Define the following types of chemical changes or reactions with a suitable example of each.

(a) Direct combination reaction or synthesis

(b) Decomposition reaction

(c) Displacement reaction or substitution reaction

(d) Double decomposition reaction

(a) A chemical reaction in which two or more elements or compounds react to form one new compound is called a Direct combination reaction or synthesis.

For example,

Hydrogen burns in air to give water:

  2H₂ [g] + O₂ [g] ⟶ 2H₂O [l]

(b) A chemical reaction in which a chemical compound decomposes into two or more simpler substances (elements and/or compounds) is called a Decomposition reaction.

For example,

Calcium carbonate on heating decomposes to Calcium Oxide and Carbon dioxide:

  CaCO₃ $\xrightarrow{\enspace\Delta\enspace}$ CaO + CO₂

(c) A chemical reaction in which an element or radical has replaced another element in a compound is known as Displacement reaction or Substitution reaction.

For example,

Magnesium displaces Copper from Copper [II] sulphate solution:

Mg + CuSO₄ ⟶ MgSO₄ + Cu

(d) A chemical reaction in which both reactants [compounds] are decomposed to give two new compounds by exchanging their radicals is known as Double decomposition reaction.

For example,

Silver nitrate + Potassium chloride ⟶ Silver chloride + Potassium nitrate

AgNO₃ + KCl ⟶ AgCl + KNO₃