- Acids,
bases and salts are used in our daily life.
- Sour
and bitter tastes of food are due to the presence of acids & bases. Acids
are sour in taste and bases are bitter.
- There
are natural and synthetic acid-base indicators used to test whether a
substance is acid or base.
- Natural
indicators: E.g. Litmus, red cabbage leaves, turmeric,
coloured petals of some flowers (Hydrangea,
Petunia & Geranium).
- Litmus
solution is a purple dye extracted from lichen (a
plant under division Thallophyta).
When the litmus solution is neutral, its colour is
purple.
Acids change the colour of blue litmus
to red.
Bases change red litmus to blue.
- A stain of curry on a white cloth turns reddish-brown
when soap (basic) is rubbed on it because the curry contains an
indicator turmeric. If washed with water, it turns yellow again.
- Synthetic
indicators: E.g. methyl orange and phenolphthalein.
UNDERSTANDING THE CHEMICAL PROPERTIES OF
ACIDS & BASES
Acids and Bases in the
Laboratory
Put
a drop of each solution given below on a watch-glass one by one and test with a
drop of different indicators.
- Hydrochloric acid (HCl)
- Calcium hydroxide [Ca(OH)2]
- Sulphuric acid (H2SO4)
- Potassium hydroxide (KOH)
- Nitric acid (HNO3)
- Magnesium hydroxide [Mg(OH)2]
- Acetic acid (CH3COOH)
- Ammonium hydroxide (NH4OH)
- Sodium hydroxide (NaOH)
Results:
|
Sample solution
|
Red litmus solution
|
Blue litmus solution
|
Phenolphthalein solution
|
Methyl orange solution
|
|
HCl
|
No change
|
Red
|
Colourless
|
Pinkish red
|
|
H2SO4
|
No change
|
Red
|
Colourless
|
Pinkish red
|
|
HNO3
|
No change
|
Red
|
Colourless
|
Pinkish red
|
|
CH3COOH
|
No change
|
Red
|
Colourless
|
Pinkish red
|
|
NaOH
|
Blue
|
No change
|
Red to pink
|
Remains yellow
|
|
KOH
|
Blue
|
No change
|
Red to pink
|
Remains yellow
|
|
Mg(OH)2
|
Blue
|
No change
|
Red to pink
|
Remains yellow
|
|
NH4OH
|
Blue
|
No change
|
Red to pink
|
Remains yellow
|
|
Ca(OH)2
|
Blue
|
No change
|
Red to pink
|
Remains yellow
|
Olfactory indicators:
These are some substances whose odour changes in
acidic or basic media. E.g. Onion, clove oil and vanilla.
Test with Onion:
- Take
some finely chopped onions in a plastic bag along with some clean cloth strips.
Tie up the bag tightly and leave overnight in the fridge.
- Take
two of these cloth strips and check their odour.
- Put
a few drops of dilute HCl solution on one strip and a
few drops of dilute NaOH solution on the other.
- Rinse
both cloth strips with water and again check their odour. The odour of onion
cloth vanishes in NaOH. The odour remains unchanged in HCl.
Test with vanilla essence:
- Take
some dilute HCl solution in one test tube and dilute NaOH solution in another.
- Add
a few drops of dilute vanilla essence to both test tubes and shake well. The odour of vanilla essence vanishes in NaOH. The odour remains unchanged in HCl.
Test with clove oil:
- Repeat
the same test using clove oil.
- The odour clove oil vanishes in NaOH. The odour remains unchanged in HCl.
How do Acids & Bases react with Metals?
Reaction of Acids with metal:
- Set
the apparatus as shown in Figure.
- Take
5 mL dilute sulphuric acid in a test tube and add some zinc granules.
- Hydrogen
gas bubbles are formed on the surface of zinc granules. Pass this gas through
soap solution. Soap bubbles filled with hydrogen are formed.
- Take
a burning candle near a gas filled bubble. Hydrogen burns with a pop sound.
- Repeat
this using HCl, HNO3 and CH3COOH.
- Bubbles
come out vigorously with strong acids.
- Zinc
also reacts with weak acids like acetic acid, but gas is formed slowly.
- When
an acid reacts with a metal, the metal displaces hydrogen atoms from the acids
as hydrogen gas and forms a compound called salt.
Acid + Metal → Salt + Hydrogen gas
H2SO4 + Zn → ZnSO4 + H2
Reaction
of Bases with metal:
- Place
some granulated zinc metal in a test tube.
- Add
2 mL sodium hydroxide solution and warm the contents of the test tube.
- Repeat
the rest of the steps as in previous experiment.
- Here
also, hydrogen is formed.
2NaOH(aq) + Zn(s) → Na2ZnO2(s) + H2(g)
(Sodium
zincate)
- However,
such reactions are not possible with all metals.
How do Metal Carbonates
and Metal Hydrogencarbonates React with Acids?
- Take 0.5 g of sodium carbonate (Na2CO3)
in test tube A and 0.5g of sodium hydrogencarbonate (NaHCO3) in
test tube B. Add 2 mL dilute HCl to both the test tubes.
- CO2 gas is produced in both test
tubes.
Test tube A:
Na2CO3(s) + 2HCl(aq) → 2NaCl(aq) + H2O(l) + CO2(g)
Test tube B:
NaHCO3(s) + HCl(aq) → NaCl(aq) + H2O(l) + CO2(g)
- Pass CO2 through lime water (calcium
hydroxide).
Ca(OH)2(aq)
+ CO2(g) → CaCO3(s) + H2O(l)
(Lime water) (White
precipitate)
On passing excess CO2:
CaCO3(s)+ H2O(l)+ CO2(g) →Ca(HCO3)2(aq)
(Soluble in water)
- Limestone,
chalk & marble are different forms of calcium carbonate. All metal
carbonates & hydrogencarbonates react with acids to give corresponding
salt, CO2 & water.
Metal carbonate/Metal hydrogencarbonate + Acid →
Salt + Carbon dioxide + Water
How do Acids and Bases React
with each other?
- Take
2 mL dilute NaOH solution in a test tube and add two drops of phenolphthalein
solution.
- The
solution becomes pink colour.
- To
this, add dilute HCl solution drop by drop. The pink colour changes and
disappears.
- Now
add a few drops of NaOH to the above mixture. The pink colour of
phenolphthalein reappears.
- The
effect of a base is nullified by an acid and vice-versa.
NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)
- The
reaction between an acid and a base to give a salt and water is called neutralisation
reaction.
Base + Acid → Salt + Water
Reaction of Metallic
Oxides with Acids
- Take
some copper oxide (CuO) in a beaker and add dilute hydrochloric acid
slowly while stirring.
- The
solution becomes blue-green colour and the copper oxide dissolves. The
colour is due to the formation of copper(II) chloride (CuCl2).
CuO + 2HCl → CuCl2 + H2O
- General
reaction between a metal oxide and an acid is
Metal oxide + Acid → Salt + Water
- Metallic
oxides react with acids to give salts & water, like the reaction of a base
with an acid. So metallic oxides are basic oxides.
Reaction of a non-metallic oxide with Base
- Calcium
hydroxide (base) reacts with carbon dioxide to
produce a salt and water.
- This
is similar to the reaction between a base and acid. It means non-metallic
oxides are acidic in nature.
WHAT DO ALL ACIDS AND ALL BASES HAVE IN
COMMON?
- All
acids have similar chemical properties.
- E.g.
All acids generate hydrogen gas on reacting with metals, so hydrogen seems to
be common to all acids.
- But
all compounds containing hydrogen are not acidic. It can be proved by the
following experiment.
· Take
solutions of glucose, alcohol, HCl, H2SO4, etc.
· Fix
two nails on a cork and place it in a 100 mL beaker. Connect the nails to the
two terminals of a 6 volt battery through a bulb and a switch.
· Pour
some dilute HCl in the beaker and switch on the current. Repeat with dilute H2SO4.
· In
both cases, bulb glows. It means there is an electric current through the acidic
solution by ions.
· Repeat
the experiment using glucose & alcohol solutions. In these cases, bulb does
not glow because glucose & alcohol solutions do not conduct electricity.
- Acids
contain H+ ion as cation and anion (Cl–
in HCl, NO3– in HNO3, SO24–
in H2SO4, CH3COO–
in CH3COOH).
- Acidic
properties are due to H+(aq) ions in solution.
- Repeat the same Activity using alkalis such as sodium
hydroxide, calcium hydroxide, etc. NaOH & KOH conduct
electricity as they are broken down into ions when dissolved in water. The
movement of the ions, Na+, K+ and OH- in
solution generate electricity.
What Happens to an Acid
or a Base in a Water Solution?
Acids produce ions only in aqueous solution. It
can be proved by the following experiment.
· Take
1g solid NaCl in a clean dry test tube.
·
To this, add some conc. sulphuric
acid.
·
HCl gas
comes out of the delivery tube. [In very humid climate, pass the HCl gas
through a guard tube (drying tube) containing calcium chloride to dry the gas].
· When
HCl gas is tested with wet blue litmus paper, it becomes red colour. But with dry
litmus paper, no colour change occurs.
· It
means dry HCl gas (absence of water) cannot produce H+ ions. So it
does not behave as an acid.
· HCl
solution (presence of water) can produce H+ ions and behave as an
acid.
HCl + H2O → H3O+ + Cl–
- H+
ions cannot exist alone. They exist after combining with water molecules.
Thus H+ ions must be shown as H+(aq) or hydronium
ion (H3O+).
H+ + H2O → H3O+
Action of base with water:
- Bases
generate hydroxide (OH–) ions in water.
- All
bases do not dissolve in water. An alkali is a base that dissolves in
water. They are soapy, bitter and corrosive. Never taste or touch them as they
cause harm.
- NaOH, KOH, Mg(OH)2, NH4OH etc. are
alkalis.
Neutralisation reaction:
Acid + Base → Salt + Water
H X + M OH → MX + HOH
H+(aq) + OH– (aq) → H2O(l)
Mixing of acid or base with water:
This process is highly exothermic. E.g.
· Take
10 mL water in a beaker. Add a few drops of concentrated H2SO4
and swirl the beaker slowly.
· Touch
the base of the beaker. It is hot. So the reaction is exothermic.
·
Repeat this activity with sodium hydroxide pellets. It is also
exothermic reaction.
- Mixing
of concentrated nitric acid or sulphuric acid with water must be done carefully.
Add the acid slowly to water with constant stirring. Otherwise, the excessive
local heating may cause the mixture to splash out and cause burns. The glass
container may also be broken.
- 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.
HOW STRONG ARE ACID OR BASE SOLUTIONS?
-
Amount of H+ ions
present in a solution can be measured by using a universal indicator. It
is a mixture of several indicators. It shows different colours at different
concentrations of hydrogen ions.
- A scale for measuring H+
ion concentration in a solution is called pH scale (p = ‘potenz’ in German = power).
-
pH is measured from 0 (very
acidic) to 14 (very alkaline).
-
Higher the H+ ion
concentration, lower is the pH value.
-
pH of a neutral solution is 7.
-
Values less than 7 represent an
acidic solution.
-
Values more than 7 represent an alkaline
solution.
- pH value from 7 to 14 represents
an increase in OH– ion concentration, i.e., increase in the strength
of alkali.
-
Generally, paper impregnated with
the universal indicator is used for measuring pH.
pH
values of various solutions:
|
Solution
|
Colour of pH paper
|
Approximate pH value
|
Nature of substance
|
|
1. Saliva (before meal)
|
Green
|
6.8 – 7.4
|
Slightly acidic to basic
|
|
2. Saliva (after meal)
|
Yellow green
|
5.8
|
Acidic
|
|
3. Lemon juice
|
Orange
|
2.2
|
Acidic
|
|
4. Colourless aerated drink
|
Yellow
|
4.0
|
Acidic
|
|
5. Carrot juice
|
Yellow green
|
6.0
|
Acidic
|
|
6. Coffee
|
Yellow
|
4.5
|
Acidic
|
|
7. Tomato juice
|
Yellow
|
4.3
|
Acidic
|
|
8. Tap water
|
Green
|
6 – 8.5
|
Varied
|
|
9. 1M NaOH
|
Dark blue
|
14
|
Basic
|
|
10. 1M HCl
|
Red
|
0
|
Acidic
|
pH of some
common substances shown on a pH paper
- The
strength of acids and bases depends on the number of H+ ions and OH–
ions produced, respectively. E.g. 1 molar hydrochloric acid and 1 molar acetic
acid (same concentration) produce different amounts of H+ ions.
- Strong
acids: Produce more H+ ions. E.g. HCl, H2SO4.
- Weak
acids: Produce less H+ ions. E.g. CH3COOH.
- Strong bases: Produce more OH- ions.
E.g. NaOH, KOH.
-
Weak bases: Produce less OH-
ions. E.g. NH4OH, Ca(OH)2.
Importance of pH in
Everyday Life
- Plants
and animals are pH sensitive. They can survive only in a narrow range of pH change.
- Our
body works at the pH range of 7.0 to 7.8.
- When
pH of rain water is less than 5.6, it is called acid rain. When it flows
into rivers, it lowers the pH of river water. It adversely affects the survival
of aquatic life.
What is the pH
of the soil in your backyard?
To
find out the pH for healthy growth of a plant, collect the
soil
from various places and check their pH as given below.
· Put about 2 g soil in a test tube and add 5 mL water.
· Shake the contents of the test tube.
· Filter the contents and collect the filtrate in a test tube.
· Check the pH of filtrate using universal indicator paper.
Ideal soil pH for the growth of plants = 6 to 7.
pH in our digestive system
- Stomach
produces hydrochloric acid.
- During
indigestion, the stomach produces too much acid and causes pain and irritation.
- To
get rid of pain, bases called antacids are used. They neutralise the
excess acid. E.g. Magnesium hydroxide (Milk of magnesia), a mild base.
pH
change as the cause of tooth decay
- Tooth
enamel is made up of calcium hydroxyapatite (a crystalline form
of calcium phosphate). It is the hardest substance in the body. It does not
dissolve in water.
- Bacteria
in the mouth produce acids by degradation of sugar and food particles. Thus enamel
is corroded at the pH below 5.5. It is called tooth decay.
- Using
toothpastes (basic) can neutralise the excess acid and prevent tooth decay.
Self-defence by organisms through chemical warfare
- Bee-sting
leaves formic acid which causes pain and irritation. Applying mild base
like baking soda on the stung area gives relief.
- Stinging
hair of nettle leaves inject methanoic acid causing
burning pain. A traditional remedy is rubbing the area with the leaf of the dock
plant.
Some naturally occurring acids
|
Natural source
|
Acid
|
Natural source
|
Acid
|
|
Vinegar
|
Acetic acid
|
Sour milk (Curd)
|
Lactic acid
|
|
Orange
|
Citric acid
|
Lemon
|
Citric acid
|
|
Tamarind
|
Tartaric acid
|
Ant sting
|
Methanoic acid
|
|
Tomato
|
Oxalic acid
|
Nettle sting
|
Methanoic acid
|
Acids in other planets: The
atmosphere of Venus is made up of thick white and yellowish clouds of sulphuric
acid.
Family of Salts
- Salts
having same positive or negative radicals belong to a family. E.g. NaCl &
Na2SO4 belong to family of sodium salts. NaCl & KCl
belong to the family of chloride salts.
|
Salts
& their Chemical formulae
|
Formed from which Acids & bases?
|
|
Potassium sulphate (K2SO4)
|
H2SO4
& KOH
|
|
Sodium sulphate (Na2SO4)
|
H2SO4
& NaOH
|
|
Calcium sulphate (CaSO4)
|
H2SO4
& CaCO3
|
|
Magnesium sulphate (MgSO4)
|
H2SO4
& Mg(OH)2
|
|
Copper sulphate (CuSO4)
|
H2SO4
& Cu(OH)2
|
|
Sodium chloride (NaCl)
|
HCl
& NaOH
|
|
Sodium nitrate (NaNO3)
|
HNO3
& NaOH
|
|
Sodium carbonate (Na2CO3)
|
H2CO3
& NaOH
|
|
Ammonium chloride (NH4Cl)
|
HCl
+ NH4OH
|
pH of Salts
- Salts
of a strong acid & a strong base are neutral (pH = 7).
- Salts
of a strong acid and weak base are acidic (pH < 7).
- Salts
of a strong base and weak acid are basic (pH > 7).
- pH
of some salt samples (soluble in distilled water) and the acid & base used
to form the salts are given below:
|
Salt
|
pH
|
Acid used
|
Base used
|
|
Sodium chloride
|
7
|
HCl
|
NaOH
|
|
Potassium nitrate
|
7
|
HNO3
|
KOH
|
|
Aluminium chloride
|
7
|
HCl
|
Al(OH)3
|
|
Zinc sulphate
|
7
|
H2SO4
|
Zn(OH)2
|
|
Copper sulphate
|
<
7
|
H2SO4
|
Cu(OH)2
|
|
Sodium acetate
|
>
7
|
CH3COOH
|
NaOH
|
|
Sodium carbonate
|
>
7
|
H2CO3
|
NaOH
|
|
Sodium hydrogen carbonate
|
>
7
|
H2CO3
|
NaOH
|
Chemicals from Common
Salt
- Common
salt (sodium chloride, NaCl) is a neutral salt formed by reaction
of HCl & NaOH solution.
- NaCl
is separated from seawater containing many salts.
- Deposits
of large crystals of solid salt are also found in several parts. These are
often brown due to impurities. This is called rock salt.
- There
are beds of rock salt formed when seas of bygone ages dried up. Rock salt is
mined like coal.
- Common
salt is used in food. It is also a raw material to make sodium hydroxide,
baking soda, washing soda, bleaching powder etc.
1. Sodium
hydroxide (NaOH)
- When
electricity is passed through an aqueous solution of sodium chloride (brine),
it decomposes to form sodium hydroxide. It is called the chlor-alkali
process because of the products formed– chlor for chlorine and alkali for
sodium hydroxide.
2NaCl(aq) + 2H2O(l) → 2NaOH(aq) + Cl2(g) + H2(g)
- Chlorine
gas is given off at the anode, and hydrogen gas at the cathode. NaOH solution
is formed near cathode.
- All
three products are useful.
- Uses
of NaOH: De-greasing metals, soaps & detergents, paper
making, artificial fibres etc.
- Uses
of Cl2: For water treatment, in swimming pools, PVC,
disinfectants, CFCs, pesticides etc.
- Uses
of H2: Fuels, margarine, ammonia for fertilisers.
Important products from the
chlor-alkali process
2. Bleaching
powder (CaOCl2)
- It
is manufactured by using chlorine produced during the electrolysis of brine.
-
Chlorine
acts on dry slaked lime to give bleaching powder.
Ca(OH)2 + Cl2 → CaOCl2 + H2O
- Actual
composition of CaOCl2 is quite complex.
Uses of Bleaching powder:
· For
bleaching cotton & 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.
· To
make drinking water free from germs.
3. Baking
soda
- Chemical
name: Sodium hydrogencarbonate (NaHCO3).
- It
is produced using sodium chloride.
- It
is a mild non-corrosive basic salt.
- When
it is heated for cooking, following reaction occurs.
Uses of Baking soda:
· To
make tasty crispy pakoras, etc.
· It
is added for faster cooking.
· To
make baking powder (baking soda + mild edible acid such as tartaric acid).
When baking powder is heated or mixed in water, following reaction occurs:
NaHCO3 + H+ → CO2 + H2O + Sodium
salt of acid
(From any acid)
CO2
causes bread or cake to rise making soft and spongy.
· It
is an ingredient in antacids. Being alkaline, it neutralises excess acid
(acidity) in stomach.
· Used
in soda-acid fire extinguishers.
4. Washing
soda (Na2CO3.10H2O)
- A
basic salt obtained from NaCl.
NaCl
→ NaHCO3 → Na2CO3 (sodium carbonate).
- Recrystallisation
of sodium carbonate → washing soda.
Na2CO3
+ 10H2O
→ Na2CO3.10H2O
Uses of washing soda:
· Used
in glass, soap and paper industries.
· To
manufacture sodium compounds such as borax.
· Used
as a cleaning agent for domestic purposes.
· For
removing permanent hardness of water.
Are the Crystals of Salts
really Dry?
-
Heat few copper sulphate crystals
in a dry boiling tube.
-
Copper sulphate becomes white and
some water droplets appear in the boiling tube.
-
When 2-3 drops of water are added,
copper sulphate crystals restore its blue colour.
-
Copper
sulphate crystals contain water of crystallisation. When heated the crystals,
this water is removed and the salt turns white.
-
Water
of crystallisation is the fixed number of water molecules present in one
formula unit of a salt. Five water molecules are present in one formula unit of
hydrated copper sulphate (Cu SO4. 5H2O).
-
Similarly,
the molecule of Na2CO3.10H2O is wet.
-
Gypsum
is also a salt possessing
water of crystallisation (CaSO4.2H2O).
Plaster
of Paris
- On
heating gypsum at 373 K, it loses water molecules and becomes a white powder.
This is called Plaster of Paris or Calcium sulphate
hemihydrate (CaSO4.
½ H2O).
- Plaster
of Paris on mixing with water, it changes to gypsum again giving a hard solid mass.
CaSO4.
½ H2O +1½ H2O → CaSO4.2H2O
(Plaster of Paris) (Gypsum)
- Only
half a water molecule is shown to be attached as water of crystallisation. It
is written in this form because two formula units of CaSO4 share one
molecule of water.
- Uses
of Plaster of Paris:
o Used
as plaster to support fractured bones.
o To
make toys and materials for decoration.
o To
make surfaces smooth.
- Plaster
of Paris gets its name from large gypsum deposits in Montmartre in Paris.
