What will be the increasing order of acid strength? 

Ethanol< Phenol< Acetic acid < Chloroacetic acid

The correct answer is option 3. Ethanol< Phenol< Acetic acid < Chloroacetic acid.

The acidity of a compound is determined by its ability to donate a proton (H⁺) in solution. Let's break down the acidity of each compound in the given options:

Ethanol (CH₃CH₂OH):

Ethanol contains a hydroxyl group (-OH), which can donate a proton to form the ethoxide ion (CH₃CH₂O⁻). However, the acidity of ethanol is relatively weak because the resulting ethoxide ion is not stabilized.

Phenol (C₆H₅OH):  

Phenol also contains a hydroxyl group, but it is attached to a benzene ring. The conjugate base of phenol, called phenoxide ion (C₆H₅O⁻), is stabilized by resonance within the benzene ring. This resonance delocalizes the negative charge over the entire ring, making the phenoxide ion more stable and thus increasing the acidity of phenol compared to ethanol.

Acetic acid (CH₃COOH):

Acetic acid contains a carboxylic acid group (-COOH), which can donate a proton to form the acetate ion (CH₃COO⁻). The conjugate base of acetic acid is stabilized by resonance between the carbonyl group (C=O) and the oxygen of the carboxylate group (-COO⁻). This resonance delocalizes the negative charge, making the acetate ion more stable and acetic acid more acidic than phenol and ethanol.

Chloroacetic acid (ClCH₂COOH):

Chloroacetic acid is similar to acetic acid but with a chlorine atom attached to the carbon of the carboxylic acid group. The electronegative chlorine atom withdraws electron density from the carboxylate group through the inductive effect, making the conjugate base (chloroacetate ion, ClCH₂COO⁻) even more stable than that of acetic acid.This increased stability of the conjugate base enhances the acidity of chloroacetic acid, making it the strongest acid among the compounds listed.

The order of increasing acidity among the given compounds is as follows: Ethanol < Phenol < Acetic acid < Chloroacetic acid

Each subsequent compound in the list is more acidic than the preceding one due to the presence of electron-withdrawing groups or resonance stabilization of the conjugate base.

Therefore, the explanation supports the answer: Ethanol < Phenol < Acetic acid < Chloroacetic acid