Chapter 2: Acids, Bases and Salts

Question 1

Classification of matter

Answer 1

On the basis of
a) composition - elements, compounds and mixtures
b) state - solids, liquids and gases
c) solubility - suspensions, colloids and solutions

Types of mixtures - homogeneous and heterogeneous

Types of compounds - covalent and ionic

Question 2

Ionisable and non-ionisable compounds

Answer 2

An ionisable compound when dissolved in water or in its molten state, dissociates into ions
almost entirely. Example: NaCl, HCl, KOH, etc.

A non-ionisable compound does not dissociate into ions when dissolved in water or in its
molten state. Example: glucose, acetone, etc.

Question 3

Examples of Acids

Answer 3

Hydrochloric acid (HCl)
Sulphuric acid (H2SO4)
Nitric acid (HNO3)

Question 4

Examples of Bases

Answer 4

Sodium hydroxide (NaOH)
Potassium hydroxide (KOH)
Calcium hydroxide (Ca(OH)2)

Question 5

Physical test

Answer 5

Given are two possible physical tests to identify an acid or a base.

Question 6

a. Taste

Answer 6

An acid tastes sour whereas a base tastes bitter.
The method of taste is not advised as an acid or a base could be contaminated or corrosive.

Question 7

b. Effect on indicators by acids and bases

Answer 7

An indicator is a chemical substance which shows a change in its physical properties,
mainly colour or odour when brought in contact with an acid or a base.
Below mentioned are commonly used indicators and the different colours they exhibit:
a) Litmus
In neutral solution - purple
In acidic solution - red
In basic solution - blue
Litmus is also available as strips of paper in two variants - red litmus and blue litmus.
An acid turns a moist blue litmus paper to red.
A base turns a moist red litmus paper to blue.

b) Methyl orange
In neutral solution - orange
In acidic solution - red
In basic solution - yellow

c) Phenolphthalein
In neutral solution - colourless
In acidic solution - remains colourless
In basic solution - pink

Question 8

Reactions of acids and bases:
a) Reaction of acids and bases with metals

Answer 8

Acid + active metal → salt + hydrogen + heat
2HCl + Mg → MgCl2 + H2(↑)
Base + metal → salt + hydrogen + heat
2NaOH + Zn → Na2ZnO2 + H2(↑)
A more reactive metal displaces the less reactive metal from its base.
2Na + Mg(OH)2 → 2NaOH + Mg

Question 9

b) Reaction of acids with metal carbonates and bicarbonates

Answer 9

Acid + metal carbonate or bicarbonate → salt + water + carbon dioxide.
2HCl + CaCO3 → CaCl2 + H2O + CO2
H2SO4 + Mg(HCO3)2 → MgSO4 + 2H2O + 2CO2
Effervescence indicates liberation of CO2 gas.

Question 10

c) Neutralisation reaction

Answer 10

1. Reaction of metal oxides and hydroxides with acids
   Metal oxides or metal hydroxides are basic in nature.
   Acid + base → salt + water + heat
   H2SO4 + MgO → MgSO4 + H2O
   2HCl + Mg(OH)2 → MgCl2 + 2H2O
2. Reaction of non-metal oxides with bases
   Non-metal oxides are acidic in nature
   Base + Non-metal oxide → salt + water + heat
   2NaOH + CO2 → Na2CO3 + H2O

Question 11

Water

Answer 11

Acids and bases in water
When added to water, acids and bases dissociate into their respective ions and help in
conducting electricity.

Question 12

Difference between a base and an alkali

Answer 12

Base-
Bases undergo neutralisation reaction with acids.
They are comprised of metal oxides, metal hydroxides, metal carbonates and metal
bicarbonates.
Most of them are insoluble in water.

Alkali -
An alkali is an aqueous solution of a base, (mainly metallic hydroxides).
It dissolves in water and dissociates to give OH− ion.
All alkalis are bases, but not all bases are alkalis.

Question 13

Hydronium ion

Answer 13

Hydronium ion is formed when a hydrogen ion accepts a lone pair of electrons from the
oxygen atom of a water molecule, forming a coordinate covalent bond.

Question 14

Dilution

Answer 14

Dilution is the process of reducing the concentration of a solution by adding more solvent (usually water) to it.

It is a highly exothermic process.
To dilute an acid, the acid must be added to water and not the other way round.

Question 15

Strength of acids and bases

Answer 15

Strong acid or base: When all molecules of given amount of an acid or a base dissociate completely in water to furnish their respective ions, H+(aq) for acid and OH−(aq) for base).

Weak acid or base: When only a few of the molecules of given amount of an acid or a base dissociate in water to furnish their respective ions, H+(aq) for acid and OH−(aq) for base).

Dilute acid: contains less number of H+(aq) ions per unit volume.
Concentrated acid: contains more number of H+(aq) ions per unit volume.

Question 16

Universal indicator

Answer 16

A universal indicator has pH range from 0 to 14 that indicates the acidity or alkalinity of a
solution.
A neutral solution has pH=7pH
pH = −log10[H+]
In pure water, [H+] = [OH−] = 10−7 mol/L. Hence, the pH of pure water is 7.

The pH scale ranges from 0 to 14.

If pH < 7 - acidic solution
If pH > 7- basic solution

Question 17

Importance of pH in everyday life

Answer 17

1. pH sensitivity of plants and animals:
   Plants and animals are sensitive to pH. Crucial life processes such as digestion of food,
   functions of enzymes and hormones happen at a certain pH value.
2. pH of a soil:
   The pH of a soil optimal for the growth of plants or crops is 6.5 to 7.0.
3. pH in the digestive system:
   The process of digestion happens at a specific pH in our stomach which is 1.5 - 4.
   The pH of the interaction of enzymes, while food is being digested, is influenced by HCl in our stomach.
4. pH in tooth decay:
   Tooth decay happens when the teeth are exposed to an acidic environment of pH
   5.5 and below.
5. pH of self-defense by animals and plants:
   Acidic substances are used by animals and plants as a self-defense mechanism. For example bee and plants like nettle secrete a highly acidic substance for self-defense. These secreted acidic substances have a specific pH.

Question 18

Manufacture of Acids and Bases

Answer 18

a) Non-metal oxide + water → acid
SO2(g) + H2O(l) → H2SO3(aq)
SO3(g) + H2O(l) → H2SO4(aq)
4NO2(g) + 2H2O(l) + O2(g) → 4HNO3(aq)
Non-metal oxides are thus referred to as acid anhydrides.

b) Hydrogen + halogen → acid
H2(g) + Cl2(g) → 2HCl(g)
HCl(g) + H2O(l) → HCl(aq)

c) Metallic salt + conc. sulphuric acid → salt + more volatile acid
2NaCl(aq) + H2SO4(aq) → Na2SO4(aq) + 2HCl(aq)
2KNO3(aq) + H2SO4(aq) → K2SO4(aq) + 2HNO3(aq)

d) Metal + oxygen → metallic oxide (base)
4Na(s) + O2(g) → 2Na2O(s)
2Mg(s) + O2(g) → 2MgO(s)

e) Metal + water → base or alkali + hydrogen
Zn(s) + H2O(steam) → ZnO(s)+ H2(g)

f) Few metallic oxides + water → alkali
Na2O(s) + H2O(l) → 2NaOH(aq)

g) Ammonia + water → ammonium hydroxide
NH3(g) + H2O(l) → NH4OH(aq)

Question 19

Salts

Answer 19

Salts
A salt is a combination of an anion of an acid and a cation of a base.
Examples - KCl, NaNO3, CaSO4, etc.
Salts are usually prepared by neutralisation reaction of an acid and a base.

Common salt
Sodium Chloride (NaCl) is referred to as common salt because it’s used all over the world for
cooking.

Question 20

Family of salts

Answer 20

Salts having the same cation or anion belong to the same family. For example, NaCl, KCl, LiCl.

Question 21

pH of salts

Answer 21

A salt of a strong acid and a strong base will be neutral in nature. pH = 7 (approx.).

A salt of a weak acid and a strong base will be basic in nature. pH > 7.

A salt of a strong acid and a weak base will be acidic in nature. pH < 7.

The pH of a salt of a weak acid and a weak base is determined by conducting a pH test

Question 22

Preparation of Sodium hydroxide

Answer 22

Chemical formula - NaOH
Also known as - caustic soda
Preparation (Chlor-alkali process):
Electrolysis of brine (solution of common salt, NaCl) is carried out.

At anode: Cl2 is released
At cathode: H2 is released
Sodium hydroxide remains in the solution.

Question 23

Bleaching powder

Answer 23

Chemical formula - Ca(OCl)Cl or CaOCl2
Preparation - Ca(OH)2(aq) + Cl2(g) → CaOCl2(aq) + H2O(l)
On interaction with water - bleaching powder releases chlorine which is responsible for bleaching action.

Question 24

Baking soda

Answer 24

Chemical name - Sodium hydrogen carbonate
Chemical formula - NaHCO3
Preparation (Solvay process) -
a. Limestone is heated: CaCO3 → CaO + CO2
b. CO_2 is passed through a concentrated solution of sodium chloride and ammonia:
NaCl(aq) + NH3(g) + CO2(g) + H2O(l) → NaHCO3(aq) + NH4Cl(aq)

Uses:
1. Textile industry
2. Paper industry
3. Disinfectant

Question 25

Washing soda

Answer 25

Chemical name - Sodium carbonate decahydrate.
Chemical formuala - (Na_2CO_3 )
Preparation: By heating NaHCO3
2NaHCO3(s) → Na2CO3(s) + CO2(g) + H2O(g)
Na2CO3(s) + 10H2O(l) → Na2CO3.10H2O(s)

Uses
1. In glass, soap and paper industries
2. Softening of water
3. Domestic cleaner

Question 26

Crystals of salts

Answer 26

Certain salts form crystals by combining with a definite proportion of water. The water that
combines with the salt is called water of crystallisation.

Question 27

Plaster of Paris

Answer 27

Gypsum, CaSO4.2H2O (s) on heating at 100°C (373K) gives CaSO4. H2O and H2O
CaSO4. H2O is plaster of paris.
CaSO4. H2O means two formula units of CaSO4 share one molecule of water.

Uses - cast for healing fractures.