A reaction mixture contains a suspected carboxylic acid contaminated with an excess of phenol. Which of the following reagents is the most suitable to confirm the presence of the carboxylic acid group by a visible test-tube reaction?
NaOH(aq)\text{NaOH(aq)}NaOH(aq)
NaHCO3(aq)\text{NaHCO}_3\text{(aq)}NaHCO3(aq)
Br2(aq)\text{Br}_2\text{(aq)}Br2(aq)
Acidified K2Cr2O7(aq)\text{K}_2\text{Cr}_2\text{O}_7\text{(aq)}K2Cr2O7(aq)