Karnataka State Education Department Subject Science 8th standard Chemicals in our daily life 2

1. Ptd By:
Raghavendra D R

2. You know that, water is one of the most
abundant and important compound in the world. It is
the major constituent of all living organisms. 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.

3. Water is made up of hydrogen and oxygen. One
molecule of water contain two atoms of hydrogen and
one atom of water which are chemically combined.
From time immemorial, water been the driving
force of every civilization and people attached great
importance to an adequate supply of water for different,
purpose like agriculture, cooking, drinking, washing, etc.
In ayurveda water has been given an important
place. Water is used to treat common problems. It is
called hydrotherapy.

4. Traditional methods of obtaining potable water
include the following. Boiling, making sunlight fall on
water, adding tulsi leaves, camphor, dropping red hot
iron balls.
There are methods of removal of colour, odour,
suspended matter and microbes from the water. Also
there are methods of removal of hardness. Water
treatments include stages of coagulation or
sedimentation, filtration, decolourisation and
sterilisation. Alum is used for sedimentation. Some of
these practices have been used in ancient civilization
also.

5. You know that chemical formula of water is
H2O. In a water molecule one atom of oxygen is
bound to two atoms of hydrogen as shown in fig.
Water is called the “Universal Solvent” because it
dissolves more substance than any other solvents.

6. we store water in different metallic containers.
We also boil water in metallic containers. Does water
react with the metallic container?
Metals like copper, aluminum and tin show no
action with water at ordinary temperature.
Many metals and their alloys are used to
prepare utensils, buckets, kitchenware and other
articles, because they do not react with water under
normal conditions.

7. Sodium and potassium are soft metals and can be
cut with a knife. They react with water, forming their
respective hydroxide liberating hydrogen gas.

8. Note:
Sodium is to be kept under kerosene as it can
react with oxygen and water vapours present in air. The
metal should always be handled with a pair of tongs
because it can even react with moisture on the hand and
cause blisters. Kerosene has no affinity towards sodium.

9. Metals like sodium, potassium react with cold
water forming respective hydroxides liberating
hydrogen gas.
(Sodium Hydroxide)

10. Other metals also react with water at different
conditions.
(Calcium Hydroxide)
(Magnesium hydroxide)
• Room Temperature
• Boiling water

11. When steam is passed over red hot metals like
zinc, iron, magnesium we get respective oxides and
hydrogen gas.

12. 1. When steam is passed over red hot coke, Mixture
of carbon monoxide and hydrogen is formed.
• As an industrial fuel.
• Used in the manufacture of hydrogen

13. 2. When steam is passed over red hot silicon,
which is a non metal, silicon dioxide and
hydrogen gas are formed.

14. Generally when a non metallic oxide dissolves in
water, acid is formed.
Carbonic acid is present in soft drinks.

15. When nitrogen dioxide, and sulphur dioxide
present in the atmosphere of certain industrial
regions, dissolve in rain water, they cause acid rain.

16. When calcium oxide reacts with water it forms
calcium hydroxide.
you might have seen this reaction when a paste for
white washing is being prepared.

17. There are many substances which absorb
moisture such as magnesium chloride, solid sodium
hydroxide, solid potassium hydroxide, anhydrous
calcium chloride, calcium nitrate and Ferric chloride.
Certain water-soluble substances, when
exposed to the atmosphere at ordinary temperature,
absorb moisture from the atmospheric air to become
moist and ultimately dissolve in the absorbed water.
Such substances are called deliquescent substances
and the phenomenon is called deliquescence.

18. Certain hydrates lose water when exposed to air
of moderate humidity. An example is sodium sulphate
. The loss of water from hydrated
compounds to the surroundings is called
efflorescence. Commonly used washing soda is
another example for efflorescent substance.

19. Water is a very common and suitable solvent.
Some impurities dissolve in it very easily. Mostly
soluble salts of Calcium and magnesium make water
hard. As the percentage of calcium and magnesium
increases in water, the degree of hardness also
increases.

20. Hard water does not produce much lather with
soap. Generally water from regions having limestone
and other mineral rocks is hard water. Calcium and
magnesium bicarbonates, sulphates and chlorides in
general cause hardness in water.
The salts causing hardness are water soluble.
Calcium carbonate does not cause hardness because it
is insoluble in water.

21. Soft water produces a rich lather with soap.
Rain water and spring water are examples of soft
water, since they contain small amount of dissolved
salts in them.

22. Hardness of water can be removed by two
methods:-
• If hard water contains Calcium and Magnesium
bicarbonates, it can be made soft by boiling.
In this reaction, Calcium bicarbonate decomposes
to insoluble calcium carbonate and carbon
dioxide.
Example :

23. • 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.
Water, so obtained is soft water. But it is unfit
for drinking as it contains excess of washing soda and
excess of sodium salts.

24. Desalinisation is an artificial process by which
saline water ( generally sea water) is converted in to
fresh water. It is suitable for human consumption.
Sometimes the process produces table salt as a by-
product. This method was invented during second
world war.

25. So Dear Students today you have studied about
chemical properties of water, purification of water, action
of water on metals and non-metals, reaction of water with
metallic and non-metallic oxides. You have also studied
about deliquescent substances, efflorescent substances
and about hard water and soft water. The methods of
removal of hardness of water. In addition to that you have
studied about Desalanisation i.e., conversation of sea
water into fresh water.