Dissolves in water to form an acidic solution — Acids and Bases Chemistry Question

STEPS:

1. Define an Acidic Solution: A solution is considered acidic when the concentration of hydrogen ions ($H^+$) or hydronium ions ($H_3O^+$) increases upon dissolving a solute. According to the Brønsted^-Lowry definition, an acid is a proton ($H^+$) donor.
2. Analyze the Functional Groups: Evaluate the chemistry of each molecule based on its specific functional group:
* $CH_3CH_2CH_2CH_3$ (A): An alkane (butane).
* $CH_3CH_2CH_2OH$ (B): An alcohol (propan^-1-ol).
* $CH_3COCH_3$ (C): A ketone (acetone/propanone).
* $CH_3COOH$ (D): A carboxylic acid (ethanoic or acetic acid).
* $CH_3CH_2CH_2NH_2$ (E): An amine (propan^-1-amine).
3. Identify the Acidic Compound: Among the choices, $CH_3COOH$ contains the carboxyl group ($-COOH$), which is a well-characterized weak acid. The hydrogen atom attached to the oxygen in this group is relatively easy to lose as a proton.
4. Describe the Dissociation: When ethanoic acid dissolves in water, it partially dissociates (ionizes) to release hydrogen ions into the solution:
$$CH_3COOH(aq) \rightleftharpoons CH_3COO^-(aq) + H^+(aq)$$
5. Conclusion: Because $CH_3COOH$ is the only molecule listed capable of donating a proton to water to increase the $H^+$ concentration, it is the only one that forms an acidic solution.

WHY_OTHERS_WRONG:

• $CH_3CH_2CH_2CH_3$ (A): This is a nonpolar hydrocarbon (alkane). It is insoluble in water and lacks any polar bonds or acidic hydrogens that could be donated.
• $CH_3CH_2CH_2OH$ (B): While this alcohol has an $-OH$ group, the $O-H$ bond is very strong. Alcohols are extremely weak acids (weaker than water) and do not dissociate in aqueous solution, resulting in a neutral pH.
• $CH_3COCH_3$ (C): This ketone has a $C=O$ double bond but no hydrogen atoms attached to an oxygen. The hydrogens attached to the carbons are not acidic in water, so the resulting solution is neutral.
• $CH_3CH_2CH_2NH_2$ (E): This is an amine, which is a Brønsted^-Lowry base. Instead of donating a proton, it accepts one from water to form $OH^-$ ions, making the solution basic rather than acidic.