NOTES
Cynesure Institute
Class 10
2. Acid, Bases and Salts
Basic Properties of acids, bases and salts:
Acids
1. Sour in taste
Bases
1. Bitter in taste &
Salts
1. Salty or pungent
soapy to touch
2. Give H+ or H3O+
2. Give OH- ions in
ions in aqueous
Water
solutions
aqueous solutions
3. Turn litmus paper
red to blue
3. Turn litmust paper blue
to red
2. Acid + Base
= Salt +
3. Shows no colour
change in litmus
***
Types of Acids
ON THE BASIS OF ORIGIN, acids are classified as:
a. Organic acids:
 Acids derived from living organisms like plants and animals are called organic
acids.
 They are weak acids and are not harmful for living organisms.
 For example: citric acid is present in fruits like orange and lemon,
Acetic acid present in vinegar,
Oxalic acid present in tomato,
Tartaric acid present in tamarind,
Lactic acid present in sour milk and curd.
b. Mineral acids:
 They are also called inorganic acids.
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 They are dangerous and corrosive.
 Special precautions have to be taken while handling them.
 For example: sulphuric acid (H2SO4), hydrochloric acid (HCl) etc.
ON THE BASIS OF THEIR STRENGTH, acids are classified as:
a. Strong acids:
 Strong acids are those acids which completely dissociate into its ions in
aqueous solutions.
 Example: nitric acid (HNO3), sulphuric acid (H2SO4), hydrochloric acid (HCl).
b. Weak acids:
 Weak acids are those acids which do not completely dissociate into its ions in
aqueous solutions.
 For example: carbonic acid (H2CO3), acetic acid (CH3COOH).
ON THE BASIS OF THEIR CONCENTRATION, acids are classified as:
a. Dilute acids: Have a low concentration of acids in aqueous solutions. In the
solution, amount of water is more than acid.
b. Concentrated acids: Have a high concentration of acids in aqueous
solutions. In the solution, amount of water is less than acid.
***
Types of Bases
Alkalies:
 Water soluble bases are called alkalies.
 For example: Sodium hydroxide (NaOH), potassium hydroxide (KOH).
ON THE BASIS OF THEIR STRENGTH, bases are classified as:
a. Strong bases:
 Strong bases are those bases which completely dissociate into its ions in
aqueous solutions.
 Example: sodium hydroxide (NaOH), potassium hydroxide (KOH).
b. Weak bases:
 Weak bases are those bases which do not completely dissociate into its ions in
aqueous solutions.
 For example: ammonium hydroxide (NH4OH).
***
Why acids and bases are good conductor of electricity?
Acids and bases conduct electricity because they produce ions in water. There is a
flow of electric current through the solution by ions.
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Dinesh
Maurya
***
Indicators
Indicators are those chemical substances which behave differently in acidic and basic
medium and help in determining the chemical nature of the substance.
Acid base indicators indicate the presence of an acid or a base by a change in
their colour or smell.
Types of Indicators:
Indicators can be natural or synthetic.
1. Olfactory indicators: These are those indicators whose odour changes in acidic or
basic medium. Example: onion
2. Onion: Smell of onion diminishes in a base and remains as it is in an acid.
3. Vanilla essence: The odour of vanilla essence disappears when it is added to a base.
The odour of vanilla essence persists when it is added to an acid.
4. Turmeric: In acids, yellow colour of turmeric remains yellow. In bases, yellow colour of
turmeric turns red.
5. Litmus: Litmus is a natural indicator.
 Litmus solution is a purple dye which is extracted from lichen.
 Acids turn blue litmus red.
 Bases turn red litmus blue.
 Water is essential for acids and bases to change the colour of litmus paper.
 Remember that litmus paper will act as an indicator only if either the litmus paper is
moist or the acid or base is in the form of aqueous solution.
 This is because acids and bases release H+ and OH- ions respectively in aqueous
solutions.
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6. Phenolphthalein: Phenolphthalein remains colourless in acids but turn pink in bases.
7. Methyl orange: Methyl orange turns pink in acids and becomes yellow in bases.
***
Chemical Properties of Acids and Bases:
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REACTION OF ACIDS AND BASES WITH WATER:
Acids
Release H+ or H3O+ ions in water
HCl
H2O
NaOH
Dinesh
Maurya
Bases
Release OH- ions in water
H+
+
Cl-
H2O
OH-
+
Na+
***
REACTION OF ACIDS AND BASES WITH METALS:
 Metals displace hydrogen from the acids and form salt and hydrogen gas.
 This is a displacement reaction.
 So, acids react with only those metals which are placed above hydrogen in the reactivity
series so that metals can displace hydrogen from acids.
Acid +
gas
Metal
Zn
Metal
Salt +
+
H2SO4
Acid
Hydrogen gas
ZnSO4
Salt
+
H2
Hydrogen
Bases react with some metals to form salt and hydrogen gas.
Zn
+
Metal
2NaOH
Base
Na2ZnO2
+
Sodium zincate
Bases do not react with all the metals to form salt and water.
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H2
Hydrogen gas
***
REACTION OF ACIDS AND BASES WITH METAL CARBONATES:
 Acids react with metal carbonate to form salt, water and release carbon dioxide.
 Limestone, chalk and marble are different forms of calcium carbonate
Na2 CO3
+
2HCl
2NaCl
+
CO
+
H2O
Bases do not react with metal carbonates.
Metal carbonate/Metal hydrogencarbonate + Acid
dioxide + Water
Salt + Carbon
***
REACTION OF ACIDS AND BASES WITH METAL BICARBONATES:
Acids react with metal bicarbonate to form salt, water and release carbon dioxide.
NaHCO3
+
HCl
NaCl
+ CO2 + H2O
Bases do not react with metal bicarbonates.
***
Reaction Of Acids With Bases: NEUTRALISATION REACTION:
Acids react with bases to form salt and water.
Base + Acid
Salt + Water
All acids generate H+(aq) and all bases generate OH–(aq),
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HX+
MOH
H+(aq) + OH–
(aq)
MX + HOH
H2O(l)
Eg.
HCl + NaOH
NaCl + H2O
***
REACTION OF ACIDS WITH METALLIC OXIDES:
Metallic oxides are basic. Therefore, acids react with metallic oxides to form salt and
water.
Metal oxide + Acid
Salt + Water
HCl + CuO
CuCl2 + H2O
***
REACTION OF BASES WITH NON-METALLIC OXIDES:
Non-metallic oxides are acidic in nature. Bases react with non- metallic oxides to
form salt and water. Example: CO2
Ca(OH)2 + CO2
CaCO3 + H2O
1. Neutral oxides: Oxides which are neither acidic nor basic are called neutral oxides.
Example: CO
***
The process of dissolving an acid or a base in water is a highly exothermic one.
Care must be taken while mixing concentrated nitric acid or sulphuric acid with water.
The acid must always be added slowly to water with constant stirring.
If water is added to a concentrated acid, the heat generated may cause the mixture to
splash out and cause burns.
 The glass container may also break due to excessive local heating.
 Mixing an acid or base with water results in decrease in the concentration of ions
(H3O+/OH) per unit volume. Such a process is called dilution and the acid or the base is
said to be diluted.




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****
Cynesure institute
Dinesh
Maurya
 It is used to find out the strength of acids and bases i.e., how strong or weak the acid or a
pH
base is.
 p in pH stands for ‘potenz’ in German.
 The strength of acids and bases depends on the number of H+ ions and OH- ions
produced respectively.
pH scale: A scale for measuring hydrogen ion concentration in a solution is called pH
scale.
On pH scale, we measure pH from 0 to 14.
pH
Less than 7
Equal to 7
More than 7
Type of solution
Acidic
Neutral
Basic
 More the hydrogen ion (or hydronium ion) concentration, lower is the pH value.
 More the hydroxyl ion concentration, higher is the pH value.
 Acids which produce more hydrogen ions are said to be strong acids and acids which
produce less hydrogen ions are said to be weak acids.
 In other words, strong acids have a lower pH value than weak acids.
 Bases which produce more hydroxyl ions are said to be strong bases and bases which
produce less hydroxyl ions are said to be weak bases.
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 In other words, strong bases have a higher pH value than weak bases.
***
Importance of pH to Living organism:
 Living organisms are pH sensitive.
 Human body works within a pH range of 7.0 to 7.8.
 Rain water with a pH less than 5.6 is called acid rain. This acid rain if it flows into river
water makes the survival of aquatic life difficult.
 Plants also require a specific pH range of soil for their healthy growth.
pH of our digestive system:
 Our stomach produces hydrochloric acid for digestion of food.
 But during indigestion, excess of acid is produced in the stomach and therefore, the pH
decreases.
 This causes pain and irritation.
 So, to neutralise this excess acid, a mild base is used.
 This mild base works as an antacid.
 An antacid is any substance, generally a base or basic salt, which counteracts stomach
acidity.
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Tooth decay:
Dinesh
 Tooth decay starts when the pH of the mouth is lower than 5.5.
Maurya
 Tooth enamel is made up of calcium phosphate which is the hardest substance in the
body.
 It does not dissolve in water, but is corroded when the pH in the mouth is below 5.5.
 If food particles remain in the mouth after eating, bacteria present in our mouth produce
acid by degradation of sugar.
 This decreases the pH of mouth and hence tooth decay occurs.
 The best way to prevent this is to clean the mouth after eating food.
 Using toothpastes, which are generally basic, for cleaning the teeth can neutralise the
excess acid and prevent tooth decay.
Sting of Insect:
 pH is also significant as it is used in self defense by animals and plants.
 Bees use an acid called mellitin in their sting. When they sting, acid is released below the
skin which causes pain, swelling and irritation.
 To neutralise the effect, a mild base like baking soda can be used.
 Red ant bite releases formic acid in the cells of our skin, which caused pain.
Soil:




If the soil used for cultivation is more acidic or basic, plant will not grow properly.
Ph value of soil should be between 6.5 to 7.3.
Acidic soil can be neutralized by adding lime (CaO).
Basic soil can be neutralized by adding gypsum.
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***
More about Salts:
 The salt formed by the combination of hydrochloric acid and sodium hydroxide solution
is called sodium chloride.
 Seawater contains many salts dissolved in it.
 Sodium chloride is separated from these salts.
 Deposits of solid salt are also found in several parts of the world.
 These large crystals are often brown due to impurities. This is called rock salt.
 Beds of rock salt were formed when seas of bygone ages dried up.
 Rock salt is mined like coal.
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Uses of Common salt
***
Dinesh
Maurya
 Used in food to add salty taste.
 The common salt is an important raw material for various materials of daily use like
sodium hydroxide, baking soda, washing soda, bleaching powder and many more.
PREPARATION OF SOME IMPORTANT AND USEFUL COMPOUNDS
1. Sodium hydroxide (NaOH)
 Preparation of sodium hydroxide is known as Chlor Alkali process.
 In this process, electricity is passed through an aqueous solution of Sodium chloride
(called brine).
 Sodium chloride decomposes to form sodium hydroxide.
 Chlorine gas is formed at the anode, and hydrogen gas at the cathode.
 Sodium hydroxide solution is formed near the cathode.
2NaCl(aq) + 2 H2O (l)
2NaOH(aq) + Cl2(g) + H2(g)
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2. Bleaching powder (CaOCl2):
Bleaching powder is represented as CaOCl2, though the actual composition is quite
complex.
Bleaching powder is produced by the action of chlorine on dry slaked lime.
Ca(OH)2




+
Cl2
CaOCl2
Uses:
for bleaching cotton and linen in the textile industry
for bleaching wood pulp in paper factories
for bleaching washed clothes in laundry
as an oxidising agent in many chemical industries
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+
H2O
 for disinfecting drinking water to make it free of germs.
3. Baking soda(NaHCO3):
 Sodium hydrogen carbonate (NaHCO3) is known as baking soda.
 A mixture of common salt, water, carbon dioxide and ammonia is taken.
 When the mixture reacts, it forms ammonium chloride and baking soda.
NaCl
+
H2O
+
CO2 +
NH3
NH4Cl
+
NaHCO3
Uses
 For making baking powder, which is a mixture of baking soda (sodium
hydrogencarbonate) and a mild edible acid such as tartaric acid.
 When baking powder is heated or mixed in water, the following reaction takes place –
NaHCO3 + H+
CO2 + H2O + Sodium salt of acid
Carbon dioxide produced during the reaction causes bread or cake to rise making
them soft and spongy.
 Sodium hydrogencarbonate is also an ingredient in antacids. Being alkaline, it
neutralises excess acid in the stomach and provides relief.
 It is also used in soda-acid fire extinguishers.
4. Washing soda (Na2CO3 . 10H2O):
 Sodium carbonate (Na2CO3 . 10H2O) is known as washing soda.
 In the first step, sodium carbonate is obtained by heating baking soda.
2NaHCO3
Na2CO3 + H2O + CO2
Then washing soda is produced by recrystallisation of sodium carbonate.
Na2CO3
+
10H2O
Na2CO3 .10H2O
Water of crystallisation: It is the fixed number of water molecules present in one
formula unit of a salt.
Uses:




Sodium carbonate (washing soda) is used in glass, soap and paper industries.
It is used in the manufacture of sodium compounds such as borax.
Sodium carbonate can be used as a cleaning agent for domestic purposes.
It is used for removing permanent hardness of water.
5. Plaster of Paris (CaSO4. ½ H2O):
 Calcium sulphate hemihydrates (CaSO4. ½ H2O) is known as plaster of paris.
 Plaster of Paris is prepared by heating Gypsum at 373K.
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CaSO4. 2H2O
CaSO4. ½ H2O + 1½ H2O
Uses: Plaster of Paris is used for making toys, materials for decoration and for
making surfaces smooth.
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