ACIDS, BASES & SALTS
1.
INTRODUCTION
On the basis of their of
their chemical properties, all the compounds can be classified into three
groups:
1.
Acids,
2.
Bases, and
3.
Salts
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| Acid bases effect |
2.
Indicators for testing Acids and Bases
An
indicators is a “dye” that change colour when it is put into an acid or a base.
An indicators gives different colours in acid and base.
The
three most common indicators to test for acids and bases are: litmus, methyl orange, phenolphthalein.
i.
Litmus
a.
An acid turns
blue litmus to red.
b.
A base (or
alkali) turns red litmus to blue.
ii.
Methyl Orange
a.
Methyl orange
indicator gives red colour in acid solution.
b.
Methyl orange
indicator gives yellow colour in basic solution.
iii.
Phenolphthalein
a.
Phenolphthalein
indicator is colourless in acid solution.
b.
Phenolphthalein
indicator gives pink colour in basic solution.
Litmus
is a natural indicator:
Litmus solution is a purple dye which is
extracted from a type of plant called ‘lichen’. Lichen is a plant belonging to
the division Thallophyta. When litmus solution is neither acidic nor basic (it
is neutral), then its colour is purple.
Turmeric is also a natural indicator. Turmeric
contains a yellow dye.
3.
Acids:
1. Acids are those chemical substances which have a
sour taste.
Ex: Mango, raw grapes, lemon, orange, and
tamarind (imli).
2. Acids change the colour of blue litmus to red.
3. The acids present in plant materials and animals
are called organic acids.
Ex:
Acetic acid or ethanoic acid, lactic acid, tartaric acid, oxalic acid and formic
acid
Sources
of these organic acids are as following:
I.
Acetic acid
is found in vinegar (sirka).
II.
Citric acid
is present in citrus fruits such as Lemon and orange.
III.
Lactic acid
in present in sour milk (or Curd).
IV.
Tartaric acid
is present in Tamarind and unripe graps,
V.
Oxalic acid
is present in tomatoes.
VI.
Formic acid
is present in ant string and nettle leaf sting.
4. Acid solution conduct electricity (they are
electrolytes).
5. Acid react with metal to form hydrogen gas.
The
acids prepared from the minerals of the earth are called mineral acids.
Ex: hydrochloric
acid, sulphuric acid and nitric acid.
Strong
acids and weak acids
i.
All the
mineral acids are strong acids.
Hydrochloric acid, sulphuric acid and nitric
acid are strong acid.
HCl
H+ + Cl-
Hydrochloric acid hydrogen ion
ii.
The organic
acids are weak acids.
Acetic
acid (ethanoic acid), formic acid, citric acid, tartaric acid and carbonic acid
are week acids.
CH3COOH
CH3COO‑ + H+
Acetic acid acetic
ions hydrogen ions
·
Dilute Acid:
Contains only a small amounts of acid and a
large amount of water.
·
Concentrated Acid:
A concentrated acid contains a large amount of
acid and a small amount of water.
1. Acid react with metal to form hydrogen gas.
When an acid reacts with
a metal, then a salt and hydrogen gas are formed. That is
Metal + acid
salt + Hydrogen
gas
When dilute sulphuric acid reacts with zinc metal, then zinc sulphate
and hydrogen gas are formed.
Zn (s) + H2SO4
ZnSO4 + H2
Zinc sulphuric
acid zinc
sulphate hydrogen gas
(a metal) (Diluteacid) (A salt)
Most of the acids reacts with metals to form salts and evolve
hydrogen gas. This shown that hydrogen gas is common to all acids.
2. Acid reacts with metal carbonates and metal
hydrogen carbonates to form carbon dioxide gas
When
the acid reacts with a metal carbonate (or metal hydrogen carbonates) then a
salt, carbon dioxide gas and water are formed:
Metal carbonates + acid
salt + carbon
dioxide + water
Metal hydrogen carbonates + acid
salt + carbon
dioxide + water
i.
When dilute
hydrochloric acid reacts with sodium carbonate, then sodium chloride, carbon
dioxide and water are formed:
Na2CO3 + 2HCl
2NaCl + CO2 + H2O
Sodium carbonate
The carbon dioxide gas is formed in the form of brisk effervescence
(the rapid escape of small bubbles of gas from the liquid)
ii.
When dilute
hydrochloric acid reacts with sodium hydrogencarbonate, then sodium chloride,
carbon dioxide and water are formed:
Na2CO3(s) + HCl(aq)
NaCl(aq) + CO2 + H2O
Sodium carbonate Sodium
chloride
Carbon dioxide reacts with lime water (calcium
hydroxide solution) as follows:
a.
When carbon
dioxide gas is passed through lime water, the lime water turns milky due to the
formation of a white precipitate of calcium carbonate.
Ca(OH)2 + CO2
CaCO3 + H2O
Calcium Hydroxide Calcium
Carbonate
(Lime Water) (White
ppt) (Make lime water milky)
b.
If excess
of carbon dioxide gas is passed through lime water, then the white precipitate
formed first dissolves due to the formation of a solution salt calcium hydrogen
carbonates, and the solution becomes clear again:
CaCO3(s) + CO2
H2O + Ca(HCO3)2(aq)
Calcium Carbonate calcium hydrogen
carbonate
(White ppt) (Solution in water)
(Insoluble in water)
3. Acids react with base (or alkalis) to form salt
and water
When an acid reacts with
a base, then a salt and water are formed. That is
Acid + base
Salt + water
The reaction between an acid and
base to form salt and water is called a neutralisation reaction.
Ex: when hydrochloric acid
reacts with sodium hydroxide solution, then a neutralisation reaction takes
place to form sodium chloride and water:
NaOH + HCl
NaCl + H2O
Sodium hydroxide Hydrochloride Sodium Chloride water
(Base) (Acid) (Salt)
4. Acid react with metal oxide to form salt and
water
Acids react with metal
oxides to form salt and water
Metal oxide + Acid
Salt + water
Copper (II) oxide is a metal
oxide. Dilute hydrochloric acid reacts with copper (II) oxide to form copper
(II) chloride and water.
CuO + 2HCl
CuCl2 + H2O
Cooper (II) Oxide Hydrochloride
acid Copper (II) chloride
(Black) (Blue-green)
5. Acid have corrosive nature
The
mineral acids cause severe burns on the skin and attack and eat up materials
like cloth, wood, metal structures and stonework, so they are said to be
corrosive.
The
strong bases (or alkalis) such as sodium hydroxide are also very corrosive, and
attack and destroy our skin.
6. What do all acids have in common
All acid is a substance
which dissociates (or ionises) on dissolving in water to produce hydrogen ions
[H+(aq) ions]
For example:
An aqueous solution of
hydrochloric acid dissociates (or ionises) to form hydrogen ions (along with
chloride ions).
HCl (aq)
H+(aq) + Cl- (aq)
Hydrochloride acid hydrogen ions chloride ions
It
is the presence of hydrogen ions [H+(aq) ions] in hydrochloric acid
sodium which makes it behave like an acid.
Please
note that hydrogen ions do not exist as H+ ions in solution, they
attach themselves to the polar water molecules to form hydronium ions, H3O+.
That is
H+ + H2O
H3O+
Hydrogen ion hydronium
ion
Sulphuric
acid (H2SO4)
H2SO4
2H+ + SO42-
Nitric acid (HNO3)
HNO3
H+ + NO3-
Acetic Acid (CH3COOH)
CH3COOH
CH3COO- + H+
Note: Glucose & Alcohol solution does not conduct
electricity.
Glucose and alcohol solution do not produce
hydrogen ions or some other ions in solution. So due to the absence of ions,
glucose and alcohol solution do not conduct electricity.
The aqueous solution of an acid conducts
electricity due to the presence of charged particles called ions in it.
For example:
When hydrochloric acid (HCl) is dissolved in
water, then its solution contains hydrogen ions, H+ ions and Cl-
(aq) ions. A solution of hydrochloric acid conduct electricity.
7. Uses of mineral acids in industry
1.
Sulphuric
acid is used in the manufacture of fertilisers (like ammonium sulphate),
paints, dyes, chemicals, plastics, synthetic fibres, detergents, explosives and
car batteries.
2.
Nitric acid
is used for making fertilisers (like ammonium nitrate) explosives (like TNT :
tri-Nitro-Toluene), dyes and plastics.
3. Hydrochloride acid is used for removing oxide
film from steel objects (before they are galvanised) and removing ‘scale’
deposits from inside the boilers. It is also used in dye-stuffs, textile, and
food and leather industries.
4.
Bases
Bases
are those chemical substance which have a bitter taste. Bases are the chemical
opposites of acids.
A
base is a chemical substance which can neutralise an acid. All the metal oxide
and metal hydroxide are bases.
For
example:
Sodium
oxide (Na2O) is metal oxide, so it is bases, calcium oxide (CaO) and
calcium hydroxide [Ca(OH)2], ammonium hydroxide (NH4OH)
is also a base thought it is not a metal hydroxide.
Sodium carbonate (Na2CO3),
Calcium carbonate (CaCO3) and sodium hydrogen carbonate (NaHCO3) are also bases.
Water soluble bases: Alkalis
Most of the bases do not dissolve in water but
some bases dissolved in water. Those bases which dissolve in water without
chemical reaction have a special name. They are called alkalis. Thus a base
which is soluble in water is called an alkali.
Some common water soluble bases are: sodium
hydroxide (NaOH), potassium hydroxide (KOH), calcium hydroxide [Ca(OH)2],
ammonium hydroxide (NH4OH) and magnesium hydroxide [MG(OH)2].
1. What do all the bases have in common
When a bases is dissolved in water, it always
produces hydroxide ions (OH- ions).
NaOH + water
Na+ + OH‑
Sodium Hydroxide Sodium Ions hydroxide
ions
Similarly:
KOH +
water
K+ + OH-
And
Mg(OH)2 + water
Mg2+ + OH‑
A common property of all the bases is that they
all produce hydroxide ions when dissolved in water.
A bases which completely ionises in water and
thus produce a large amount of hydroxide ions is called a strong bases.
Ex: NaOH, KOH, etc.
A base which is property ionises in water and
thus produces a small amount of hydroxide ions is called a weak base.
Ex: NH4OH, Ca(OH)2, Mg(OH)2 etc.
2. Properties of bases:
1.
Bases have
bitter taste.
2.
Bases feel
soapy to touch
3.
Bases turn
red litmus to blue
4.
Bases conduct
electricity in solution (they are electrolytes)
5.
Bases react
with some metal to form hydrogen gas.
6.
Base react
with acids to form salt and water.
NaOH + H2SO4
Na2SO4 + 2H2O
7.
Base react
with non-metal oxides to form salt and water.
Non-metal + base
Salt + Water
Ex:
Ca(OH)2 + CO2
CaCO3 + H2O
Calcium hydroxide calcium
carbonate
(Base) (Salt)
3. Uses of bases:
Some of the important
uses of bases are given below:
1.
Sodium
hydroxide is used in the manufacture of soap, paper and a synthetic fibre
called ‘rayon’.
2.
Calcium
hydroxide is used in the manufacture is bleaching powder.
3.
Magnesium hydroxide
is used as an ‘antacid’ to neutralise excess acid in the stomach and cure
indigestion.
4. Sodium hydrogen carbonate is used as baking soda
in cooking food, for making baking powders, as an antacid to cure indigestion
and in soda- acid fire extinguishers.
5. Sodium carbonate is used as washing soda and for
softening hard water.
5.
STRENGTH OF ACID AND BASE SOLUTION: pH SCALE
Water
is slightly ionised in to hydrogen ions (H+) and hydroxide (OH‑).
In pure water, the concentrations of hydrogen ions and hydroxide ions are
equal.
6.
Impotence of pH in everyday life
The
pH plays an important role in many activities of our everyday life. For example
pH of gastric juices in the stomach is important in the process of digestion;
the pH changes in mouth can become a cause of tooth decay; the growth of plants
and survival of animals also depends on the proper maintenance of pH; and many
animals, and some plants make use of acids and basic liquids for sell defence
purposes.
1. pH in our digestive system:
pH level of our body
regulates our digestive system. In
case of indigestion our stomach produces acid in a very large quantity because of which we feel pain and
irritation in our stomach. To get relief from this
pain antacids are used. These antacids neutralises the excess acid and we get relief.
2. pH of Acid Rain :
When pH of rain water is less than 5.6 it is
called Acid Rain. When this acidic rain flows into rivers these also get
acidic, which causes a threat to the survival of aquatic life.
3. pH of Soil :
Plants
require a specific range of pH for their healthy growth. If pH of soil of any
particular place is less or more than normal than the farmers add suitable
fertilizers to it.
Our
body functions between the range of 7.0 to 7.8 living organisms can survive
only in the narrow range of pH change.
4. Tooth decay and pH:
Bacteria present in the mouth produce acids by
degradation of sugar and food particles remaining in the mouth. Using
toothpaste which is generally basic can neutralise the excess acid and prevent
tooth decay.
Bee
sting or Nettle sting contains methanoic acid which causes pain and irritation.
When we use a weak base like baking soda on it we get relief.
Neutral Salts: Strong Acid + Strong base
pH value is 7
eg. NaCl, CaSO4
Acidic Salts: Strong Acid + weak base
pH value is less than 7
eq. NH4Cl, NH4NO3
Basic Salts: Strong base + weak acid
pH value is more than 7
eg. CaCO3, CH3COONa
7.
Chemicals from Common Salt
– Sodium chloride is called as common salt used in our
food. It is derived from seawater.
– Rock Salt is the brown coloured large crystals. This s
mined like coal.
– Common Salt is an important raw material for many
materials of daily use such as.
Sodium hydroxide
Washing Soda
Bleaching Power.
Sodium Hydroxide
Preparation: Prepared by the method called chlor-alkali Called
chlor-alkali because we get chlorine and a base in this.
2NaCl(aq) + 2H2O
(l)
2NaOH(aq) + Cl2(g) + H2(g)
Used in industry, medicines
and cosmetics Household, Bleaching
fabric
Bleaching Power
Preparation
Ca(OH)2 + Cl2
CaOCl2
+ H2O
Calcium
hydroxide chlorine bleaching water
Powder
Uses in textile, factories and laundry, used as
disinfectant
Baking Soda
Common name – Sodium Hydrogen Carbonate
Preparation
NaCl + H2O
+ CO2 + NH3
NH4Cl + NaHCO3
Sodium Water Carbon
Ammonia Sodium hydrogen
Chloride dioxide
carbonate
On heating NaHCO3 produces:
NaHCO3
Na2CO3
+ H2O
+ CO2
Sodium Hydrogen carbonate sodium
carbonate water carbon dioxide
CO2 produced causes dough to rise and make
cakes, pastries spongy.
Uses:
In household, ingredients of antacid
In making baking power
On heating baking powder produces
NaHCO3 + H+
CO2
+
H2O + Sodium Salt of acid
Washing Soda
Preparation:
Recrystallization of sodium carbonate
Na2CO3 + 10H2O
Na2CO3. 10H2O
Uses:
– Used in glass, soap and paper industry
– Cleaning agent for domestic purposes.
– Removal of hardness of water.
– Manufacturer of borax.
Water of crystallisation:
Fixed no. of water molecules present in one formula unit
of a salt.
On heating copper sulphate crystals water droplets
appear, formula of hydrated copper sulphate – CuSO4. 5H2O.
Plaster of Paris
Gypsum also contains water of crystallisation.
Formula of gypsum (Plaster of Paris) – CaSO4.2H2O
On heating gypsum at 373k it becomes CaSO4.½H2O
is plaster of paris.
Plaster of Paris is used as plaster for fractured bones.
When plaster of Paris is mixed with water it changes to
gypsum.
CaSO4.½ H2O + 1½ H2O
CaSO4.2H2O
Uses of plaster of Paris:
Making toys, decorative material and smooth surfaces
