According to the textbook context, a solution of slaked lime is used for whitewashing walls. This slaked lime (calcium hydroxide) is produced by the reaction of calcium oxide (quick lime) with water. The context explicitly states: 'Calcium oxide reacts vigorously with water to produce slaked lime (calcium hydroxide) releasing a large amount of heat.' Therefore, substance 'X' refers to quick lime (CaO), which when reacted with water produces the whitewashing solution. This is a combination reaction where two reactants combine to form a single product.