Notes on NCERT Science Class 10 Ch 2 Acids, Bases and Salts

Class 10 Science Note Acids Bases and Salts

1. Acids, Bases and Salts
1) Acids are sour in taste, turn blue litmus red, and dissolve in water to
release H
+
ions
HCl – Hydrochloric Acid, H
2SO
4 – Sulphuric Acid, HNO
3 – Nitric Acid.

Acids may be classified as:
Organic Acids Mineral Acids
Acids which are derived from plants and
animals, they are known as Organic
Acids.
Mineral acids are inorganic
acids
Example, Citric Acid, Tartaric acid, oxalic
acid, acetic acid.
Sulphuric Acid, Hydrochloric
acid, Nitric Acid.

Strong Acids Weak Acids
Strong acid is an acid that
completely dissociates into ions in
aqueous solutions.
Weak acid is the one which does
not dissociate completely into ions
in aqueous solutions
Example: Sulphuric Acid,
Hydrochloric Acid.
Example: Acetic Acid.


Acids can also be as Dilute Acid and Concentrated Acids. The one which
has low concentration of acids in aqueous solution, they are known as
Dilute Acids whereas the one which has high concentration of acids in
aqueous solution, are known as Concentrated Acids.

Dilution of Acids: It is advisable to add acid to water and not vice versa
because large amount of heat is released if water is added to acid. This
released heat is large enough to cause harm.

2) Bases are bitter in taste, have soapy touch, turn red litmus blue and
give hydroxide ions in solution. e.g. NaOH, KOH etc.
Bases produce hydroxide ions in aqueous solutions. Bases which are water
soluble are known asAlkalis.

Bases are classified as Strong Base and Weak Base. Strong base is the one
which dissociates completely into its ions in aqueous solution. Example:
NaOH.

Weak base is the one which does not dissociate completely into its ions in
aqueous solutions.Example: Ammonium Hydroxide,NH4OH

3) A salt is a compound which is formed by neutralization reaction
between an acid and base. e.g. sodium chloride.

4) Indicators are substances that which indicate acidic or basic nature of
the solution using colour change. For Example, litmus solution, methyl
orange, phenolphthalein, etc
Acids convert blue litmus paper red in colour. Bases turn red litmus blue.
Phenolphthalein remains colourless in presence of acids but turn pink in
presence of bases.
Olfactory indicators: Substances whose odour (smell) changes in acidic or
basic media are called olfactory indicators. Vanilla, Onion, Clove can be
used as olfactory indicators.

5) Chemical properties of acids:
i) Acids react with active metals to give hydrogen gas.
Zn + H2SO4  ZnSO4 + H2

ii) Acids react with metal carbonate and metal hydrogen carbonate to give
carbon dioxide.
NaHCO3 +HCl  NaCl + H2O + CO2

iii) Acids react with bases to give salt and water. This is called as
neutralization reaction.
NaOH + HCl  NaCl +H2O

iv) Acids react with metals oxides to give salt and water.
CuO + H2SO4  CuSO4 + H2O

2. 6) Chemical properties of Bases:
i) Reaction with Metals: Certain reactive metals such as Zinc, Aluminium,
and Tin react with alkali solutions on heating and hydrogen gas is evolved.
2NaOH + Zn  Na2ZnO2 +H2

ii) Reaction with acids: Bases react with acids to form salt and water.
KOH +HCl  KCl +H2O

iii) Reaction with Non -metallic oxides: These oxides are generally acidic
in nature. They react with bases to form salt and water.
2NaOH + CO2  Na2CO3 + H2O

7) pH Scale: The concentration of hydrogen ions in solution is expressed in
terms of pH. The pH of a solution is defined as the negative logarithm of
hydrogen ion concentration in moles per litre. pH = - log [H
+
]

For water or neutral solutions, pH = 7, For acids pH < 7 For
bases pH > 7

Importance of pH
 Human body works at a pH of about 7.0 to 7.8.
Stomach has a pH of about 2 due to presence of hydrochloric acid in it. It
is needed for the activation of pepsin protein required for protein
digestion.

When we eat food containing sugar, then the bacteria present in our
mouth break down the sugar to form acids. This acid lowers the pH in the
mouth. Tooth decay starts when the pH of acid formed in the mouth falls
below 5.5. This is because then the acid becomes strong enough to attack
the enamel of our teeth and corrode it. This sets in tooth decay. The best
way to prevent tooth decay is to clean the mouth thoroughly after eating
food.

 Soil pH and plant growth: Most of the plants grow best when the pH of the
soil is close to 7. If the soil is too acidic or basic, the plants grow badly or do not
grow at all. The soil pH is also affected by the use of chemical fertilizers in the
field. If the soil is too acidic then it is treated with materials like quicklime or
slaked lime

8) Some Important Chemical Compounds:
Salts: When acid and base neutralize, salts are formed. Strong acid and strong
base combines to form neutral salt.
Strong acid + Strong base →Neutral salt
NaOH + HCl → NaCl + H
2O
Formation of Neutral Salt

Strong acid and weak base combine to form acidic salt.
Example: Hydrochloric Acid and ammonium hydroxide combine to form
ammonium chloride. Other examples, sodium hydrogen carbonate, sodium
hydrogen sulphate etc.
Strong acid + Weak base →Acidic salt
HCl + NH
4OH → NH
4Cl + H
2O
Formation of Acidic Salt

Similarly, weak acid and strong base combine to form basic salt.
Example: Acetic Acid and sodium hydroxide combine to form sodium acetate.
Other examples are calcium carbonate, potassium cyanide etc.
Weak acid + Strong base →Basic salt
CH
3COOH + NaOH → CH
3COONa + H
2O
Formation of Basic Salt
a) Common Salt (NaCl)
Sodium chloride is known as common salt. Its main source is sea water.
It is also exists in the form of rocks and is called rock salt.
It is used for preparing sodium hydroxide, baking soda, washing soda etc.

3. b) Sodium Hydroxide or Caustic Soda (NaOH)
 It is prepared by passing electricity through an aqueous solution
of sodium chloride also known as brine. This process is known as chlor-
alkali process.
2NaCl (aq) + 2 H
2O (l)  2NaOH (aq) + Cl
2
(g) + H
2
(g)

Properties: 1. It is white translucent solid.
2. It is readily soluble in water and gives a strong alkaline solution.

c) Bleaching Powder (CaOCl2)
Its chemical name is calcium oxychloride. It is prepared by passing chlorine gas
through dry slaked lime.
Ca(OH) 2 + Cl2  CaOCl2 + H2O

Uses – 1. For bleaching cotton and linen in textile industry and wood pulp in
paper industry
2. For disinfecting drinking water.

d) Baking Soda (NaHCO3): Chemical name is Sodium hydrogen carbonate.
It is prepared by passing CO2 gas through brine solution saturated with
ammonia.
NaCl + H2O +CO2 +NH3  NH4Cl + NaHCO3
Properties:
1. It is white crystalline solid and sparingly soluble in water at room
temperature.
2. On heating it decomposes to give sodium carbonate and carbon
dioxide.
3. It reacts with acids to give carbon dioxide gas.
4. It aqueous solution is weak alkaline due to hydrolysis.

Uses: 1. It is used in soda – acid fire extinguisher.
2. It acts as mild antiseptic and antacid.
3. It is used as a component of baking powder. In addition to sodium
hydrogen carbonate baking soda contains tartaric acid.

e) Washing Soda (Na2CO3.10 H2O)
Chemical name is sodium carbonate decahydrate. [Deca means ten, hydrate -
water]
It is prepared by heating baking soda. Recrystallisation of sodium carbonate gives
washing soda.
2NaHCO3
 Na2CO3
+H2O +CO2
Na
2CO
3 +10 H
2O  Na
2CO
3.10 H
2O
Uses:
1. It is used for removing permanent hardness of water.
2. It is used in glass, soap and paper industries.
3. It can be used as a cleaning agent for domestic purposes.

f) Plaster of Paris (CaSO
4.
1
2
H
2O)
Its chemical name is calcium sulphate hemihydrates. It is obtained by heating
Gypsum upto 373K.
CaSO4.2H2O  CaSO4.
1
2
H2O + 1
1
2
H2O

On treatment with water it is again converted into gypsum and sets as a hard
mass.
CaSO
4∙
1
2
H2O + 1
1
2
H2O  CaSO
4
.2H
2O

Uses: 1. It is used by doctors for setting fractured bones.
2. It is used for making statues, models and other decorative materials.

 Water of crystallisation is a fixed number of water molecules present in one
formula unit of a salt.
When hydrated salts are heated strongly, they lose their water of crystallisation.
On strong heating, blue copper sulphate crystals turn white (due to the loss of
water of crystallisation).
CuSO
4∙5H
2O
Hydrated copper sulphate
(Blue)

Heat
CuSO
4
Anhydrous Copper Sulphate
(White)
+5H
2O
Examples:
 One unit of copper sulphate contains five water molecules (CuSO
4.5H
2O).
 Sodium carbonate crystals contains 10 molecules of water of
crystallisation(Na
2CO
3.10H
2O)