What are the chemical changes that are reversible?

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Solution

1) Thermal decomposition of salts such as hydrates copper-sulphate results in the dehydration of the salt.
$C u S O_{4} .5 H_{2} O \to C u S O_{4} (s) + 5 H_{2} O (g)$
2) Decomposition of ammonium chloride is also a reversible reaction. Ammonium chloride decomposes on heating to ammonia and hydrogen chloride gas.
$N H_{4} C l \to N H_{3} + H C l$
However, on cooling, white solid of ammonium chloride is formed.
$N H_{3} (g) + H C l (g) \to N H_{4} C l (s)$
3) Formation of water by combustion of hydrogen.
$H_{2} + O_{2} \to H_{2} O$
Electrolytic decomposition of water gives back $H_{2}$ and $O_{2}$ gas
$H_{2} O + H e a t \to H_{2} + O_{2} .$

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