pH of Solutions Explained

The pH scale is used to determine how acidic or basic a solution is. The scale typically ranges from 0 to 14.

• A pH lower than seven means the solution is acidic. Acidic solutions have a higher concentration of hydrogen ions ($\text{H}^+$).
• A pH equal to seven means the solution is neutral. Pure water has a pH of 7.
• A pH greater than seven means the solution is basic or alkaline. Basic solutions have a higher concentration of hydroxide ions ($\text{OH}^-$).

The question asks us to identify the compound dissolved in water that produces a solution with a pH lower than seven, i.e., an acidic solution.

Compound Analysis in Water

Let's examine how each given compound behaves when dissolved in water:

• Common salt ($\text{NaCl}$): This is sodium chloride, which is a salt formed from a strong acid ($\text{HCl}$) and a strong base ($\text{NaOH}$). When dissolved in water, it dissociates into $\text{Na}^+$ and $\text{Cl}^-$ ions. These ions do not react significantly with water to produce excess $\text{H}^+$ or $\text{OH}^-$. Therefore, a solution of common salt in water is generally neutral, with a pH close to 7.
• Vinegar: Vinegar is mainly a dilute solution of acetic acid ($\text{CH}_3\text{COOH}$) in water. Acetic acid is a weak acid. When it dissolves in water, it partially ionizes, releasing hydrogen ions ($\text{H}^^+$): $\text{CH}_3\text{COOH} \rightleftharpoons \text{CH}_3\text{COO}^- + \text{H}^+$. The increase in the concentration of $\text{H}^+$ ions makes the solution acidic, resulting in a pH lower than seven.
• Sugar (Sucrose, $\text{C}_{12}\text{H}_{22}\text{O}_{11}$): Sugar is a covalent compound. When dissolved in water, sugar molecules disperse throughout the water but do not ionize to produce significant amounts of $\text{H}^+$ or $\text{OH}^-$ ions. A sugar solution in water is generally neutral, with a pH close to 7.
• Ethanol ($\text{C}_2\text{H}_5\text{OH}$): Ethanol is an alcohol. It dissolves in water due to hydrogen bonding. However, it does not ionize in a way that significantly affects the pH of the solution, unlike acids or bases. Ethanol solutions are generally neutral, with a pH close to 7.

Solution pH Determination

Based on the analysis, among the given options, only Vinegar forms an acidic solution when dissolved in water because it contains acetic acid. An acidic solution has a pH lower than seven.

Thus, the compound dissolved in water to give a solution of pH lower than seven is Vinegar.