(a) What is meant by chemical reaction? Explain with the help of an example.
(b) Give one example each of a chemical reaction characterised by:
(i) evolution of a gas
(ii) change in colour
(iii) formation of a precipitate
(iv) change in temperature
(v) change in state.

(a) Chemical reaction is the process in which atoms of some elements rearrange—that is, undergo a chemical change—to form a new product with different properties. This involves breaking of old bonds between the reacting atoms and making of new chemical bonds between the rearranged atoms.

For example, the burning of magnesium ribbon in air. In this reaction, magnesium ribbon, on heating, combines with oxygen to form a white powder of magnesium oxide, which is totally different in its properties from magnesium and oxygen.
$\mathrm{2Mg}\left(s\right)+{\mathrm{O}}_2\left(g\right)\stackrel{heat}{\to }2\mathrm{MgO}\left(s\right)$

(b)
(i) Evolution of gas: When zinc granules react with dilute sulphuric acid in a flask, hydrogen gas evolves and zinc sulphate solution is formed.

${\mathrm{Zn+\; H}}_2{\mathrm{SO}}_4{\to \; ZnSO}_4{+\; H}_2\uparrow$

(ii) Change in colour: When sulphur dioxide gas is passed through acidified potassium dichromate solution, orange colour of potassium dichromate solution changes to green colour.

$3{\mathrm{SO}}_2\left(\mathrm{g}\right)+{\mathrm{K}}_2{\mathrm{Cr}}_2{\mathrm{O}}_7\left(\mathrm{aq}\right)\underset{}{\overset{}{+{\mathrm{H}}_2{\mathrm{SO}}_4\left(\mathrm{aq}\right)\to }}{\mathrm{K}}_2{\mathrm{SO}}_4\left(\mathrm{aq}\right)+{\mathrm{Cr}}_2\left({\mathrm{SO}}_4{)}_3\right(\mathrm{aq})+{\mathrm{H}}_2\mathrm{O}(\mathrm{l})$

(iii) Formation of a precipitate: When potassium iodide is added to lead nitrate solution (colourless and clear), yellow precipitate of lead iodide solution is formed.

$\mathrm{Pb}\left({\mathrm{NO}}_3{)}_2\right(\mathrm{aq})+2\mathrm{KI}(\mathrm{aq})\underset{}{\overset{}{\to }}{\mathrm{PbI}}_2(\mathrm{s})+2{\mathrm{KNO}}_3(\mathrm{aq})$

(iv) Change in temperature: When quicklime reacts with water, slaked lime is formed with a release of large amount of heat, causing change in temperature in the reaction mixture.

$\mathrm{CaO}+{\mathrm{H}}_2\mathrm{O}\underset{}{\to \mathrm{Ca}(\mathrm{OH}{)}_2+\mathrm{Heat}}$

(v) Change in state: The combustion reaction of candle wax (solid) melts initially to produce molten wax (liquid) and then produces carbon dioxide (gas) and water vapour (liquid). So, in this reaction, state of matter gets changed from solid to liquid and gas.