Acidity of phenols

Acid + $NaHCO_3 \rightarrow$ salt + $H_2CO_3$; the reaction goes forward only if the acid is stronger than $H_2CO_3$.
o-Nitrophenol is less acidic than $H_2CO_3$, so it is not soluble in sodium hydrogen carbonate.
2,4,6-Trinitrophenol, benzoic acid and benzenesulphonic acid are soluble in $NaHCO_3$.

The acidity of a phenol depends on the stability of its phenoxide ion.
An electron-withdrawing group ($-NO_2$) disperses the negative charge of the phenoxide ion and increases acidity; an electron-donating group ($-CH_3$) intensifies the charge and decreases acidity.
p-Nitrophenol: the nitro group at the para position withdraws electrons by both −I and −R effects, so the charge is delocalised onto the nitro group; most acidic ($pK_a$ = 7.1).
m-Nitrophenol: at the meta position only the −I effect operates; less acidic than the para isomer ($pK_a$ = 8.3).
Phenol: no substituent ($pK_a$ = 10.0).
Methyl phenol (cresol): the +I and hyperconjugation effects of $-CH_3$ make it the least acidic ($pK_a \approx$ 10.2).
Acidity order: d > c > a > b