Question

Write the differences between the reactions of NaOH in water and CaO inwater.

Answer 1

Sr No.	Point	NaOH in water	CaO in water
1.	Reaction & equation	Dissolution/ionisation: NaOH(s) → Na + (aq) + OH − (aq). Dissolving solid NaOH in water produces free OH⁻ ions.	Chemical hydration (slaking): CaO(s) + H2O(l) → Ca(OH)2 (s or aq). CaO reacts with water to form slaked lime (calcium hydroxide).
2.	Nature of process	Primarily a physical dissolution with ionic dissociation (solid → solvated ions). The process is strongly exothermic for NaOH.	A chemical combination (oxide + water → new hydroxide compound), known as “slaking” of lime, is exothermic.
3.	Solubility / final medium	NaOH is highly soluble → forms a clear, strongly alkaline solution (high [OH−]).	Ca(OH)2 is only sparingly soluble: a saturated “lime water” is alkaline; excess Ca(OH)2 may remain as a suspension. Lime water's behaviour (and reaction with CO2) is described in the files.
4.	Heat & safety	Considerable heat is released on dissolving solid NaOH; handle carefully (NaOH is corrosive).	Heat is released during slaking (can be appreciable); handle with care (CaO/Ca(OH)2 are caustic).
5.	Observations / uses	Rapid temperature rise; gives a clear caustic solution used where strong alkali is required. Warning: corrosive.	Forms slaked lime (paste or milky suspension if excess); used in mortar and lime water testing (turns milky with CO2).