Question

Give one example each of a chemical reaction characterised by:
(a) evolution of a gas
(b) formation of a precipitate
(c) change in temperature
​Write equations of the reactions which take place in each case.

Answer 1

(a) When zinc granules react with dilute sulphuric acid in a flask, hydrogen gas is evolved and zinc sulphate solution is formed.
$\mathrm{Zn}\left(\mathrm{s}\right)+{\mathrm{H}}_2{\mathrm{SO}}_4\left(\mathrm{aq}\right)\to {\mathrm{ZnSO}}_4\left(\mathrm{aq}\right)+{\mathrm{H}}_2\left(\mathrm{g}\right)$

(b) When potassium iodide is added to lead nitrate solution (colourless and clear), a yellow precipitate of lead iodide solution is formed.
$\mathrm{Pb}\left({\mathrm{NO}}_3{)}_2\right(\mathrm{aq})+2\mathrm{KI}(\mathrm{aq})\to {\mathrm{PbI}}_2(\mathrm{s})+2{\mathrm{KNO}}_3(\mathrm{aq})$

(c) When quick lime reacts with water, slaked lime is formed along with release of a large amount of heat, causing a change in temperature in the reaction mixture.
$\mathrm{CaO}\left(\mathrm{s}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\to \mathrm{Ca}\left(\mathrm{OH}{)}_2\right(\mathrm{aq})+\mathrm{Heat}$