilled water after evaporation.
The salinity of water is more in the Dead Sea. It is actually a salt lake as it has a single source of water and is not connected to the ocean. It is landlocked and this causes the water to evaporate. This has led to a steady increase in its degree of salinity. Now the salinity is so high such that the marine life cannot survive in it. This is why it is called the Dead Sea.
Every year 4.6 million children die due to diarrhea. Access to clean water improves hygiene and health.
RO purifiers are the purifiers that can remove the dissolved impurities and germs. They also improved the taste of water. RO stands for the name of the technology, ‘reverse osmosis’, used in these purifiers. Some RO purifiers also have a UV (ultraviolet) unit that destroys the germs present in water.
Distilled water and boiled water have no taste. The pleasant taste of drinking water is due to the presence of dissolved substances which include air, carbon dioxide and minerals.
About 90% of the available surface water has already been tapped mainly for agriculture and irrigation.
The largest source of water pollution in India is untreated sewage. On an average, a person uses 135 litres of water per day for washing clothes, cooking, bathing, etc.
Plastic sheets are used in agriculture to grow vegetables. At the end of the season, these plastic sheets are ploughed back into the soil. The plastic sheets break into tiny pieces and get eaten by earth worms, which is harmful to their health and that of soil.
Micro-plastics can be found in almost every freshwater source. They have been found from the freezing waters of the Arctic and Antarctic to the bottom of the deep-sea floor upto 5,000 metres deep. Micro-plastics have been found in bottled water and tap water around the world.
Choose the best answers:
1. Water changes to ice at
(a) 0oC
(b) 100oC
(c) 102oC
(d) 98oC
2. Solubility of carbon dioxide in water is high when the
(a) Pressure is low
(b) Pressure in high
(c) Temperature is high
(d) None of the above
3. The gas collected at the cathode on electrolysis of water is
(a) Oxygen
(b) Hydrogen
(c) Nitrogen
(d) Carbon dioxide
4. Which of the following is a water pollutant?
(a) Lead
(b) Alum
(c) Oxygen
(d) Chloride
5. Permanent hardness of water is due to the presence of ________________.
(a) Sulphates and chloride
(b) Dust particles
(c) Carbonates and bicarbonates
(d) Other soluble particles
Fill in the blanks:
1. Water is colourless, odourless and ____________.
2. The boiling point of water is ____________.
3. Temporary hardness of water can be removed by _________________ of water.
4. The density of water is maximum at _______________.
5. Loading speeds up the process of _______________.
State true or false. If false, correct the statement:
1. Sewage should be treated well before being discharged it into water bodies.
2. Sea water is suitable for irrigation as it contains dissolved salts.
3. Excessive use of chemical fertilizers depletes the soil and causes water pollution.
4. The density of water will not change at all temperature
5. Soap lathers well in hard water.
Match the following:
1. Universal solvent – Water pollutant
2. Hard water – Kills germs
3. Boiling – Ozonisation
4. Sterilization – Water
5. Sewage – Stomach ailments
Answers:
Choose the best answers:
1. 00C 2. pressure is high 3. Hydrogen 4. Lead 5. sulphates and chlorides
Fill in the blanks:
1. Tasteless 2. 1000C 3. Boiling 4. 40C 5. Sedimentation
State true or false. If false, correct the statement:
1.True
2.False
Correct answer: Sea water is not suitable for irrigation as it contains dissolved salts
3.True
4.False
Correct answer: The density of water will change at all temperature
5.False
Correct answer: Soap lathers well in soft water
Match the following:
1.Universal solvent- Water
2.Hard water- Stomach ailments
3.Boiling- Ozonisation
4.terilization- Kills germs
5.Sewage- Water Pollutant