Q1.
Define (i) Acid (ii) Bases (iii) Salt
Acid:
A substance which turns blue litmus to red sour in taste and liberates hydrogen
ions in aqueous solution. Metals react with acids to form salts and liberate
hydrogen gas [Most metals don’t react with Nitric Acid to liberate hydrogen as
it is highly oxidizing in nature--- Mg and Mn reacts with Nitric Acid to
liberate hydrogen] .Many acids are corrosive such as, HNO3, H2SO4,
HCl, etc.
Base:
A substance that are bitter in taste and change the colour of red litmus to
blue .Bases reacts with an acid to form a salt and water only. If dissolved in
water, they give hydroxyl ions (OH–) ions.
Salt: A chemical compound formed when the hydrogen
from acid has been replaced by a metal. A salt is also produced when an acid reacts with a base in
neutrilisation reaction. Salts are named according to anion or cation they are
formed of.. For example, Sodium salts or Chloride salts .
Q2.
Why is it advised to clean mouth after
consuming food? or
pH
change is a cause for toot decay. Explain.
Acid is produced in the mouth due to the
degradation of remains of sugar and food particles after eating, by certain
bacteria. As a result the pH of mouth is lowered. Tooth enamel [ made up of
calcium phosphate- Hardest substance in our body] gets corroded when the pH in
the mouth lowered than 5.5 . So, tooth decay starts when the pH of the
decreases below 5.5
Therefore,
it is advised to clean the mouth using basic material e.g toothpastes and mouth wash.
Q3.
How sodium hydroxide is produced? or
What
is ‘Chlor-alkali’ process? or
What
do we obtain by electrolysis of brine?
Sodium hydroxide is produced by electrolysis [
passing of electricity] of aqueous solution of sodium chloride (NaCl) [ called
Brine ]. Electrolysis of brine results in the decomposition of NaCl and
formation of NaOH.
2NaCl
(aq) + 2H2O(liq) → 2NaOH (aq) + Cl2(g) + H2(g)
The
process is named ‘chlor-alkali’ process because the products formed -
chlor for chlorine and alkali for sodium hydroxide. Chlorine is given off at
the anode while hydrogen at cathode.
Q4.
What is ‘water of crystallization’?
Water of recrystallization is a fixed number
of water molecules present in one formula unit of salt. These are called
hydrated salt. For example,
(Na2CO3.10H2O) → 10 water molecules present as water of
crystallization.
CuSO4.5H2O → 5 water molecules present as
water of crystallization.
CaSO4.2H2O → 2 water molecules present as
water of crystallization.
The
water of crystallization remain in chemical combination with crystal. It
doesn’t make the crystal wet . It is essential for the maintenance of
crystalline properties of the crystal. such as its shape and colour .It can be
removed by sufficient heat. By loosing water molecules the crystal looses its
colour and shape as well.
Q5.
What are hydrated salts? Give an example.
Salts containing a fixed number of water
molecules in their crystal structure are called hydrated salts.
A
molecule of sodium carbonate (Na2CO3.10H2O)
contains ten molecules of water. This is known as hydrated salt of sodium
carbonate [ called Washing Soda].
Q6.
Name some chemicals obtained by using sodium
chloride (common salt) as a raw material.
Sodium
hydroxide (NaOH),
Baking
soda (NaHCO3),
Bleaching
powder (CaOCl2), etc.
Washing
soda (Na2CO3.10H2O),
Q7.
What are strong and weak acids?
acids that give rise to more H+ ions are
said to be strong acids and vice-versa acids. A strong acid has pH value closer
to zero while acids with higher pH values or closer to 7 are weak acids.
Q8.
What is ‘Plaster of Paris’?
Q9.
when
gypsum is heated at 3730 K, it loses some of its water molecules and becomes
calcium sulphate hemihydrates (CaSO4.1/2 H2O). This is
called ‘Plaster of Paris’.
CaSO4.2 H2O)
→ CaSO4.1/2 H2O + 3/2 H2O
Q10.
What is Universal Indicator?
Universal Indicator [ UI ] is a mixture of several indicators. The
universal indicator shows different colours at different concentration of
hydrogen ions in a solution. Hence with the help of a UI we can judge how strong a given acid or base
is.
Q11.
Why do HCl, HNO3, etc. show acidic
characters in aqueous solutions while solutions of compounds like alcohol and
glucose do not show acidic character?
HCl and HNO3 etc. disintegrate
in presence of water to form hydrogen ions (H+). Since hydrogen ions can’t
exist alone they combine with H2O to form hydronium ions (H3O+).
The reaction can be given as follows:
HCl
→ H+ + Cl–
H+ +
H2O → H3O+
Because
of this property HCl and HNO3 show acidic character in aqueous
solutions. On the other hand, alcohol and glucose can not disintegrate or dissociate
in water to form hydrogen ions. Hence, they do not show acidic character.
Additional
Information: Also these (alcohol and glucose) are not capable of conducting
electricity as these don’t produces ions which are essential for conduction of
electricity.
Q12.
Why
does an aqueous solution of acid conduct electricity?
When dissolved in water, acids disintegrate to
form ions e.g.,
HCl
+ H2O → Cl– + H3O+
These
ions are responsible for electrical conductivity.
Q13.
Why does dry HCl gas not change the colour of
the dry litmus paper?
We know that the colour of the litmus is
changed by H+ ions of an acid. Dry HCl does not disintegrate to
give H+ ions. It is only in the aqueous medium that an acid disintegrate
to give ions. Since both HCL and litmus paper don’t contain water the colour of
litmus paper does not change.
