Ethanol is soluble in water due to the presence of a

1. Analyze the question

The question asks for the specific functional group responsible for the solubility of ethanol ($C_2H_5OH$) in water.

2. Explain the concept

Solubility in water is largely determined by the ability of a molecule to form hydrogen bonds with water molecules.

• Ethanol consists of an alkyl chain (ethyl group) and a functional group attached to it.
• The hydroxyl group (-OH) is polar. Because oxygen is highly electronegative, it creates a dipole moment, allowing the hydrogen atom in the -OH group to form hydrogen bonds with the oxygen atoms in water molecules.
• This interaction between the solute (ethanol) and the solvent (water) is what makes ethanol miscible (soluble) in water in all proportions.

3. Show the solution

• Option A (Carbonyl group): This is found in aldehydes and ketones ($C=O$). While polar, it is not the group present in ethanol.
• Option B (Hydroxyl group): This is the $-OH$ group. It is the defining functional group of alcohols. It allows for hydrogen bonding, which is the primary reason for the solubility of short-chain alcohols like ethanol in water.
• Option C (Phenyl group): This is a benzene ring ($C_6H_5-$). It is non-polar and hydrophobic, which would actually decrease solubility in water.
• Option D (Methyl group): This is $-CH_3$. It is non-polar and hydrophobic.

Since ethanol is an alcohol, it contains the hydroxyl group, which facilitates its dissolution in water.

4. Identify the answer

The correct option is B.

5. Exam Tip

Remember the "Like Dissolves Like" rule: Polar substances dissolve in polar solvents. Since water is a highly polar solvent, it dissolves substances that can form hydrogen bonds. Whenever you see an alcohol (ending in "-ol"), immediately associate it with the Hydroxyl (-OH) group and its ability to hydrogen bond!