Mostly all the chemical changes are irreversible but some are reversible also eg.-  dimerization of nitrogen dioxide. Nitrogen dioxide dimerizes to give dinitrogen tetroxide. Upon heating the reaction reverses and nitrogen dioxide is formed again, decomposition of ammonium chloride and some salts are reversible chemical changes too.

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No they are not chemical changes. The reversible chemical changes are oxygen and hydrogen and carbon - di - oxide .
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