Page 109 - Chemistry ICSE Class X

Study of Acids, Bases and Salts 95

Let us say it Again

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— Acid. A hydrogen-containing compound which gives free hydrogen ions (H O or H ) when dissolved in water is called
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an acid.
 Naturally-occurring acids are weak, mineral acids are strong. For example, citric acid and lactic acid are naturally-
occurring acids. Hydrochloric acid, sulphuric acid etc., are mineral acids.
 Acids turn blue litmus to red.
 Acids give hydrogen gas when reacted with active metals, viz, zinc, magnesium, iron, etc.
 Acids decompose carbonates to liberate carbon dioxide.
 Acids which are almost completely dissociated/ionised when dissolved in water are called strong acids. Hydrochloric
acid (HCl), sulphuric acid (H SO ), nitric acid (HNO ) are typical strong acids.
2 4 3
 The acids which are partially dissociated/ionised when dissolved in water are called weak acids. Acetic acid (CH COOH),
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carbonic acid (H CO ), sulphurous acid (H SO ) are weak acids.
2
3
3
2
— Basicity of an acid. The number of free hydrogen ions (H ) produced by one molecule of an acid is called its basicity.
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 Acids having basicity of one are called monobasic acid. HCl(aq), HNO (aq), CH COOH(aq) are monobasic acids.
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3
 Acids having basicity of two are called dibasic acids. H SO , oxalic acid, carbonic acid (H CO ) are dibasic acids.
2 4 2 3
 Acids having basicity of three are called tribasic acids. Phosphoric acid (H PO ) is a tribasic acid.
3 4
— Preparation of acid. Acids can be prepared
 By synthesis
+H O
hv
2
H (g) + Cl (g) o 2HCl(g) o 2HCl(aq)
2 2
 By the action of water on an acidic oxide
CO (g) + H O(l) o H CO (aq)
2 2 2 3
SO (g) + H O(l) o H SO (aq)
3 2 2 4
 By the displacement of more volatile acid from their salts by less volatile acid
+H O
'
2
NaCl(s) + H SO o NaHSO + HCl(g) o HCl(aq)
2 4 4
 By the oxidation of nonmetals
S + 6HNO (conc.) o H SO (aq) + 2H O + 6NO (g)
2
2
2
4
3
P + 5HNO o H PO 4 + H O + 5NO 2
2
3
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— Base. A compound which gives free hydroxide ions (OH ) when dissolved in water is called a base. For example, sodium
–
hydroxide, sodium carbonate etc. are bases.
 Base turn red litmus to blue.
 Base have bitter taste.
 The bases which are highly soluble in water and contain one or more hydroxyl ions in their molecules are called
alkalis. NaOH, KOH are alkalis.
 Bases which are almost completely dissociated/ionised in their solutions are called strong bases. NaOH, KOH, Ba(OH)
2
are strong bases.
 Bases which are partially dissociated/ionised in their solutions are called weak bases. NH OH, Ca(OH) , Mg(OH) are
4 2 2
weak bases.
— Acidity of a base. The number of hydroxide ions furnished by one molecule of the base in its aqueous solution is called
its acidity.
 The acidity of a base may be 1, 2 or 3.
 The bases having acidity of 1 are called monoacid bases, those having acidity of 2 are called diacid bases and those
having acidity of 3 are called triacid bases.