Give reasons for the following:
(a) Sulphuric acid forms two types of salts with NaOH.
(b) Red-brown vapours are produced when concentrated sulphuric acid is poured on it
(c) A piece of wood becomes black when concentrated sulphuric acid is poured on it.
(d) Brisk effervescence is seen when oil of vitriol is added to sodium carbonate.
[4 MARKS]

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Solution

Each option: 1 Mark

(a) Sulphuric acid forms two types of salts with NaOH because it is a dibasic acid.
$N a O H + H_{2} S O_{4} \to N a H S O_{4} S o d i u m h y d r o g e n s u l p h a t e + H_{2} O 2 N a O H + H_{2} S O_{4} \to N a_{2} S O_{4} S o d i u m s u l p h a t e + 2 H_{2} O$

(b) When conc. $H_{2} S O_{4}$ is added to hydrogen bromide (HBr), red brown vapours of bromine gas are produced due to the oxidation of hydrogen bromide.
$H_{2} S O_{4} (c o n c .) \to H_{2} O + S O_{2} + O 2 H B r + O \to H_{2} O + B r_{2}$
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$H_{2} S O_{4} (c o n c .) + 2 H B r \to 2 H_{2} O + S O_{2} + B r_{2}$
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(c) A piece of wood becomes black when conc. $H_{2} S O_{4}$ is poured on it due to the dehydrating action of the acid. Wood, which is largely cellulose $[C_{6} H_{10} O_{5}]_{N}$ gets charred by conc. $H_{2} S O_{4}$ .
$C_{6} H_{10} O_{5} C o n c . H_{2} S O_{4} \to 6 C + 5 H_{2} O$

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(ii) When solution of sulphur dioxide is exposed to air, it gets converted to sulphuric acid.
(iii) When diluting concentrated sulphuric acid, the acid should be added to water and not water to the acid.
(iv) When concentrated sulphuric acid is exposed to air, its volume increases and it becomes slightly dilute.
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(vi) When barium chloride is added to dilute sulphuric acid, a white precipitate is formed.
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(xii) Concentrated sulphuric acid must be stored in airtight bottles.
(xiii) Cotton clothes get burnt with concentrated sulphuric acid.
(xiv) H₂SO₄ cannot be prepared by heating sodium sulphate with conc. HCl or HNO₃.

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