Two
nails are fitted on a cork and are kept it
in a 100 mL beaker. The nails are then connected to the two terminals
of a 6-volt battery through a bulb and a switch. Some dilute HCl is
poured in the beaker and the current is switched on. The same
experiment is then performed with glucose solution and alcohol
solution.
Observations:
It will be
observed that the bulb glows in the HCl solution and does not glow in
the glucose solution.
Result:
HCl
dissociates into H+
and Cl−
ions. These ions conduct electricity in the solution resulting in the
glowing of the bulb. On the other hand, the glucose solution does not
dissociate into ions. Therefore, it does not conduct electricity.
Conclusion:
From this
activity, it can be concluded that all acids contain hydrogen but not
all compounds containing hydrogen are acids.
That is
why, though alcohols and glucose contain hydrogen, they are not
categorised as acids.
Question 7:
Why does distilled water not conduct electricity, whereas rain water
does?
Distilled
water is a pure form of water and is devoid of any ionic species.
Therefore, it does not conduct electricity. Rain water, being an
impure form of water, contains many ionic species such as acids and
therefore it conducts electricity.
Question 8:
Why do acids not show acidic behaviour in the absence of water?
Acids do
not show acidic behaviour in the absence of water because the
dissociation of hydrogen ions from an acid occurs in the presence of
water only. It is the hydrogen ions that are responsible for the
acidic behaviour.
Question 9:
Five
solutions A, B, C, D and E when tested with universal indicator
showed pH as 4, 1, 11, 7 and 9, respectively. Which solution is
(a) neutral?
(b) strongly
alkaline?
(c) strongly
acidic?
(d) weakly
acidic?
(e) weakly
alkaline?
Arrange
the pH in increasing order of hydrogen-ion concentration.
(a) Neutral →
Solution D with pH 7
(b) Strongly
alkaline →
Solution C with pH 11
(c) Strongly
acidic →
Solution B with pH 1
(d) Weakly
acidic →
Solution A with pH 4
(e) Weakly
alkaline →
Solution E with pH 9
The pH can be arranged
in the increasing order of the concentration of hydrogen ions as: 11
< 9 < 7 < 4 < 1
Question 10:
Equal
lengths of magnesium ribbons are taken in test tubes A and B.
Hydrochloric acid (HCl) is added to test tube A, while acetic acid
(CH3COOH)
is added to test tube B. In which test tube will the fizzing occur
more vigorously and why?
The
fizzing will occur strongly in test tube A, in which hydrochloric
acid (HCl) is added. This is because HCl is a stronger acid than
CH3COOH
and therefore produces hydrogen gas at a faster speed due to which
fizzing occurs.
Question 11:
Fresh milk
has a pH of 6. How do you think the pH will change as it turns into
curd? Explain your answer.
The pH of
milk is 6. As it changes to curd, the pH will reduce because curd is
acidic in nature. The acids present in it decrease the pH.
Question 12:
A milkman adds a very small amount of baking soda to fresh milk.
(a) Why
does he shift the pH of the fresh milk from 6 to slightly alkaline?
(b) Why
does this milk take a long time to set as curd?
(a) The milkman shifts the pH of the fresh milk from 6 to slightly
alkaline because in alkaline condition, milk does not set as curd
easily.
(b) Since this milk is slightly basic than usual milk, acids produced
to set the curd are neutralized by the base. Therefore, it takes a
longer time for the curd to set.
Question 13:
Plaster of Paris
should be stored in a moisture-proof container. Explain why?
Plaster of Paris
(POP) should be stored in a moisture-proof container because Plaster
of Paris, a powdery mass, absorbs water (moisture) to form a hard
solid known as gypsum.
Question 14:
What is a neutralization reaction? Give two examples.
A reaction in which an acid and base react with each other to give
a salt and water is termed as neutralization reaction. In this
reaction, energy is evolved in the form of heat.
For example:
(i)
(ii) During indigestion (caused due to the production of excess of
hydrochloric acid in the stomach), we administer an antacid
(generally milk of magnesia,
which
is basic in nature). The antacid neutralizes the excess of acids and
thus gives relief from indigestion.
Question 15:
Give two
important uses of washing soda and baking soda.
Two important uses of washing soda and baking soda are as follows:
(1) Washing soda:
(a) It is used in glass, soap, and paper industries.
(b) It is used to remove permanent hardness of water.
(2) Baking soda:
(a) It is used as baking powder. Baking powder is a mixture of
baking soda and a mild acid known as tartaric acid. When it is heated or
mixed in water, it releases CO2 that makes bread or cake fluffy.
(b) It is used in soda-acid fire extinguishers.