The reaction of quicklime (CaO) with water to form slaked lime, Ca(OH)2, is an example of a/an:

An exothermic reaction is a chemical reaction that releases heat (energy) into its surroundings. An endothermic reaction absorbs heat from its surroundings.

When water is added to quicklime (calcium oxide), it reacts vigorously to form slaked lime (calcium hydroxide). A large amount of heat is released, and the water may even boil and hiss:

CaO (s) + H2O (l) → Ca(OH)2 (aq) + heat

This reaction is also a combination reaction, because two substances combine to form a single product. It is a common example in NCERT Class 10 Science.

A solution of slaked lime in water is used for whitewashing walls. After the whitewash is applied, the calcium hydroxide reacts slowly with carbon dioxide in the air to form a thin, shiny layer of calcium carbonate:

Ca(OH)2 + CO2 → CaCO3 + H2O

This reaction takes two to three days, and it gives the walls their white, shining finish. A clear solution of calcium hydroxide is called lime water. It turns milky when carbon dioxide is passed through it, which is the standard test for CO2.

Other examples of exothermic reactions include the burning of fuels such as coal and natural gas, respiration (in which glucose combines with oxygen to release energy), and the neutralisation of an acid with a base. Examples of endothermic reactions include photosynthesis, the decomposition of calcium carbonate into quicklime and carbon dioxide on heating, and dissolving ammonium chloride in water.

Quicklime is made by heating limestone (CaCO3) in a lime kiln; this decomposition reaction is endothermic.

Why not the other options?

• ✗An endothermic reaction absorbs heat; this reaction releases it.
• ✗A decomposition reaction breaks a compound into simpler substances; here two substances combine.
• ✗A redox reaction involves the transfer of electrons; the oxidation states do not change here.