Class X: Chemistry
Chapter 2: Acids, Bases and Salts
Top Concepts
1. Definition of acids , bases and salts:
Acids Bases Salts
Sour in taste
Bitter in taste &
soapy to touch
Acid + Base ® Salt
+ Water
Give H+ or H3O+
ions in aqueous
solutions
Give OH- ions in
aqueous solutions
2. On the basis of origin, acids are classified as:
a. Organic acids: Acids derived from living organisms like plants and
animals are called organic acids. They are weak acids and are not
harmful for living organisms. For example: citric acid is present in
fruits, acetic acid present in vinegar, oxalic acid present in
tomato, tartaric acid present in tamarind, lactic acid present in
sour milk and curd.
b. Mineral acids: They are also called inorganic acids. They are
dangerous and corrosive. Special precautions have to be taken
while handling them. For example: sulphuric acid (H2SO4),
hydrochloric acid (HCl) etc.
3. On the basis of their strength, acids are classified as:
a. Strong acids: Strong acids are those acids which completely
dissociate into its ions in aqueous solutions. Example: nitric acid
(HNO3), sulphuric acid (H2SO4), hydrochloric acid (HCl).
b. Weak acids: Weak acids are those acids which do not completely
dissociate into its ions in aqueous solutions. For example:
carbonic acid (H2CO3), acetic acid (CH3COOH).
4. On the basis of their concentration, acids are classified as:
a. Dilute acids: Have a low concentration of acids in aqueous
solutions.
b. Concentrated acids: Have a high concentration of acids in
aqueous solutions.
5. Alkalies: Water soluble bases are called alkalies. For example: Sodium
hydroxide (NaOH), potassium hydroxide (KOH).
6. On the basis of their strength, bases are classified as:
a. Strong bases: Strong bases are those bases which completely
dissociate into its ions in aqueous solutions. Example: sodium
hydroxide (NaOH), potassium hydroxide (KOH).
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b. Weak bases: Weak bases are those bases which do not
completely dissociate into its ions in aqueous solutions. For
example: ammonium hydroxide (NH4OH).
7. On the basis of their concentration, bases are classified as:
a. Dilute bases: Have a low concentration of alkali in aqueous
solutions.
b. Concentrated bases: Have a high concentration of alkali in
aqueous solutions.
8. Acids and bases conduct electricity because they produce ions in
water. There is a flow of electric current through the solution by ions.
9. Indicators are those chemical substances which behave differently in
acidic and basic medium and help in determining the chemical nature
of the substance. Acid base indicators indicate the presence of an acid
or a base by a change in their colour or smell.
10. Indicators can be natural or synthetic.
11. Olfactory indicators: These are those indicators whose odour changes
in acidic or basic medium. Example: onion
12. Onion: Smell of onion diminishes in a base and remains as it is in an
acid.
13. Vanilla essence: The odour of vanilla essence disappears when it is
added to a base. The odour of vanilla essence persists when it is added
to an acid.
14. Turmeric: In acids, yellow colour of turmeric remains yellow. In bases,
yellow colour of turmeric turns red.
15. Litmus: Litmus is a natural indicator. Litmus solution is a purple dye
which is extracted from lichen. Acids turn blue litmus red. Bases turn
red litmus blue. Water is essential for acids and bases to change the
colour of litmus paper. Remember that litmus paper will act as an
indicator only if either the litmus paper is moist or the acid or base is
in the form of aqueous solution. This is because acids and bases
release H+ and OH- ions respectively in aqueous solutions.
16. Phenolphthalein: Phenolphthalein remains colourless in acids but turn
pink in bases.
17. Methyl orange: Methyl orange turns pink in acids and becomes yellow
in bases.
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18. Reaction of acids and bases with water:
Acids Bases
Release H+ or H3O+ ions in water Release OH- ions in water
HCl + H2O ® H3O+ + Cl- H2O NaOH(s) Na (aq) OH (aq) + −
¾¾¾® +
19. Reaction of acids and bases with metals:
Metals displace hydrogen from the acids and form salt and hydrogen
gas. This is a displacement reaction. So, acids react with only those
metals which are placed above hydrogen in the reactivity series so that
metals can displace hydrogen from acids.
Zn + H2SO4 ® ZnSO4 + H2
Metal Acid Salt Hydrogen gas
Bases react with some metals to form salt and hydrogen gas.
Zn + 2 NaOH ® Na2ZnO2 + H2
Metal Base Sodium Hydrogen gas
zincate
(salt)
Bases do not react with all the metals to form salt and water.
20. Reaction of acids and bases with metal carbonates:
Acids react with metal carbonate to form salt, water and release
carbon dioxide.
2 3 2 2 Na CO + 2HCl®2NaCl+CO +H O
Bases do not react with metal carbonates.
21. Reaction of acids and bases with metal bicarbonates:
Acids react with metal bicarbonate to form salt, water and release
carbon dioxide.
3 2 2 NaHCO +HCl®NaCl+CO +H O
Bases do not react with metal bicarbonates.
22. Reaction of acids with bases: Neutralisation reaction:
Acids react with bases to form salt and water.
HCl + NaOH ® NaCl + H2O
or
H+ +OH- ® H2O
23. Reaction of acids with metallic oxides:
Metallic oxides are basic. Therefore, acids react with metallic oxides to
form salt and water.
HCl + CuO ® CuCl2 + H2O
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24. Reaction of bases with non-metallic oxides: Non – metallic oxides are
acidic in nature. Bases react with non- metallic oxides to form salt and
water. Example: CO2
Ca(OH)2 + CO2 ® CaCO3 + H2O
25. Amphoteric oxides: Oxides which show acidic as well as basic
properties. For example: ZnO, Al2O3
HCl + ZnO ® ZnCl2 + H2O
Zn + 2 NaOH ® Na2ZnO2 + H2O
26. Neutral oxides: Oxides which are neither acidic nor basic are called
neutral oxides. Example: CO
27. pH: It is used to find out the strength of acids and bases i.e., how
strong or weak the acid or a base is. p in pH stands for ‘potenz’ in
German. The strength of acids and bases depends on the number of
H+ ions and OH- ions produced respectively.
28. pH scale: A scale for measuring hydrogen ion concentration in a
solution is called pH scale.
29. On pH scale, we measure pH from 0 to 14.
pH value:
pH Type of solution
Less than 7 Acidic
Equal to 7 Neutral
More than 7 Basic
30. More the hydrogen ion (or hydronium ion) concentration, lower is the
pH value.
31. More the hydroxyl ion concentration, higher is the pH value.
32. Variation in pH:
33. Acids which produce more hydrogen ions are said to be strong acids
and acids which produce less hydrogen ions are said to be weak acids.
In other words, strong acids have a lower pH value than weak acids.
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34. Bases which produce more hydroxyl ions are said to be strong bases
and bases which produce less hydroxyl ions are said to be weak bases.
In other words, strong bases have a higher pH value than weak bases.
35. Living organisms are pH sensitive. Human body works within a pH
range of 7.0 to 7.8.
36. Rain water with a pH less than 5.6 is called acid rain. This acid rain if it
flows into river water makes the survival of aquatic life difficult.
37. Plants also require a specific pH range of soil for their healthy growth.
38. pH of our digestive system: Our stomach produces hydrochloric acid
for digestion of food. But during indigestion, excess of acid is produced
in the stomach and therefore, the pH decreases. This causes pain and
irritation. So, to neutralise this excess acid, a mild base is used. This
mild base works as an antacid. An antacid is any substance, generally
a base or basic salt, which counteracts stomach acidity.
39. Tooth decay: Tooth decay starts when the pH of the mouth is lower
than 5.5. Tooth enamel is made up of calcium phosphate which is the
hardest substance in the body. It does not dissolve in water, but is
corroded when the pH in the mouth is below 5.5. If food particles
remain in the mouth after eating, bacteria present in our mouth
produce acid by degradation of sugar. This decreases the pH of mouth
and hence tooth decay occurs. The best way to prevent this is to clean
the mouth after eating food. Using toothpastes, which are generally
basic, for cleaning the teeth can neutralise the excess acid and prevent
tooth decay.
40. pH is also significant as it is used in self defence by animals and
plants. Bees use acids in their sting. To neutralise the effect a mild
base like baking soda can be used.
41. Sodium hydroxide (NaOH)
Preparation:
Chlor Alkali process:
In this process, electricity is passed through an aqueous solution of
Sodium chloride (called brine). Sodium chloride decomposes to form
sodium hydroxide. Chlorine gas is formed at the anode, and hydrogen
gas at the cathode. Sodium hydroxide solution is formed near the
cathode.
2NaCl(aq) + 2 H2O (l) ® 2NaOH(aq) + Cl2(g) + H2(g)
42. Bleaching powder: Bleaching powder is represented as CaOCl2, though
the actual composition is quite complex.
Preparation: Bleaching powder is produced by the action of chlorine on
dry slaked lime.
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Ca(OH)2 + Cl2 ® CaOCl2+ H2O
43. Baking soda: Sodium hydrogen carbonate (NaHCO3)
Preparation:
NaCl + H2O + CO2+ NH3 ® NH4Cl + NaHCO3
44. Washing soda: Sodium carbonate Na2CO3 .10H2O
Preparation:
In the first step, sodium carbonate is obtained by heating baking
soda.
2 NaHCO3 ¾¾¾®
Heat
Na2CO3 + H2O + CO2
Then washing soda is produced by recrystallisation of sodium
carbonate.
Na2CO3 + 10H2O ® Na2CO3 .10H2O
45. Plaster of Paris: Calcium sulphate hemihydrate CaSO4. ½ H2O
Preparation: Plaster of Paris is prepared by heating Gypsum at 373K.
CaSO4. 2H2O
Heat at 373K
¾¾¾¾¾® CaSO4. ½ H2O + 1½ H2O
46. Water of crystallisation: It is the fixed number of water molecules
present in one formula unit of a salt.