In keto-enol tautomerism, a carbonyl compound with a hydrogen atom on its alpha-carbon rapidly equilibrates with its corresponding enol.

$$C{H_3} - CHCl - C{H_2} - C{H_3},$$       Priority order is $$ - Cl > - {C_2}{H_5} > - C{H_3} > - H$$

In \[C{{H}_{3}}\overset{\begin{smallmatrix} O \\ \parallel \end{smallmatrix}}{\mathop{-C-}}\,C{{H}_{2}}\overset{\begin{smallmatrix} O \\ \parallel \end{smallmatrix}}{\mathop{-C-}}\,C{{H}_{2}}-C{{H}_{3}},\]       \[-C{{H}_{2}}-\]   group is flanked on both sides by electron- withdrawing groups and hence its hydrogens are most acidic. Once a carbanion is formed, it is stabilised due to resonance.

Two or more compounds having the same molecular formula but different physical, chemical properties and structural formula are called isomers.

$$\left( {\text{i}} \right)C{H_3}F{\text{ - }}s{p^3}$$    hybridised carbon, tetrahedral shape $$\left( {{\text{ii}}} \right)HC \equiv N{\text{ - }}sp$$    hybridised carbon, linear shape.

$$\eqalign{ & C{H_2} = CH - C \equiv CH; \cr & {\text{No}}\,{\text{of}}\,\sigma \,{\text{bonds}} = \,2 + 1 + 1 + 1 + 1 + 1 = 7; \cr & {\text{No}}\,{\text{of}}\,\pi \,{\text{bonds}} = 1 + 2 = 3 \cr} $$

$${C_2}{H_2}$$  have acidic nature but it is less acidic than water. Phenol is more acidic than water due to resonance stabilisation of phenoxide ion. $${H_2}C{O_3}$$  is most acidic due to resonating stabilisation of carbonate ion $$\left( {CO_3^{2 - }} \right).$$
Thus, the correct order is
$${C_2}{H_2} < {H_2}O < phenol < {H_2}C{O_3}$$

Crystallisation is based on the difference in the solubilities of the compound and the impurities in a suitable solvent.