Combination and decomposition

Combination: two or more substances join to make one product. Think of an aloo paratha: potato, flour, spices and ghee go in, and one paratha comes out. Many in, one out.

• Burning coal: C + O₂ → CO₂.
• Quick lime and water: CaO + H₂O → Ca(OH)₂ (slaked lime).

Drop quick lime (calcium oxide) into water and the beaker gets hot. A reaction that gives out heat is exothermic. Burning natural gas is exothermic, and so is respiration: the glucose from your food reacts with oxygen in your cells and gives you energy.

A real use: slaked lime solution is used for whitewashing walls. Over two or three days it reacts with carbon dioxide in the air to form a thin layer of calcium carbonate, which gives the walls their shiny white finish.

Decomposition is the opposite: one substance breaks into two or more simpler ones. Breaking needs energy, and the book shows three ways to supply it:

Heating green ferrous sulphate crystals: the colour changes and you can smell burning sulphur. Heating lead nitrate gives off brown fumes of nitrogen dioxide: 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂.

In electrolysis of water, the gas at one electrode is collected in twice the volume of the gas at the other. Read it straight from the equation: 2H₂O → 2H₂ + O₂ gives two hydrogen for every one oxygen.

White silver chloride turns grey in sunlight, because silver forms. Silver bromide does the same. These reactions were used in black and white photography.

Reactions that take in energy are endothermic, and most decomposition needs heat, light or electricity. But don't make it a rule: the book points out that vegetable matter rotting into compost is a decomposition that gives out heat.