★ We have studied matter on the basis of composition, state,
solubility, types of mixture, types of compound.
★ Now in this chapter we will categorise some compounds on
the basis of their nature as acids, bases and salts.
1. Introduction
The chemicals which we come across are usually categorised as
acids, bases and salts.
Earlier this classification was based on different taste of chemicals.
Acids were first recognized as substances that taste sour. Vinegar tastes sour because it is a
dilute solution of acetic acid. Citric acid is responsible for the sour taste of a lemon.
Bases, sometimes called alkalis, are characterised by their bitter taste and they are soapy
to feel. Most hand soaps and commercial preparation for unclogging drains are highly basic.
Substances having taste similar to that of common salt are called salts. However, some
substances have very unpleasant taste and even may be poisonous. Usually salts are
prepared by chemical reaction between acids and bases.
[51]
, Now, the question arises – how to test that the given substance is an acid or a base without
tasting it? In this chapter, we shall discuss the common characteristics of acids and bases
and their chemical nature. We shall also discuss how salts are produced and important
applications of some of these salts.
2. Acids
The term "acid" is derived from the latin word "acidus" meaning sour in taste.
According to Arrhenius theory, "An acid is a substance which when dissolved in water,
ionizes and releases hydrogen ions [H+(aq)] in solution".
Example,
(i) HCl(aq) ⎯⎯→ H+(aq) + Cl–(aq)
Hydrochloric acid Hydrogen ion Chloride ion
OR
HCl(g) + H2O() ⎯⎯→ H3O+(aq) + Cl–(aq)
(ii) H2SO4(aq) ⎯⎯→ 2H+(aq) + SO42–(aq)
Sulphuric acid Hydrogen ion Sulphate ion
3. Bases
According to Arrhenius theory "Those substances which
give hydroxide or hydroxyl ion (OH–) in their aqueous
solution are called bases".
For example,
(i) NaOH(aq) → Na+(aq) + OH–(aq)
Sodium hydroxide Sodium ion Hydroxyl ion
(ii) KOH(aq) → K+(aq) + OH–(aq)
Potassium hydroxide Potassium ion Hydroxyl ion
★ In earlier classes, we have studied about some common indicators like – litmus paper.
★ As we know, acid turn blue litmus paper red and base turns red litmus paper blue.
4. Chemical properties of acids and bases
1. Action of indicators
Indicators: It is a chemical which indicates the nature of solution by changing its
colour/odour.
[52]
, Natural indicators: Some plants are natural indicators. For example, onion extract.
Litmus is obtained from a plant called lichens.
Litmus paper is made by soaking paper in litmus solution. It turns red in acid and blue in
alkali.
Turmeric (haldi) is commonly used as a spice in kitchen. It is yellow in colour and stains
a white cloth yellow. If the yellow stain is rubbed with soap solution, the colour of the
stain changes to brownish red. Why does this happen?
The soap solution always contains a small amount of chemical called sodium hydroxide.
It is the sodium hydroxide which changes the colour of turmeric stain from yellow to
brownish red. Now, if we rub stain with lemon, it again changes to yellow colour. The
lemon contains another substance called citric acid, which neutralises the effect of
sodium hydroxide.
Olfactory indicators: Indicators giving different odour in acidic and basic medium are
called olfactory indicators.
Examples: Onion extract, vanilla and clove oil.
What causes indicators to show different colours in acids and bases?
Explanation
Indicators undergo some structural changes in the presence of acids or bases which causes
them to show different colours.
Note: Litmus is the most commonly used indicator in the lab.
Aim
To test acids and bases in the laboratory using acid-base indicators that changes colour.
Materials required
Watch glass, test tube, hydrochloric acid, sulphuric acid, nitric acid, acetic acid, sodium
hydroxide, calcium hydroxide, potassium hydroxide, magnesium hydroxide, ammonium
hydroxide, red and blue litmus solution, phenolphthalein solution, methyl orange solution.
Method
(i) Collect the following samples from the science laboratory-hydrochloric acid (HCl),
sulphuric acid (H2SO4), nitric acid (HNO3), acetic acid (CH3COOH), sodium hydroxide
(NaOH), calcium hydroxide [Ca(OH)2], potassium hydroxide (KOH), magnesium
hydroxide [Mg(OH)2], and ammonium hydroxide (NH4OH).
[53]
, (ii) Put a drop of each of the above solutions on a watch-glass and test with a drop of the
following indicators.
Red litmus solution, Blue litmus solution, Phenolphthalein solution, Methyl orange
solution.
Observation
The changes observed are as follows:
Methyl
Red Blue
Phenolphthalei orange
Sample solution litmus litmus
n solution solutio
solution solution
n
Hydrochloric acid (HCl) No effect Red Colourless Red
Sulphuric acid (H2SO4) No effect Red Colourless Red
Nitric acid (HNO3) No effect Red Colourless Red
Acetic acid (CH3COOH) No effect Red Colourless Red
Sodium hydroxide(NaOH) Blue No effect Pink Yellow
Calcium hydroxide (Ca(OH)2) Blue No effect Pink Yellow
Potassium hydroxide (KOH) Blue No effect Pink Yellow
Magnesium hydroxide (Mg(OH)2) Blue No effect Pink Yellow
Ammonium hydroxide (NH4OH) Blue No effect Pink Yellow
Conclusion
Acids turn blue litmus red but have no effect on red litmus.
Bases turn red litmus blue but have no effect on blue litmus.
Phenolphthalein is colourless in acidic medium and turns pink in basic medium.
Methyl orange is yellow in basic medium and red in acidic medium.
★ Colour of indicator in acidic/basic medium.
Neutral medium Acidic medium Basic medium
Indicator
(pH = 7) (pH < 7) (pH > 7)
Red litmus paper
No change No change Red to Blue
(Red)
Blue litmus paper
No change Blue to red No change
(Blue)
Phenolphthalein
No change No change Pink
(Colourless)
[54]
solubility, types of mixture, types of compound.
★ Now in this chapter we will categorise some compounds on
the basis of their nature as acids, bases and salts.
1. Introduction
The chemicals which we come across are usually categorised as
acids, bases and salts.
Earlier this classification was based on different taste of chemicals.
Acids were first recognized as substances that taste sour. Vinegar tastes sour because it is a
dilute solution of acetic acid. Citric acid is responsible for the sour taste of a lemon.
Bases, sometimes called alkalis, are characterised by their bitter taste and they are soapy
to feel. Most hand soaps and commercial preparation for unclogging drains are highly basic.
Substances having taste similar to that of common salt are called salts. However, some
substances have very unpleasant taste and even may be poisonous. Usually salts are
prepared by chemical reaction between acids and bases.
[51]
, Now, the question arises – how to test that the given substance is an acid or a base without
tasting it? In this chapter, we shall discuss the common characteristics of acids and bases
and their chemical nature. We shall also discuss how salts are produced and important
applications of some of these salts.
2. Acids
The term "acid" is derived from the latin word "acidus" meaning sour in taste.
According to Arrhenius theory, "An acid is a substance which when dissolved in water,
ionizes and releases hydrogen ions [H+(aq)] in solution".
Example,
(i) HCl(aq) ⎯⎯→ H+(aq) + Cl–(aq)
Hydrochloric acid Hydrogen ion Chloride ion
OR
HCl(g) + H2O() ⎯⎯→ H3O+(aq) + Cl–(aq)
(ii) H2SO4(aq) ⎯⎯→ 2H+(aq) + SO42–(aq)
Sulphuric acid Hydrogen ion Sulphate ion
3. Bases
According to Arrhenius theory "Those substances which
give hydroxide or hydroxyl ion (OH–) in their aqueous
solution are called bases".
For example,
(i) NaOH(aq) → Na+(aq) + OH–(aq)
Sodium hydroxide Sodium ion Hydroxyl ion
(ii) KOH(aq) → K+(aq) + OH–(aq)
Potassium hydroxide Potassium ion Hydroxyl ion
★ In earlier classes, we have studied about some common indicators like – litmus paper.
★ As we know, acid turn blue litmus paper red and base turns red litmus paper blue.
4. Chemical properties of acids and bases
1. Action of indicators
Indicators: It is a chemical which indicates the nature of solution by changing its
colour/odour.
[52]
, Natural indicators: Some plants are natural indicators. For example, onion extract.
Litmus is obtained from a plant called lichens.
Litmus paper is made by soaking paper in litmus solution. It turns red in acid and blue in
alkali.
Turmeric (haldi) is commonly used as a spice in kitchen. It is yellow in colour and stains
a white cloth yellow. If the yellow stain is rubbed with soap solution, the colour of the
stain changes to brownish red. Why does this happen?
The soap solution always contains a small amount of chemical called sodium hydroxide.
It is the sodium hydroxide which changes the colour of turmeric stain from yellow to
brownish red. Now, if we rub stain with lemon, it again changes to yellow colour. The
lemon contains another substance called citric acid, which neutralises the effect of
sodium hydroxide.
Olfactory indicators: Indicators giving different odour in acidic and basic medium are
called olfactory indicators.
Examples: Onion extract, vanilla and clove oil.
What causes indicators to show different colours in acids and bases?
Explanation
Indicators undergo some structural changes in the presence of acids or bases which causes
them to show different colours.
Note: Litmus is the most commonly used indicator in the lab.
Aim
To test acids and bases in the laboratory using acid-base indicators that changes colour.
Materials required
Watch glass, test tube, hydrochloric acid, sulphuric acid, nitric acid, acetic acid, sodium
hydroxide, calcium hydroxide, potassium hydroxide, magnesium hydroxide, ammonium
hydroxide, red and blue litmus solution, phenolphthalein solution, methyl orange solution.
Method
(i) Collect the following samples from the science laboratory-hydrochloric acid (HCl),
sulphuric acid (H2SO4), nitric acid (HNO3), acetic acid (CH3COOH), sodium hydroxide
(NaOH), calcium hydroxide [Ca(OH)2], potassium hydroxide (KOH), magnesium
hydroxide [Mg(OH)2], and ammonium hydroxide (NH4OH).
[53]
, (ii) Put a drop of each of the above solutions on a watch-glass and test with a drop of the
following indicators.
Red litmus solution, Blue litmus solution, Phenolphthalein solution, Methyl orange
solution.
Observation
The changes observed are as follows:
Methyl
Red Blue
Phenolphthalei orange
Sample solution litmus litmus
n solution solutio
solution solution
n
Hydrochloric acid (HCl) No effect Red Colourless Red
Sulphuric acid (H2SO4) No effect Red Colourless Red
Nitric acid (HNO3) No effect Red Colourless Red
Acetic acid (CH3COOH) No effect Red Colourless Red
Sodium hydroxide(NaOH) Blue No effect Pink Yellow
Calcium hydroxide (Ca(OH)2) Blue No effect Pink Yellow
Potassium hydroxide (KOH) Blue No effect Pink Yellow
Magnesium hydroxide (Mg(OH)2) Blue No effect Pink Yellow
Ammonium hydroxide (NH4OH) Blue No effect Pink Yellow
Conclusion
Acids turn blue litmus red but have no effect on red litmus.
Bases turn red litmus blue but have no effect on blue litmus.
Phenolphthalein is colourless in acidic medium and turns pink in basic medium.
Methyl orange is yellow in basic medium and red in acidic medium.
★ Colour of indicator in acidic/basic medium.
Neutral medium Acidic medium Basic medium
Indicator
(pH = 7) (pH < 7) (pH > 7)
Red litmus paper
No change No change Red to Blue
(Red)
Blue litmus paper
No change Blue to red No change
(Blue)
Phenolphthalein
No change No change Pink
(Colourless)
[54]
