Q.Why phenol is more acidic than ethyl alcohol ?

Step 1. Compare the two ionisation equilibria (section 11.4.4). Ethanol: CH3-CH2-OH + H2O <=> CH3-CH2-O(-) + H3O+. Phenol: C6H5-OH + H2O <=> C6H5-O(-) + H3O+. Both equilibria lie mostly to the left, but the position of each equilibrium -- and hence the relative acid strength -- depends on how well each conjugate base (ethoxide vs. phenoxide) is stabilised.

Step 2. Ethoxide is destabilised. The ethyl group's alkyl substituent exerts an electron-DONATING inductive (+I) effect, which pushes MORE electron density onto the already-negative oxygen of the ethoxide ion. Since like charges concentrated together are destabilising, this +I effect makes ethoxide LESS stable, so ethanol barely ionises at all and behaves as essentially neutral in aqueous solution.

Step 3. Phenoxide is stabilised. In phenoxide, the oxygen's negative charge can delocalise into the aromatic ring through resonance: the lone pair on oxygen conjugates with the ring's pi system, spreading the negative charge out onto the ortho and para ring carbons as well (five resonance contributing structures in total, as shown in Fig. 11.1/section 11.4.4). Spreading a charge out over a larger volume (multiple atoms) is stabilising, so phenoxide is considerably more stable than a simple, non-delocalised alkoxide like ethoxide.

Step 4. Conclude. Because its conjugate base is comparatively stabilised (by resonance) rather than destabilised (by induction), phenol ionises to a measurably greater extent than ethanol, and so shows a genuine (if still weak) acidic character that ethanol lacks.

Comparing the relative stability of phenoxide (resonance-stabilised) versus ethoxide (inductively destabilised) conjugate bases, per section 11.4.4's ionisation-equilibrium analysis

• Describing phenol's acidity without mentioning resonance/delocalisation specifically -- the explanation must be conjugate-base stability, not merely 'phenol has a benzene ring'.
• Forgetting to explain why ethanol is comparatively LESS acidic too (the destabilising +I effect of the alkyl group), giving only half the comparison.