Arrange the following in increasing order of acid strength.
(A) Phenol
(B) Ethanol
(C) p-Nitrophenol
(D) Water
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (B) < (D) < (A) < (C)
An acid is stronger when its anion is more stable. A lower pKa means a stronger acid.
(B) Ethanol — weakest (pKa 15.9). It is a weaker acid than water: water gives a proton to the ethoxide ion and sets ethanol free.
(D) Water — second. It is a stronger acid than ethanol, but phenol, which reacts with aqueous NaOH, is stronger than water.
(A) Phenol — third (pKa 10.0). The negative charge of the phenoxide ion is delocalised over the ring, so phenol is about a million times more acidic than ethanol.
(C) p-Nitrophenol — strongest (pKa 7.1). The electron withdrawing nitro group at the para position spreads the negative charge of the phenoxide ion further.
NCERT (2026-27 edition):
7.4.4 Chemical Reactions – “Alcohols are, however, weaker acids than water.”
“The reaction of phenol with aqueous sodium hydroxide indicates that phenols are stronger acids than alcohols and water.”
“In substituted phenols, the presence of electron withdrawing groups such as nitro group, enhances the acidic strength of phenol. This effect is more pronounced when such a group is present at ortho and para positions.”
“From the above data, you will note that phenol is million times more acidic than ethanol.”
Table 7.3: pKa Values of some Phenols and Ethanol
