1: Which gas is usually liberated when an acid
reacts with a metal? Illustrate with an example. How will you test for the
presence of this gas?
Ans: Hydrogen gas is
usually liberated when an acid reacts with a metal.fig 2.1 page 19
Take few pieces of
zinc granules and add 5 ml of dilute H2SO4. Shake it and
pass the gas produced into a soap solution. The bubbles of the soap solution
are formed. These soap bubbles contain hydrogen gas.
2H2SO4 + Zn → Zn(SO4)2 + 2H2
We can test the
evolved hydrogen gas by its burning with a pop sound when a candle is brought
near the soap bubbles.
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.
CaCO3 (s) + HCI(aq) →
CaCI2(aq)
+ CO2 + H2 O (I)
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+).
HCI
--------> H+ + CI – H+ + H2 O --> H3 O+
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.
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