Introduction
In our daily
life, we come across many substances that taste sour or bitter. The sour taste
of lemon, curd, or vinegar is due to acids, while the bitter taste of soap or
baking soda is due to bases. These substances are extremely important not only
in laboratories but also in cooking, medicine, agriculture, industries, and
even inside our own body.
Acids and
bases have opposite chemical properties. When they react with each other, they
neutralise one another and form salt and water. This process is known as neutralisation.
Indicators –
Substances that Identify Acids and Bases
Indicators
are substances that help us identify whether a solution is acidic or basic by
showing a colour change.
Types of
Indicators
1. Natural
Indicators
These are
obtained from natural sources.
Litmus
- Obtained
from lichens.
- Purple
in neutral solution.
|
Indicator |
In Acid |
In Base |
|
Blue
Litmus |
Turns Red |
No change |
|
Red Litmus |
No change |
Turns Blue |
Turmeric
- Remains
yellow in acid.
- Turns
reddish-brown in base.
Other
natural indicators include:
- Red
cabbage
- Hydrangea
flowers
- Petunia
flowers
- Geranium
flowers
2. Synthetic
Indicators
These are
chemically prepared indicators.
Phenolphthalein
- Colourless
in acid
- Pink in
base
Methyl
Orange
- Red in
acid
- Yellow
in base
Olfactory
Indicators
Some
substances change their smell in acidic or basic medium. These are called
olfactory indicators.
Examples
- Onion
- Vanilla
essence
- Clove
oil
Observation
- In
acidic medium → smell remains unchanged.
- In
basic medium → smell disappears.
This happens
because bases affect the smell-producing substances present in them.
Chemical
Properties of Acids and Bases
1. Reaction
of Acids with Metals
When acids
react with metals:
- Hydrogen
gas is evolved.
- A salt
is formed.
General
Reaction
Example:
Zinc and Sulphuric Acid
Observations
- Bubbles
are formed.
- Hydrogen
gas burns with a pop sound.
This
confirms the presence of hydrogen gas.
Reaction of
Bases with Metals
Some strong
bases react with metals to produce hydrogen gas.
Example
Product
Formed
- Sodium
zincate
Not all
metals show this reaction.
2. Reaction
of Acids with Metal Carbonates
Acids react
with metal carbonates to form:
- Salt
- Carbon
dioxide
- Water
General
Reaction
Example
Observation
- Effervescence
due to release of carbon dioxide gas.
Reaction of
Acids with Metal Hydrogencarbonates
Example
Test for
Carbon Dioxide Gas
Carbon
dioxide turns lime water milky.
Reaction
Observation
- White
precipitate of calcium carbonate appears.
When excess
carbon dioxide is passed, the milkiness disappears because soluble calcium
bicarbonate is formed.
Important
Forms of Calcium Carbonate
- Limestone
- Marble
- Chalk
All are
chemically calcium carbonate.
3.
Neutralisation Reaction
When an acid
reacts with a base, both neutralise each other to form salt and water.
General
Reaction
Example
Observation
The pink
colour of phenolphthalein disappears when acid is added to the basic solution.
4. Reaction
of Metallic Oxides with Acids
Metal oxides
react with acids to form salt and water.
General
Reaction
Example:
Copper Oxide and Hydrochloric Acid
Observations
- Black
copper oxide dissolves.
- Blue-green
solution forms.
Conclusion
Metal oxides
are basic in nature.
5. Reaction
of Non-Metallic Oxides with Bases
Non-metallic
oxides react with bases to form salt and water.
Example:
Carbon dioxide reacts with calcium hydroxide.
Conclusion
Non-metallic
oxides are acidic in nature.
Why Do Acids
and Bases Show Similar Properties?
Acids
Produce Hydrogen Ions
Acids
release hydrogen ions in aqueous solution.
Example
Hydrogen
ions cannot exist independently. They combine with water molecules to form
hydronium ions.
Bases
Produce Hydroxide Ions
Examples
Alkalis
Bases that
dissolve in water are called alkalis.
Examples:
- Sodium
hydroxide
- Potassium
hydroxide
- Ammonium
hydroxide
They are:
- Soapy
to touch
- Bitter
in taste
- Corrosive
in nature
Why Do
Acidic Solutions Conduct Electricity?
Acids
conduct electricity because they contain ions that carry electric current.
Observation
- Bulb
glows in acid solution.
- Bulb
does not glow in glucose or alcohol solution.
This proves
that acids ionise in water while glucose and alcohol do not.
Why Dry HCl
Does Not Show Acidic Behaviour
Dry HCl gas
does not contain free hydrogen ions.
Only in the
presence of water are H⁺ ions
produced.
Therefore:
- Dry
blue litmus → no colour change
- Wet
blue litmus → turns red
Dilution of
Acids and Bases
Mixing acids
or bases with water is highly exothermic.
Safety Rule
Always add:
Acid into
water
and never
water into acid.
Reason
Large heat
is produced which may cause splashing and burns.
pH Scale
The pH scale
measures how acidic or basic a solution is.
Range of pH
|
pH Value |
Nature |
|
0–6 |
Acidic |
|
7 |
Neutral |
|
8–14 |
Basic |
- Lower
pH → stronger acid
- Higher
pH → stronger base
Strong and
Weak Acids
Strong Acids
Produce
large amount of H⁺ ions.
Examples:
- Hydrochloric
acid
- Sulphuric
acid
Weak Acids
Produce
fewer H⁺ ions.
Example:
- Acetic
acid
Importance
of pH in Everyday Life
1. pH in Our
Digestive System
Our stomach
produces hydrochloric acid for digestion.
Problem
Excess acid
causes:
- Acidity
- Pain
- Irritation
Remedy
Antacids
like magnesium hydroxide neutralise excess acid.
2. Tooth
Decay
Tooth decay
begins when pH falls below 5.5.
Cause
Bacteria
produce acids from food particles.
Prevention
- Brushing
teeth regularly
- Using
basic toothpaste
3. Bee Sting
and Nettle Sting
Bee sting
contains methanoic acid.
Remedy
Apply baking
soda or another mild base.
4. Soil pH
Plants
require a specific pH range for proper growth.
Acidic Soil
is Treated With
- Quick
lime (CaO)
- Slaked
lime [Ca(OH)₂]
- Chalk
(CaCO₃)
Naturally
Occurring Acids
|
Natural Source |
Acid Present |
|
Vinegar |
Acetic
acid |
|
Lemon |
Citric
acid |
|
Tamarind |
Tartaric
acid |
|
Tomato |
Oxalic
acid |
|
Curd |
Lactic
acid |
|
Ant sting |
Methanoic
acid |
Salts
Salts are
compounds formed during neutralisation reactions.
Family of
Salts
Salts having
common positive or negative radicals belong to the same family.
Examples
- NaCl
and Na₂SO₄ → sodium family
- NaCl
and KCl → chloride family
pH of Salts
|
Type of Salt |
Nature |
|
Strong
acid + Strong base |
Neutral |
|
Strong
acid + Weak base |
Acidic |
|
Strong
base + Weak acid |
Basic |
Common Salt
(NaCl)
Common salt
is obtained from:
- Sea
water
- Rock
salt deposits
It is an
important raw material used in industries.
Chlor-Alkali
Process
Electrolysis
of brine produces:
- Sodium
hydroxide
- Chlorine
gas
- Hydrogen
gas
Reaction
Uses
- Sodium
hydroxide → soap and paper industries
- Chlorine
→ disinfectants, PVC
- Hydrogen
→ fuel and fertilisers
Bleaching
Powder
Prepared by
passing chlorine gas through dry slaked lime.
Reaction
Uses
- Bleaching
clothes
- Disinfecting
drinking water
- Oxidising
agent
Baking Soda
Chemical
Name:
Sodium
Hydrogencarbonate (NaHCO₃)
Preparation
of Baking Soda
Heating of
Baking Soda
Importance
CO₂ released
helps cakes and bread become soft and fluffy.
Uses of
Baking Soda
- Making
baking powder
- Antacids
- Fire
extinguishers
Washing Soda
Chemical
Formula:
Nature
Basic salt
Uses of
Washing Soda
- Cleaning
agent
- Softening
hard water
- Manufacture
of glass, soap and paper
- Manufacture
of borax
Water of
Crystallisation
The fixed
number of water molecules present in one formula unit of a salt is called water
of crystallisation.
Copper
Sulphate Crystals
Hydrated
copper sulphate:
Observation
- Blue
crystals turn white on heating.
- Blue
colour returns when water is added.
Gypsum
Chemical
Formula:
Plaster of
Paris (POP)
Prepared by
heating gypsum at 373 K.
Formula
Preparation
of POP
POP with
Water
Uses
- Supporting
fractured bones
- Decorative
materials
- Toys
and statues
