Class 8 Basic Science Chapter 14 Important Questions Kerala Syllabus Water

The comprehensive approach in SCERT Class 8 Basic Science Textbook Solutions Chapter 14 Water Important Questions ensure conceptual clarity.

Water Extra Questions and Answers Class 8 Basic Science Chapter 14 Kerala Syllabus

Water Class 8 Important Questions

Question 1.
Match the property of water in Column A with its definition or consequence in Column B.

Column A	Column B
(i) Surface Tension	(a) Allows many substances to dissolve in it.
(ii) High Heat Capacity	(b) Causes ice to float on water.
(iii) Universal Solvent	(c) Allows water to absorb much heat without a large temperature rise.
(iv) Anomalous Expansion	(d) Force on the water surface minimizing its area.

Answer:
(i) – d, (ii) – c, (iii) – a, (iv) – b

Column A	Column B
(i) Surface Tension	(d) Force on the water surface minimizing its area.
(ii) High Heat Capacity	(c) Allows water to absorb much heat without a large temperature rise.
(iii) Universal Solvent	(a) Allows many substances to dissolve in it.
(iv) Anomalous Expansion	(b) Causes ice to float on water.

Question 2.
Match the condition with its effect on the boiling point of water.

Condition	Effect on Boiling Point
(i) Increased Pressure (e.g., pressure cooker)	(a) Boiling point remains 100°C.
(ii) Decreased Pressure (e.g., high altitude)	(b) Boiling point increases (above 100°C).
(iii) Normal Pressure(sea level, 1 atm)	(c) Boiling point decreases (below 100°C).

Answer:
(i) – b, (ii) – c, (iii) – a

Condition	Effect on Boiling Point
(i) Increased Pressure (e.g., pressure cooker)	(b) Boiling point increases (above 100°C).
(ii) Decreased Pressure (e.g., high altitude)	(c) Boiling point decreases (below 100°C).
(iii) Normal Pressure(sea level, 1 atm)	(a) Boiling point remains 100°C.

Question 3.
Match the type of water hardness with its cause and removal method.

Type of Hardness	Cause / Removal
(i) Soft Water	(a) Caused by chlorides/sulphates; cannot be removed by boiling.
(ii) Temporary Hardness	(b) Soap lathers well; contains few dissolved salts.
(iii) Permanent Hardness	(c) Caused by bi-carbonates; can be removed by boiling.

Answer:
(i) – b, (ii) – c, (iii) – a

Type of Hardness	Cause / Removal
(i) Soft Water	(b) Soap lathers well; contains few dissolved salts.
(ii) Temporary Hardness	(c) Caused by bi-carbonates; can be removed by boiling.
(iii) Permanent Hardness	(a) Caused by chlorides/sulphates; cannot be removed by boiling.

Question 4.
Statement (i): Water has a high surface tension, which helps it wet clothes easily.
Statement (ii): Adding soap or heating water decreases its surface tension.
a) Both statements (i) and (ii) are correct.
b) Statement (i) is correct, but statement (ii) is incorrect.
c) Statement (ii) is correct, but statement (i) is incorrect.
d) Both statements (i) and (ii) are incorrect.
Answer:
(c) Statement (ii) is correct, but statement (i) is incorrect. (High surface tension prevents water from wetting clothes easily).

Question 5.
Statement (i): When water freezes to form ice, its volume increases and its density decreases.
Statement (ii): Because ice is denser than water, it floats on water.
a) Both statements (i) and (ii) are correct.
b) Statement (i) is correct, but statement (ii) is incorrect.
c) Statement (ii) is correct, but statement (i) is incorrect.
d) Both statements (i) and (ii) are incorrect.
Answer:
(b) Statement (i) is correct, but statement (ii) is incorrect. (Ice floats because it is less dense than water).

Question 6.
Statement (i): The boiling point of water increases as atmospheric pressure decreases.
Statement (ii): Food cooks faster in a pressure cooker because the increased pressure raises the boiling point of water.
a) Both statements (i) and (ii) are correct.
b) Statement (i) is correct, but statement (ii) is incorrect.
c) Statement (ii) is correct, but statement (i) is incorrect.
d) Both statements (i) and (ii) are incorrect.
Answer:
(c) Statement (ii) is correct, but statement (i) is incorrect. (Boiling point decreases as pressure decreases).

Question 7.
Why is water called the “universal solvent”?
Answer:
Water is called the universal solvent because it has the ability to dissolve a wide variety of substances, more than many other liquids.

Question 8.
Why does steam at 100°C cause more severe burns than boiling water at 100°C?
Answer:
When water boils and turns into steam at 100°C, it absorbs extra heat energy called latent heat without changing its temperature. This latent heat is used for the change of state. Steam at 100°C contains this additional latent heat energy compared to water at 100°C, so it transfers more heat upon contact, causing more severe burns.

Question 9.
Why does ice float on water, and why is this important for aquatic life?
Answer:
When water freezes, its volume increases (anomalous expansion), causing its density to decrease. Because ice is less dense than liquid water, it floats. This is important because lakes freeze from the top down, forming an insulating layer of ice that prevents the entire water body from freezing, allowing aquatic life to survive underneath.

Question 10.
Describe a simple experiment to distinguish between hard water and soft water using soap.
Answer:
Take two separate flasks or bottles.

• Pour equal amounts of the water samples (one hard, one soft) into each flask.
• Add equal-sized pieces of soap to both flasks.
• Shake both flasks well.
• Observation: The flask containing soft water will produce a good amount of lather (foam). The flask containing hard water will produce very little lather or scum.

Question 11.
How can you demonstrate that the boiling point of water decreases when pressure is reduced?
Answer:

• Fill a boiling tube half with water and boil it using a flame.
• After turning off the flame, immediately close the tube with a rubber cork fitted with a glass tube connected to a syringe.
• Pull back the piston of the syringe.
• Observation: The water inside the tube will start boiling again, even though it has cooled slightly.
• Conclusion: Pulling the piston reduces the pressure inside the tube, which lowers the boiling point, causing the water to boil at the lower temperature.

Question 12.
Describe an activity to demonstrate surface tension using a razor blade or needle.
Answer:
Fill a glass or tumbler completely with still water, perhaps even slightly overfull so the surface bulges.

• Carefully and gently place a clean, dry razor blade or needle flat onto the surface of the water.
• Observation: The blade or needle will float on the surface without sinking, supported by the “skin” created by surface tension.

Question 13.
Water (H₂O) can be decomposed by electrolysis. Write the balanced chemical equation for this reaction.
Answer:

Question 14.
During the electrolysis of water using a Hoffmann voltameter, identify the products formed at the positive electrode (anode) and the negative electrode (cathode).
Answer:
Positive electrode (anode): Oxygen gas (O₂).
Negative electrode (cathode): Hydrogen gas (H₂).

Question 15.
When quicklime (CaO) is added to water (H₂O), slaked lime (Ca(OH)₂) is formed. Write the balanced chemical equation for this reaction.
Answer:
CaO + H₂O → Ca(OH)₂.

Question 16.
What is the chemical formula for a molecule of water, and what does it indicate about the atoms present?
Answer:
The chemical formula is H₂O. It indicates that one molecule of water contains two hydrogen atoms and one oxygen atom.

Question 17.
During the electrolysis of water, twice the volume of hydrogen gas is produced compared to oxygen gas. Based on the formula of water (H₂O), explain why this ratio occurs.
Answer:
The formula H₂O shows that each water molecule contains two hydrogen atoms for every one oxygen atom. When water is split during electrolysis, these atoms combine to form H₂ and O₂ molecules. Since there are twice as many hydrogen atoms available, they form twice the volume (or number of molecules) of hydrogen gas (H₂) compared to oxygen gas (O₂).

Question 18.
What are the chemical formulas for the salts typically responsible for:
a) Temporary hardness (as bicarbonates)
b) Permanent hardness (as chlorides)
Answer:
(a) Calcium bicarbonate: Ca(HCO₃)₂;
Magnesium bicarbonate: Mg(HCO₃)₂.

(b) Calcium chloride: CaCl₂;
Magnesium chloride: MgCl₂.

Question 19.
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?
c. What will be the ratio of the volume of the gases?
Answer:
a. Hydrogen and oxygen
b. Hydrogen
c. Ratio of hydrogen and oxygen – 2 : 1

Question 20.
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

Question 21.
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.

Question 22.
Name two metals which react with cold water.
Answer:
Potassium, sodium

Question 23.
Which is the metal react with hot water? Which is the gas produced? Write the chemical equation.
Answer:
Magnesium; Hydrogen.
Mg + H₂O MgO + H₂

Question 24.
Why does water is known as universal solvent?
Answer:
Since water can dissolves various substances and widely used for preparing solutions it is a universal solvent.

Question 25.
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.

Question 26.
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.

Question 27.
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.

Question 28.
Why is air continuously introduced in to the water in an aquarium?
Answer:
Aquatic plants and animals make use of oxygen dissolved in water. Water get polluted as the amount of oxygen in it decreases. Air is introduced in to water to make up the quantity of oxygen.

Question 29.
Write two remedies to prevent water pollution.
Answer:

1. Stop dumping of waste in water resources.
2. Reduce the use of fertilisers and pesticides.

Question 30.
Write two methods to reduce the surface tension of water
Answer:

1. Add soap to water
2. Boil the water.

Question 31.
Complete the given chemical equation.
Mg + H₂O → MgO + …………………
Answer:
H₂

Question 32.
Identify the property of water shown in each of the following situations.
a. Water is used in radiators of automobiles.
b. Small water drops assume spherical shape.
Answer:
a. Heat capacity of water
b.Surface tencsion of water

Question 33.
Two samples of water are given.
Sample 1 – Soap gives lather easily.
Sample 2 – Soap does not give lather easily.
a. Which sample represents hard water?
b. Which among the following salts causes hardness of water?
(Sodium carbonate, Magnesium chloride, Zinc hydroxide, Ammonium chloride)
c. Which type of hardness of water can be removed by boiling?
Answer:
a. Sample 2
b. Magnesium Chloride (MgCl₂)
c. Temporary hardness of water

Question 34.
Water pollution is a burning issue now-a-day.
a. How do aquatic animals and plants get oxygen for respiration?
b. The wastes dumped into rivers and ponds result in the destruction of aquatic life. Explain.
c. Write any other activity that causes water pollution.
Answer:
a. Aquatic animals and plants make use of the oxygen and carbon dioxide dissolved in water.

b. The Wastes dumped in the rivers and Ponds decay using the oxygen dissolved in them. This depletes the amount of oxygen and results in the destruction of aquatic life.

c. Excessive use of detergents Rampant use of fertilizers.

Question 35.
Boling point of water is …………………..
Answer:
100°c.

Question 36.
Following are the observations
when a student added soap solution to two samples of water collected from two different sources.

Sample	Observation
Water collected from source 1	Lathers easily
Water collected from source 2	Does not lather easily

a. Which one of these is hard water?
b. Which salts are responsible for the hardness of water?
Answer:
a. 2.
b. Ca or Mg.

Question 37.
Water is used to cool hot objects. Which property of water is utilised here?
Answer:
Heat capacity of water, (or Specific heat capacity)

Question 38.
Water pollution is a major issue we are facing nowadays. Write any two activities that cause water pollution.
Answer:
Dumping of wastes in water resources Rampant use of fertilizers

Question 39.
Water taken in a round bottomed flask is heated continuously and the temperature is noted.
a. What is the boiling point of water?
b. Eventhough we are heating the flask continuously the temperature remains constant at 100° C. Why?
c. Steam causes more severe burns than boiling water. Why?
Answer:
a. The temperature at which water boils at normal atmospheric pressure is called boiling point of water -100°C

b. Once boiling starts the temperature will not change because all the heat supplied will be used for the change of state.

c. When water starts boiling all the heat supplied will be used for the change of state. Hence all heat supplied will be contained in the steam at the same temperature. So steam causes more severe burns.

Question 40.
Soap is added to two samples of water taken in two test tubes and shaken well.
a. Soap lathers easily in water taken in one test tube. Which type of water is it?
b. Soap does not lather easily in water taken in the second test tube Write the name of a salt which causes this behaviour.
c. Suggest any one method to remove this behaviour of water.
Answer:
a. Soft water
b. Bicarbonates, chlorides and sulphates of calcium and magnesium
c. Temporary hardness is removed by boiling and permanent hardness by any chemical methods.

Question 41.
The volume of water When it solidifies
(increases, decreases, does not change)
Answer:
Increases

Question 42.
Water pollution is a major issue we face today
a. Write down any two situations in which water in your locality gets polluted.
b. Suggest a method to prevent water pollution.
Answer:
a. Plastic thrown into the soil
b. Chemical Pesticides and Chemical fertilizers.

Question 43.
In water from certain sources soap lathers easily. But in water from some other sources soap does lather easily
a. What is the name of water in which soap doesn’t lather easily?
b. Give an example of any salt responsible for this defect of water.
c. Write a method to remove this defect.
Answer:
a. Hard water
b. Bicarbonate, Chloride or Sulphate of Calcium and Magnesium
c. Boil water / add suitable Chemicals

Question 44.
Water is known as the ……………………. solvent.
a) Polar
b) Universal
c) Common
d) Basic
Answer:
b) Universal

Question 45.
Hardness caused by sulphates of calcium and magnesium is known as:
a) Temporary hardness
b) Basic hardness
c) Permanent hardness
d) Chemical hardness
Answer:
c) Permanent hardness

Question 46.
The special force due to the mutual attraction between water molecules that causes the surface to behave like a stretched film is called:
a) Cohesion
b) Adhesion
c) Surface Tension
d) Capillary action
Answer:
c) Surface Tension

Question 47.
Water has its maximum density at ……………………. °C.
a) 0
b) 100
c) 4
d) 37
Answer:
c) 4

Question 48.
When a water solution is tested with litmus paper, it turns blue litmus red. The solution is:
a) Neutral
b) Basic
c) Acidic
d) Alkaline
Answer:
c) Acidic

Question 49.
State any two uses of water’s high heat capacity in daily life.
Answer:
Two uses of high heat capacity:

1. Used as a coolant in car radiators,
2. Helps in maintaining the body temperature within a limit.

Question 50.
How is temporary hardness different from permanent hardness in water?
Answer:
Temporary Hardness is caused by bicarbonates of Calcium/Magnesium and can be removed by boiling. Permanent Hardness is caused by sulphates or chlorides of Calcium/Magnesium and cannot be removed by boiling.

Question 51.
Explain why food takes a longer time to cook in high mountain regions compared to sea level.
Answer:
In high mountain regions, the atmospheric pressure is lower. Since the boiling point is related to pressure, water boils at a temperature lower than 100°C. Cooking takes longer because the cooking medium (water) is at a lower maximum temperature.

Question 52.
Explain why steam causes a more severe burn than boiling water at 100°C.
Answer:
Steam contains more energy than boiling water at the same temperature (100°C). This extra heat is called latent heat of vaporization. When steam condenses on the skin, it releases this large amount of latent heat, causing a much more severe burn.

Question 53.
List three problems caused by water pollution, as mentioned in the chapter.
Answer:
Three problems caused by water pollution: (Any three of the following):

1. Destruction of aquatic plants,
2. Damage to the food chain,
3. Soil pollution,
4. Spread of water-borne diseases (leading to sickness and death).

Question 54.
Explain the Density Anomaly of water and its importance for the survival of aquatic life in cold regions.
Answer:
The maximum density of water is at 4°C. When water is cooled below 4°C and freezes into ice, its volume increases, which means its density decreases. Because ice is less dense than water, it floats on the surface. This floating ice forms an insulating layer that prevents the water underneath from freezing completely, allowing fish and other aquatic life to survive below the surface.