ACID, BASES AND SALTS
ACID-BASE INDICATORS:- acid-base indicators are substances
that indicate the presence of acid or base in a solution.
Neutral colour
|
Colour with acids
|
Colour with base
|
|
Litmus paper
|
purple
|
red
|
blue
|
Methyl orange
|
orange
|
red
|
Yellow
|
phenolphthalein
|
colourless
|
colourless
|
pink
|
1.
Why should curd
and sour substances not be kept in brass and copper vessels?
Ans:- Curd and other sour substances contain acids. Therefore,
when they are kept in brass and copper vessels, the metal reacts with the acid
to liberate hydrogen gas and harmful products, thereby spoiling the food.
2.
Metal compound A reacts
with dilute hydrochloric acid to produce effervescence. The gas evolved
extinguishes a burning candle. Write a balanced chemical equation for the
reaction if one of the compounds formed is calcium chloride.
3.
Why do HCl, HNO3,
etc., show acidic characters in aqueous solutions while solutions of compounds
like alcohol and glucose do not show acidic character?
Ans:-The
dissociation of HCl or HNO3 to form hydrogen ions always occurs in
the presence of water. Hydrogen ions (H+) combine with H2O
to form hydronium ions (H3O+).
The reaction is as follows:
Although aqueous solutions of glucose and alcohol contain
hydrogen, these cannot dissociate in water to form hydrogen ions. Hence, they
do not show acidic character.
4.
Why does an aqueous
solution of an acid conduct electricity?
Ans:-Acids
dissociate in aqueous solutions to form ions. These ions are responsible for
conduction of electricity.
5.
Why does dry HCl gas not
change the colour of the dry litmus paper?
Ans:-Colour
of the litmus paper is changed by the hydrogen ions. Dry HCl gas does not
contain H+ ions. It is only in the aqueous solution that an acid
dissociates to give ions. Since in this case, neither HCl is in the aqueous
form nor the litmus paper is wet, therefore, the colour of the litmus paper
does not change.
6.
While diluting an acid,
why is it recommended that the acid should be added to water and not water to
the acid?
Ans:-Since
the process of dissolving an acid in water is exothermic, it is always
recommended that acid should be added to water. If it is done the other way,
then it is possible that because of the large amount of heat generated, the
mixture splashes out and causes burns.
7.
How is the concentration
of hydronium ions (H3O+) affected when a solution of an
acid is diluted?
Ans:-When
an acid is diluted, the concentration of hydronium ions (H3O+)
per unit volume decreases. This means that the strength of the acid decreases.
8.
How is the concentration
of hydroxide ions (OH−) affected when excess base is dissolved in a
solution of sodium hydroxide?
Ans:-The
concentration of hydroxide ions (OH−) would increase when excess
base is dissolved in a solution of sodium hydroxide.
9.
Under what soil condition
do you think a farmer would treat the soil of his fields with quick lime
(calcium oxide) or slaked lime (calcium hydroxide) or chalk (calcium
carbonate)?
Ans:-If
the soil is acidic and improper for cultivation, then to increase the basicity
of soil, the farmer would treat the soil with quick lime or slaked lime or
chalk.
10.
Write an equation to show
the reaction between Plaster of Paris and water.
Why
plaster of paris kept in moisture-proof containers.
Ans:-The
chemical equation for the reaction of Plaster of Paris and water can be
represented as
11.
Why does distilled water
not conduct electricity, whereas rain water/tap water does?
Ans:-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.
12.
Why do acids not show
acidic behaviour in the absence of water?
Ans:-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.
13.
Fresh milk has a pH of 6.
How do you think the pH will change as it turns into curd? Explain your answer.
Ans:-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.
14.
Plaster of Paris should
be stored in a moisture-proof container. Explain why?
Ans:-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.
(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.
15.
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?
Ans:- (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) Why does this milk take a long time to set as curd?
Ans:- (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.
10 mL of a solution of NaOH is found to be completely
neutralised by 8 mL of a given solution of HCl. If we take 20 mL of the same
solution of NaOH, the amount of HCl solution (the same solution as before)
required to neutralise it will be
(a) 4 mL (b) 8mL (c) 12 mL (d) 16 mL
(d) 16 mL of HCl solution
will be required
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