- Raman Ayan
- Feb 15, 2024
- 8 min read
Updated: Mar 3, 2024
Ramancoeducation online
Afzala janpati biraul Darbhanga, City br
NEW TRENDS IN ICSE CHEMISTRY - 8
CHAPTER - 8. Water
Answers
Tick (✔) The correct choice amongest the following 1. (ii) Potable water |
2. (iii) Sewage |
3. (i) Sedimentation |
4. (i) Chlorine gas |
5. (iv) Boiling |
6. (ii) Aquatic plants |
7. (a) Hydrogen |
8. (d) Hoffmans voltameter |
9. (a) Hydrogen |
10. (a) Respire |
11. (c) At 100°C |
12. (b) permanent hardness |
13. (d) boiling |
14. (a) potable water |
15. (b) water borne diseases |
16. (b) Ganga Action Plan |
17. (b) Zinc |
18. (c) Alum |
19. (a) 0°C |
20. (b) Sticky scum with soap solution |
21. (d) 126 J/g |
Fill in The Blanks 1. Aquatic |
2. Metabolic |
3. coolant |
4. nutrients |
5. Roots |
6. lowered |
7. impurities |
8. 0°c, 4°C |
9. 0° c |
10. increases |
11. bad |
12. universal |
13. blue, pink |
14. hydroxide |
15. sulphurous acid |
16. sulphuric acid |
17. gold, silver |
18. hydrogen |
19. fertilizers |
20. oil slick |
21. soft water |
22. Cholera, diarrohea |
23. algae |
24. intermolecular |
25. 4 |
26. universal |
27. increase |
28. gases |
Write True or False for each statment.Rewrite the Statement Correctly 1. False |
2. True |
3. False |
4. False |
5. False |
6. True |
7. True |
8. False (Liquid water is heavier than solid ice) |
9. True |
10. False (Pure water has a flat taste) |
11. False (Detergents are non-biodegradable) |
Match The Columns 1. 1.Typhoid 2.Thermal pollution 3.Hard water 4.Soft water |
2. 1.70% water 2.Drinking water 3.90% water 4.Rain water |
Answer The Followings Questions 1. • Synthetic detergent -----> Household waste • Pesticides -----> Agricultural fields • Lead and Mercury -----> Industrial waste |
2. lg/cc at 4°C |
3. Volume increases when water freezes. |
4. Jaundice, cholera |
5. Upper layer of ice acts as an insulator and protects them from further cold. |
6. Because water is universal solvent, it dissolves something of everything. |
7. Because it doesn't form lather with soap and it deposits in the boiler tubes as hard as stone-like layer. |
8. All metabolic reactions take place in water. It helps in transportation of nutrients. |
9. It is the process in which chemicals like alum is added to coagulate the floating matter so that impurities become heavier and settle down. |
10. 2H2 + 02 ----> H2O Because water turns anhydrous copper sulphate blue. |
11.  |
12. Two main steps as involved in purification of water: I. Removal of suspended impurities. II. Destruction of harmful germs and bacteria. I. Removal of suspended impurities : 1. Sedimentation : The process in which heavy particles like clay etc. settle down at the bottom due to the effect of gravity. 2. Loading : It is the process in which chemicals like alum is added to coagulate the floating matter so that impurities become heavier and settle down. 3. Filtration : It is the process in which water is pumped through beds of sand and gravel to remove the suspended matter. II. Destroying harmful germs : 1. Aeration : This process involves passing of air at high pressure to oxidise all the organic matter and germs which remain dissolved. 2. Chlorination : This process involves disinfection of water by using bleaching powder which releases chlorine and kills germs. |
Select the Odd and Out Giving reason 1. Nitrification- Others are related to purification of water. |
2. Potable water- Others are different sources of water. |
3. Rain water - Others have dissolved impurities |
4. Gravitation- Other terms are related to purification of water. |
5. Anaemia - It is due to deficiency of iron. |
WATER
EXERCISES
A. ANSWER THE FOLLOWING QUESTIONS IN ABOUT TWENTY WORDS.
Q. 1. List five uses of water.
Ans. The uses of water are as follows :
(i) Water is used in cooking.
(ii) Water is used for drinking almost by all living organisms.
(iii) Water is used in industries for various purposes.
(iv) Water is used for cleaning clothes, utensils etc.
(v) Water provides a medium for recreational sports such as
swimming, boating, water skiing etc.
Q.. 2 Why is natural water not considered safe for drinking ?
Ans. The natural water is not considered safe for drinking because
it is contaminated with many impurities that come naturally and also in
the form of different chemicals from industries. Due to these reasons, we
should avoid drinking natural water.
Q.. 3. How is river water purified to m e fit for drinking ?
Ans. River water is purified by chlorination to make it fit for drinking.
In this method chlorine is added to water in specific amount to kill the
germs present in it.
Q.. 4. What is the imp tance of water for humans, animals
and plants ?
Ans. Water is very important humans, animals and plants in the
following ways :
(i) Water regulates body temperature by the process of sweating and
evaporation.
(ii) Water inside the bodies of plants, animals and humans transport
minerals, gases and many products of digestion and carries them
at the place of need in the body.
(iii) Water provides a medium for all biochemical reactions inside our
bodies to take place.
(iv) Water is also needed by plants for their growth and carrying out
photosynthesis.
Q. 5. What is the effect of water on the following and under
what condition :
(a) Sodium metal (b) Iron ?
Ans. (a) Sodium metal reacts with water at room temperature. This
reaction is so fast that hydrogen so produced catches fires.
room temp.
2Na (s) + 2H 2
O (l) → 2NaOH (aq) + H2 (g)
vigorous
Sodium Water Sodium hydroxide Hydrogen
(b) Iron reacts with water only when steam is passed over red-hot
iron. The reaction is reversible.
steam
3Fe (s) + 4H 2
O (g) Fe 3
O 4 (s) + 4H2 (g)
Iron Steam Ferrosoferric Hydrogen
(Red hot) oxide
WATER 2
Q.. 6. raw a labelled diagram of the apparatus used for the
electrolysis of water.
Ans. The labelled diagram used for electrolysis of water is as follows :
water
Electrolysis of water
Acidulated
Electrodes
Glass
tubes
Oxygen
Hydrogen
Q. 7. Why do icecubes float on the surface of water ? Give
reason.
Ans. In ice, water molecules form a three dimensional structure with
large open space between them. Ice occupies more volume than an equal
mass of water. That is why ice is lighter than water and floats over it.
3 TEACHER’S MANUAL :: The World of Science–CHEMISTRY— 8
Q. 8. What is meant by the water of crystallisation ?
Ans. The water molecules present in the crystal lattice of the
substance (per molecule of the substance) is called its water of
crystallisation. Different salts contain different number of water molecules
as the water of crystallisation.
Q. 9. How will you find out if the given sample of water is
soft or hard ? How is hard water softened ?
Ans. Hard water does not form lather with the soap solution. Hard
water can be softened by the following methods :
(i) By boiling : By this method, temporary hardness can be
removed.
(ii) By adding washing soda : Calcium and magnesium ions
present in hard water react with sodium carbonate to produce insoluble
carbonates. These insoluble substances are removed by filtration.
(iii) Ion-exchange resin method : Hardness of water can also
be removed by passing hard water through a column packed with
ion-exchange resins. The ion-exhange resins exchange calcium and
magnesium ions (Ca 2+ and Mg 2+) present in hard water for sodium and
potassium ion (Na + and K + ).
Ion-exchange method can remove both the types of hardness of
water.
Q.. 10. Give one example of hydrated sal
Ans. Sodium carbonate decahydrate or washing soda
(Na 2
CO 3
.10H2
O) is an example of hydrated salt.
Q.. 11. Wha are colloids ? Mention two characteristics of
colloids.
Ans. A homogeneous-looking heterogeneous mixture in which
particles having size between 1 and 100 nm* are dispersed in a
continuous medium is called a colloid. Colloids are commonly called
sols.
Milk, Office paste (gum), Blood, Toothpaste, Jelly, etc., are some
examples of colloids.
The two characteristics of colloids are as follows :
(i) Colloid pa ticles are large enough to cause scattering of light.
(ii) The constituents of a colloid cannot be separated by simple
physical methods like filtration.
Q. 12. How is the temporary hardness of water removed ?
Ans. Temporary hardness of water can easily be removed by boiling.
On boiling, calcium and magnesium bicarbonates decompose to give the
precipitate of calcium and magnesium carbonates. The precipitates are
removed by filtration.
boiling
Ca(HCO 3
) 2 → CaCO 3 (s) + H2
O (l) + CO2 (g)
Calcium bicarbonate Calcium cabonate Water Carbon
(in temporary precipitate (can be dioxide
hard water) removed by filtration)
boiling
Mg(HCO 3
) 2 → MgCO 3 (s) + H2
O (l) + CO2 (g)
Magnesium bicarbonate Magnesium carbonate Water
(in temporary precipitate
hard water) (removed by filtration)
Q. 13. Why can’t we use hardwater for washing our clothes ?
Ans. Soap is the sodium salt of stearic acid (C17
H35
COO Na, sodium
stearate). When soap is added to hard water, it reacts with the calcium
and magnesium ions present in hard water to form insoluble calcium or
magnesium stearate (white and curdy precipitate).
WATER 4
*nm is the symbol for nanometre : 1 nm = 10–9 m
CaCl 2 + 2C17
H35
COONa → 2NaCl + C17
H35
COO) 2
Ca
Calcium chloride Sodium stearate Sodium chloride Calcium stearate
MgCl 2 + 2C17
H35
COONa → 2NaCl + (C17
H35
COO) 2
Mg
Magnesium Sodium stearate Sodium chloride Magnesium
chloride stearate
As a result, soap will not produce lather with water until all the
calcium and magnesium ions have precipitated. This is the reason why
hard water cannot be used for washing (laundry) purposes.
Q. 14. How does water react with the following ?
(a) K 2
O (b) CaO (c) CO 2
(d) Fe (e) Zn
Write balanced chemical equation for the reactions.
Ans.
(a) K 2
O (s) + H2
O (l) → 2KOH (aq)
Potassium oxide Water Potassium hydroxide
(b) CaO (s) + H2
O (l) → Ca(OH) 2 (aq)
Calcium oxide Water Calcium hydroxide
(Lime) (Limewater)
(c) CO 2 (g) + H2
O (l) → H 2
CO 3 (aq)
Carbon dioxide Water Carbonic acid
5 TEACHER’S MANUAL :: The World of Science–CHEMISTRY— 8
steam
(d) 3Fe (s) + 4H2
O (g) Fe 3
O 4 (s) + 4H2 (g)
Iron Steam Ferrosoferric Hydrogen
(Red hot) oxide
boiling
(e) Zn (s) + H 2
O (l) → ZnO (s) + H 2 (g)
Zinc Boiling Zinc oxide Hydrogen
powder water
Q. 15. Describe the characteristic properties of colloids.
Ans. Some characteristic properties of a colloid are described below :
l Heterogeneous nature. A colloid
(or sol) is heterogeneous in nature. The
particles in a colloid can be seen only
with a powerful microscope.
l Particle size. The size of particles in a
colloid lies between 10 –9 m and 10 –7 m.
l Separation of colloidal particles
by filtration. Colloidal particles
can easily pass through the pores of a filter paper. Therefore,
colloidal particles cannot be separated by ordinary filtration.
Particles in a colloid
l Brownian movement of colloidal
particles. The zig-zag motion of particles
in a colloid is called Brownian motion.
l Light scattering by colloidal particles.
Colloids scatter a light beam passing
through them. The scattering of light by
the particles in a colloid is called tyndall
effect. The Tyndall effect is shown in the figure.
Tyndall effect shown by colloid
Brownian motion
of colloidal
B. TICK (4) THE CORRECT CHOICE.
1. The volume ratio of hydrogen and oxygen in water is
(a) 2 : 1 (b) 1 : 2
(c) 1 : 8 (d) 8 : 1
2. How much of the Earth’s surface is covered with water ?
(a) about two-third (b) about three-fourth
(c) about one-fourth (d) about one-third
3. Pure water is an/a
(a) acidic oxide (b) amphoteric oxide
(c) basic oxide (d) none of these
4. Water is used as a coolant in an automobile because of its
(a) high density (b) high specific heat
(c) high boiling point (d) high heating capacity
5. Temporary hardness is due to the presence of
(a) calcium bicarbonate (b) calcium chloride
(c) magnesium chloride (d) magnesium sulphate
6. Both temporary and permanent types of hardness can be
removed by using
(a) washing soda (b) lime
(c) ion-exchange resin (d) hydrochloric acid
WATER 6
7. The loss of dissolved oxygen from water in water bodies is called
(a) photosynthesi
(c) de xygenation
(b) xygenation
(d) eutrophicatio
Ans. 1. (a) 2. (b) 3. (b) 4. (b) 5. (a) 6. (c) 7. (d)
7 TEACHER’S MANUAL :: The World of Science–CHEMISTRY— 8
C. FILL IN THE BLANK SPACES IN THE FOLLOWING STATEMENTS.
1. Water is an essential component of all living beings.
2. Seeds need water for germination.
3. Pure water boils at 100 °C under normal pressure.
4. Water helps in transporting vitamins and minerals from one
place to another.
5. When water is electrolysed, the gas produced at cathode is
hydrogen.
6. Density of water is maximum at 4 °C.
7. Ice at 0 °C is cooler than water at the same temperature.
8. Permanent hardness of water can be removed by adding
washing soda to it.
9. The major source of water pollution in our cities is sewage.
10. Chlorination is the process of killing bacteria present in water
by adding chlorine to it.
D. WRITE T FOR TRUE AND F FOR FALSE STATEMENT.
1. Boiling point of water is the highest of all liquids.
2. Pure water is a good conductor of electricity.
3. Water is denser than ice.
4. High specific heat of water makes it suitable for use in desert
coolers.
5. Ion-exchange method is used for removing permanent hardness
only.
6. Hard water should not be used for drinking.
7. Permanent hard water contains calcium or magnesium
bicarbonate.
8. Supersaturated solution is less concentrated than an unsaturated
solution.
Ans. 1. False 2. False 3. True 4. True 5. False
6. True 7. False 8. False
Water, a remarkable substance composed of hydrogen and oxygen, exists in three states: gaseous, liquid, and solid. Here are some key points:
Water Cycle:
The water cycle involves the circulation of water through processes like evaporation and condensation.
Water evaporates from oceans, rivers, lakes, wells, and soil due to sunlight and heat.
Transpiration occurs when plants release water vapor from their leaves.
Condensation converts water vapor back into liquid form, leading to cloud formation.
Precipitation occurs when water droplets in clouds become heavy and fall as rain, snow, or sleet1.
Three States of Water:
Liquid State: The water we use daily is in the liquid state.
Gaseous State: Water evaporates to form vapor when heated.
Availability and Interconvertibility:
Water’s easy interconvertibility between states ensures its availability across the Earth.
Evaporation from oceans, rivers, and lakes replenishes water vapor in the atmosphere1.
Importance:
Water is essential for life, dissolving many other substances.
It sustains ecosystems, supports agriculture, and provides drinking water.
Remember, understanding water’s properties and cycles helps us appreciate its significance in our lives!
