Q1.
Define (i) Acid (ii) Bases
(iii) Salt
Ans: 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–
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from acid has been replaced by a metal.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 anon 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.
Ans:
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?
Ans:
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’?
Ans:
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.
Ans:
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.
Ans:
Sodium
hydroxide (NaOH),
Baking
soda (NaHCO3),
Bleaching
powder (CaOCl2), etc.
Washing
soda (Na2CO3.10H2O),
Q7.
What are strong and weak
acids?
Ans:
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’?
Ans:
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
Q9.
What is Universal Indicator?
Ans:
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.
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