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Kerala State Syllabus 8th Standard Basic Science Solutions Chapter 16 Water
Water is a precious natural resource. Water is essential not only for sustaining life but also for agriculture, industry, energy production and transportation. Water is formed by the combination of hydrogen and oxygen. Hydrogen and oxygen are produced by decomposition of water. Decomposition of water is done by electrolysis of water.
Basic Science Class 8 Chapter 16 Characteristics of water
Water is a substance found in nature in all the three states of matter namely solid, liquid and gas. The temperature at which a liquid boils at normal atmospheric pressure is its boiling point. The boiling point of water is 100°C. Once water starts boiling, the temperature will not change because all the heat supplied is utilised for the change of state. Water has the ability to hold more heat than other liquids. The ability to hold heat by a liquid is its heat capacity. Because of its high heat capacity water is utilised in radiators to regulate heat, to cool hot objects, to control the temperature of earth etc.
Water becomes ice in 0°C. The temperature of a liquid at which it freezes to solid at normal atmospheric pressure is called its freezing point. When water is converted to ice its volume increases and the density decreases. The unbalanced attractive force on the surface of a liquid causes the liquid, surface behaves like a stretched membrane. This is surface tension. The surface tension decreases the surface area of a liquid. The liquid drops remain like a ball to decrease the surface area. Adding soap to water is a way to decrease surface tension of water.
Class 8 Chemistry Notes Kerala Syllabus The components of water
When electricity is passed through water it decomposes to hydrogen and oxygen. For this, we can make water voltmeter shown in fig.
Iron nail and plastic bottle can be used for this. Adding a few drops of acid into water and battery are also used. When electricity is passed through it oxygen and hydrogen are filled in the test tubes. This process is called electrolysis. The apparatus used for the electrolysis of water is called Hoffmann Water Voltameter.
Kerala Syllabus 8th Standard Physics Notes Reaction of water with metals
Coldwater reacts with sodium, potassium etc. to liberate hydrogen. Magnesium reacts with hot water and iron reacts with steam to liberate hydrogen gas.
Hss Live Guru 8 Chemistry Kerala Syllabus Water the Universal solvent
Several substances dissolve in water. Since water can dissolve various substances and widely used for preparing solutions it is a universal solvent.
8th Standard Chemistry Textbook Soft water and hard water
The water in which soap does not lather easily is called hard water. Water in which soap gives lather readily is called soft water. The hardness of water containing calcium or magnesium bicarbonate is removed during boiling. This type of hardness is called temporary hardness. But the hardness of water containing the chlorides and sulphates of calcium and magnesium is not removed by boiling. Such hardness is called permanent hardness. Pure water has neither the properties of acid nor those of alkali, hence it is a neutral solvent.
Std 8 Chemistry Notes Kerala Syllabus Water pollution
The following activities cause the water pollution
- Dumping of waste in water resources.
- Rampant use of fertilisers
- Excessive use of detergents
- Insecticides getting mixed with water
Water Textbook Questions and Answers
Kerala Syllabus 8th Standard Basic Science Notes Questions 1.
When water is heated at its boiling point or melting point, its temperature does not change.
a. What is meant by boiling point and melting point?
b. What are the boiling and freezing points of water?
c. Why is there no change in temperature?
Answer:
a. The temperature at which a liquid boils at normal atmospheric pressure is its boiling point. The temperature of a liquid at which it freezes to solid at normal atmospheric pressure is called its freezing point.
b. 100°C, 0°C.
c. Once water starts boiling, the temperature will not change because all the heat supplied is utilized for the change of state. So the temperature does not vary.
8th Class Chemistry Notes Kerala Syllabus Question 2.
A definite quantity of water and coconut oil are heated in separate test tubes using the same amount of heat.
a. In which case does the temperature increase slowly?
b. What is the reason for this?
c. Write any one practical application of this property.
Answer:
a. water
b. The high heat capacity of water
c. 1. To regulates the heat of radiator in vehicles
2. To regulate the heat of earth
8th Standard Water Lesson Kerala Syllabus Question 3.
100 ml. each of coconut oil and water are taken in two beakers and kept in the freezer.
a. What difference can be observed in their volumes during freezing?
b. What do you infer from the observation?
c. When water is frozen in glass bottles, it is advised not to fill the bottles completely. Explain the reason.
Answer:
a. The volume of the beaker containing water is greater.
b. Due to the anomalous expansion of water.
c. The volume of water in the bottle filled with water is increased and the bottle is broken.
Hss Live Guru 8 Physics Kerala Syllabus Question 4.
Soap decrease the surface tension of water.
a. What is surface tension?
b. How does the decrease in surface tension benefit washing of clothes?
Answer:
a. The unbalanced attractive force on the surface of a liquid causes the liquid surface behaves like a stretched membrane. This is surface tension
b. When we add soap to water it reduces the surface tension of water and water can enter into the fine threads of the fabric.
Basic Science For Class 8 Chapter 16 Question 5.
Surface tension tends to minimise the surface area of a liquid. Suggest an experiment to prove this. (Follow the pattern: Required materials, procedure, expected observation).
Answer:
’Materials required: metallic loop, thread, soap water, pin Method of experiment: Tie a thread to a metallic loop, immerse it in soap water and create a soap film as shown in fig. a, b, c.
Prick a portion of film using a pin. The shape of remaining portion of soap film will be as such the surface area is reduced.
Basic Science Class 8 Ch 16 Kerala Syllabus Question 6.
Providing excess food for fish in an aquarium is a threat to its survival. Justify.
Answer:
When we add more food materials into water, the dissolved oxygen is utilized to decompose the food materials. So the amount of oxygen is reduced and the water is polluted.
Hsslive Guru 8th Class Chemistry Question 7.
Some substances when dissolved in water cause hardness of water.
a. Which of the following substances cause hardness of water? Sodium chloride, Calcium bicarbonate, Calcium carbonate, Calcium sulfate, Magnesium sulfate, Calcium chloride, Magnesium carbonate
a. The hardness due to which of the above salts cannot be removed by boiling?
Answer:
a.calcium bicarbonate, calcium sulfate, calcium chloride.
b. The hardness of water containing the chlorides and sulphates of calcium and magnesium is not removed by boiling.
Water Additional Questions and Answers
Kerala Syllabus 8th Standard Chemistry Notes Question 1.
a. What are the gases liberated in the water voltameter arranged for decomposition of water?
b. Which gas is liberated at the negative pole of the battery?
Answer:
a. Hydrogen and oxygen
b. Hydrogen
c. Ratio of hydrogen and oxygen – 2: 1
8th Class Biology Notes Pdf Question 2.
Heat equal quantity of water and coconut oil in two vessels.
a. The temperature of which liquid will be raised rapidly?
b. Which has more heat capacity?
Answer:
a.Coconut oil
b.water
Hss Live Guru 8th Chemistry Kerala Syllabus Question 3.
Take water in a vessel and put a needle on the surface of water carefully.
a. Will the needle be immersed in water. Justify your answer.
b. Why do the water drops assume spherical shape?
Answer:
a. No. Because of the surface tension of water.
b. The surface tension reduces the surface area.
Hsslive Guru 8th Basic Science Question 4.
Name two metals which react with cold water.
Answer:
Potassium, sodium
Hsslive Guru Chemistry 8 Kerala Syllabus Question 5.
Which is the metal reacts with hot water? Which is the gas produced? Write the chemical equation.
Answer:
Magnesium; Hydrogen.
Mg + H2O → MgO + H2
Hss Live Guru 8th Basic Science Question 6.
Why does water is known as universal solvent?
Answer:
Since water can dissolve various substances and widely used for preparing solutions it is a universal solvent.
8th Physics Guide Kerala Syllabus Question 7.
What are hard water and soft water?
Answer:
The water in which soap does not lather easily is called hard water. Water in which soap gives lather readily is called soft water.
Class 8 Physics Notes Kerala Syllabus Question 8.
Give the reason for temporary hardness of water.
Answer:
The hardness of water is due to the presence of calcium or magnesium bicarbonate. This type of hardness is called temporary hardness.
8th Standard Chemistry Textbook Kerala Syllabus Question 9.
Hardness of water is not removed even after it is boiled. What may be the reason for this?
Answer:
This is due to chloride and sulphate of magnesium and calcium are dissolved in water.
Class 8 Basic Science Chapter 16 Question 10.
Why is air continuously introduced into the water in an aquarium?
Answer:
Aquatic plants and animals make use of oxygen dissolved in water. Water gets polluted as the amount of oxygen in it decreases. Air is introduced into water to make up the quantity of oxygen.
Question 11.
Write two remedies to prevent water pollution.
Answer:
- Stop dumping of waste in water resources.
- Reduce the use of fertilisers and pesticides.
Question 12.
Write two methods to reduce the surface tension of water
Answer:
- Add soap to water
- Boil the water.