Explain the following chemical changes by giving one example of each:

(a) Double decomposition

(b) Thermal dissociation

(c) Reversible reaction

(d) Displacement

(a) Double decomposition — This is a type of chemical change in which two compounds in a solution react to form two new compounds by mutual exchange of radicals. Double decomposition reaction is also called double displacement reaction.
AB + CD ⟶ AD + CB

Eg., AgNO₃ + NaCl ⟶ AgCl ↓ + NaNO₃

(b) Thermal dissociation — A reversible decomposition reaction brought about only by heat is called thermal dissociation reaction.

Heat some solid ammonium chloride in a test tube. Two colourless gases, ammonia and hydrogen chloride, are produced. As these gases move up to the upper part of the test tube which is cooler, they combine to form ammonium chloride, which appears as a white sublimate on the upper cooler side of the test tube.

NH₄Cl ⇌ NH₃ ↑ + HCl ↑

(c) Reversible reaction — A reaction that can be reversed by changing the conditions under which the reaction is taking place is called a reversible reaction.

$\underset{\text{ red hot iron}}{\text{3Fe}} + \underset{\text{ steam}}{4\text{H}$2\text{O}} \rightleftharpoons \underset{\text{ magnetic oxide}}{\text{Fe}3\text{O}4} + \underset{\text{ hydrogen gas}}{4\text{H}2}

(d) Displacement — A chemical change in which a more active element displaces a less active element from its salt solution is called a displacement reaction.

Mg + H₂SO₄ ⟶ MgSO₄ + H₂ ↑