Karnataka Board Class 8 Science Chapter 13 Water

KSEEB Class 8 Science Chapter 13 Textual Questions & Answers

I. Four alternatives are given to each of the following incomplete statement/questions. Choose the right answer.

Question 1.
Magnesium sulfate crystals are commonly called …………..
a) Gypsum salt
b) Epsom slat
c) green vitriol
d) Alum
Answer:
b) Epsom slat

Question 2.
When you buy washing soda from the market, you will get it in powder form because ………….
a) The crystals of washing soda quickly lose water molecule
b) The shop keeper prefers to power the crystal.
c) washing soda does not form crystals
d) washing soda is a drying agent
Answer:
a) The crystals of washing soda quickly lose water molecule

Question 3.
A sample of hard water is heated in a utensil a white deposit of calcium carbonate is formed. This is due to the presence of ….
a) calcium chloride
b) magnesium sulfate
c) calcium bicarbonate
d) calcium hydroxide
Answer:
c) calcium bicarbonate

Question 4.
The salt that cause hardness in water and that liberates carbon dioxide on heating is
a) magnesium bicarbonate
b) calcium chloride
c) magnesium sulfate
d) calcium sulfate
Answer:
a) magnesium bicarbonate

Question 5.
When steam is passed over red hot …………. iron we get the following products
a) Ferric hydroxide and hydrogen
b) Ferric oxide and hydrogen
c) Ferrous hydroxide and hydrogen
d) Ferrous ferric oxide and hydrogen
Answer:
d) Ferrous ferric oxide and hydrogen

II. Fill in blanks with suitable words :

Question 1.
The chemical name of Epsom salt is ………..
Answer:
sulfate (Magnesium)

Question 2.
The chemical used to soften hard water is …………..
Answer:
sodium carbonate

III. Match the following :
KSEEB Solutions for Class 8 Science Chapter 13 Water III.1
Answer:

  1. d
  2. a
  3. b
  4. c

IV. Answer the following:

Question 1.
Water is tasteless, but we experience different tastes of water in different places. Why?
Answer:
Water obtained from different places has several salts dissolved in it, therefore, water has different taste even though pure water is tasteless.

Question 2.
Write the balanced equations for the reaction of sodium, potassium and calcium on cold water.
Answer:

  1. 2Na + 2H2O → 2NaOH + H2 ↑
  2. 2K + 2H2O → 2 KOH + H2 ↑
  3. Ca + 2H2O → Ca(OH)2 + H2 ↑

Question 3.
What happens When steam is passed over red hot iron? Write the equation.
Answer:
When steam is passed over red hot iron hydrogen is liberated along with the formation of ferric oxide.
3Fe + 4H2O → Fe ↑ + 4H2

Question 4.
Define the terms efflorescence and Deliquescent.
Answer:

  1. Efflorescence: Certain hydrates lose water when exposed to air of moderate humidity are called substances.
  2. Deliquescence: Certain water-soluble substances, when exposed to the atmosphere at ordinary temperature, absorb moisture from the atmospheric air to become moist and ultimately dissolved in the absorbed water. Such substances are called deliquescent substances.

Question 5.
Give two reasons to prove that water is a compound and not an element.
Answer:
Though water contains combustible hydrogen and supporter of combustion oxygen, water is neither a supporter of combustion nor a combustible substance. Water has a different property which hydrogen and oxygen do not have i.e., it is a very good fire extinguisher, compound does not show the properties of elements by which it is formed but it will have a different property since it is a new substance. By this one can prove that water is a compound.

Question 6.
Name three metals which do not react with water under normal conditions.
Answer:
Copper, tin and aluminium are the three metals which do not react with water under normal conditions.

Question 7.
Name the products formed when the following oxides are dissolved in water. Write balance of equation.

  1. Sulfur dioxide
  2. potassium oxide
  3. calcium oxide
  4. Carbon dioxide

Answer:

  1. When sulfur dioxide dissolves in water sulfurous acid is formed.
    SO2 + H2O → H2SO3
  2. When potassium oxide dissolves in water-potassium hydroxide is formed
    KO + H2O → KOH
  3. When calcium oxide dissolves in water calcium hydroxide is formed.
    CaO +H2O → Ca(OH)2
  4. When carbon dioxide dissolves in water carbonic acid is formed.
    CO2+ H2O → H2CO3

Question 8.
State the reasons for hardness in water.
Answer:
Presence of water soluble calcium and magnesium bicarbonate sulfates and chlorides cause hardness in water.

V. Give reasons for the following :

Question 1.
Sodium metal should be stored under Kerosene.
Answer:
Sodium is to be kept under kerosene as it can react with oxygen and water vapors present in air and kerosene has no affinity towards sodium.

Question 2.
Washing soda loses its crystal structure quickly.
Answer:
Washing soda loses its crystal structure quickly as it loses water when exposed to air of moderate humidity. It is efflorescent substance.

Question 3.
When calcium nitrate crystals are exposed to air a solution is formed?
Answer:
Calcium nitrate when exposed to air absorb moisture from the atmospheric air to become moist and dissolve in the absorbed water therefore it cannot be in the crystal form. It is deliquescent substance.

KSEEB Class 8 Science Chapter 13 Additional Questions & Answers

Question 1.
Explain the biological significance of water.
Answer:
Water is one of the most abundant and important compounds in the world. It is the major constituent of all living organism, some plants contain more percentage of water, the human body has about 65% of water. Neither plants, animals nor human beings can survive without water.

Question 2.
Water is called the universal solvent. Why?
Answer:
Water is called the universal solvent because it dissolves more substance than any other solvent.

Question 3.
What is the chemical formula of water?
Answer:
The chemical formula of water is H2O

Question 4.
Write the molecular structure of water molecule
Answer:
KSEEB Solutions for Class 8 Science Chapter 13 Water add 4
Question 5.
Many metals and their alloys are used to prepare utensils why?
Answer:
Many metals and their alloys are used to prepare utensils because they do not react with water under normal conditions.

Question 6.
What happens when potassium is dipped in water?
Answer:
When potassium is dipped in water forming respective hydroxides liberating hydrogen gas.

Question 7.
What happens when sodium is dipped in water?
Answer:
Sodium piece rushes here and there reacting with water violently. It gradually diminishes in size and finally disappears. It dissolves in water forming sodium hydroxide and hydrogen.

Question 8.
Write the equation for the reaction between potassium and water.
Answer:
2k + 2H2O →  2kOH + H2 ↑

Question 9.
Rusting is quick in rainy season why?
Answer:
Rusting is quick in the rainy season because the humidity will be more in the rainy season, which promotes rusting..

Question 10.
What happens when steam is passed over red hot metals like zinc, iron, magnesium. Write a balanced equation.
Answer:
When steam is passed over red hot metals like zinc, iron, magnesium we get respective oxides and hydrogen gas.
Zn + H2O → ZnO+ H2 ↑
3Fe + 4H2O → Fe3O4 + 4H2O ↑
Mg + H2O →  MgO+H2 ↑

Question 11.
What happens when steam is passed over red hot coke and silicon.
Answer:

  1. When steam is passed over red hot coke, a mixture of carbon monoxide and hydrogen is formed.”
    C + H2O → CO ↑ + H2↑
  2. When steam is passed over red hot silicon, which is a non-metal silicon dioxide and hydrogen gas are formed.
    Si + 2H2O → SiO2  +2H2  ↑

Question 12.
What happens when carbon dioxide dissolves in water.
Answer:
When carbon dioxide dissolves in water carbonic acid is formed.
CO2  + H2O → H2CO3 (Carbonic acid) Carbonic acid is present in soft drinks.

Question 13.
What happens when calcium oxide reacts with water. Write a balanced equation.
Answer:
When calcium oxide reacts with water it forms calcium hydroxide
CaO + H2O → Ca(OH)2 

Question 14.
Give some examples of the substances which can absorb moisture.
Answer:
The substances which absorb moisture are magnesium chloride, solid sodium hydroxide, solid potassium hydroxide, anhydrous calcium chloride, calcium nitrate Ferric chloride, and sodium nitrate.

Question 15.
Define deliquescence.
Answer:
Certain water-soluble substances, when exposed to the atmosphere at ordinary temperature, absorb moisture from the atmospheric air to become moist and ultimately dissolved in the absorbed water. Such substances are called deliquescent substances.

Question 16.
Give some examples of desiccants.
Answer:
Concentrated sulfuric acid and calcium chloride monohydrate are examples for desiccants.

Question 17.
Define Efflorescent substances.
Answer:
Certain hydrates that lose water when exposed to air of moderate humidity are called substances.

Question 18.
Give an example of efflorescent substances.
Answer:
Sodium sulfate, (NaSO410H2O) washing soda (Na2CO3H2O).

Question 19.
Calcium carbonate does not cause hardness. Why?
Answer:
Calcium carbonate does not cause hardness because it is insoluble in water.

Question 20.
Name the salts causing hardness in water.
Answer:
Calcium and magnesium bicarbonates, sulfates, and chlorides cause hardness in water.

Question 21.
What is meant by hard water?
Answer:
The water which does not give lather with soap is called hard water.

Question 22.
What is meant by soft water?
Answer:
The water which gives lather readily with soap is called soft water.

Question 23.
Why is hard water not used in industrial boilers?
Answer:
Hard water contains calcium and magnesium soluble salts. If it is used in industrial boilers, it forms insoluble salts of calcium and magnesium. They produce scales on the inner walls of the boilers, which reduce heating efficiency. There is a chance of an explosion of boilers due to foaming. It also corrodes the boilers.

Question 24.
Mention methods of removal of hardness of the water.
Answer:
Hardness can be removed by two methods.

  1. If hard water contains only magnesium and calcium bicarbonate it can be made soft by boiling. In this reaction, bicarbonates are converted into insoluble carbonate and CO2
    Ca (HCO3)2 → CaCO3 + H2O + CO2↑
    Mg (HCO3)2 → MgCO3+ H2O + CO2↑
  2.  Hardness can be removed by the addition of washing soda to water. In this reaction, the soluble salts of calcium and magnesium are converted into insoluble carbonates. The soluble sodium salts do not cause hardness.
    CaCl2+ Na2CO3  → CaCO3 + 2NaCl  ↓
    CaHCO3 +Na2CO3 → CaCO3↓ + 2NaHCO3
    CaSO4 + Na2CO3 → CaCO + Na2SO4

Question 25.
Experiment to show the removal of hardness by washing soda.
Answer:
Materials required:

  1. beaker
  2. glass rod
  3. washing soda
  4. hard water

Take some hard water in a beaker, and add a small quantity of washing soda to it stir it with a glass rod. Allow the insoluble carbonates to settle down or precipitate. Filter or decant the liquid. Water so obtained is soft water. But it is unfit for drinking as it contains an excess of washing soda and excess of sodium salts.

Question 26.
What is meant by Desalinization?
Answer:
Desalinization is an artificial process by which saline water (generally seawater) is converted into freshwater.

KSEEB Solutions for Class 8 Science