Question

which of the following best defines an acid?
○ an acid has a ph of 7.
○ an acid has hydroxide ions.
○ an acid has fewer positive ions in solution.
○ an acid has more hydrogen ions in solution.

Explanation:

• For the first option: A substance with a pH of 7 is neutral (like pure water), while acids have a pH less than 7, so this is incorrect.
• For the second option: Hydroxide ions ($\ce{OH^-}$) are characteristic of bases, not acids. Acids are associated with hydrogen ions ($\ce{H^+}$ or $\ce{H_3O^+}$), so this is wrong.
• For the third option: Acids, when dissolved in water, increase the concentration of positive hydrogen ions in the solution, so saying they have fewer positive ions is incorrect.
• For the fourth option: By the Arrhenius definition, an acid is a substance that dissociates in water to produce hydrogen ions ($\ce{H^+}$), which means acids have a higher concentration of hydrogen ions in their aqueous solutions compared to neutral or basic solutions. This matches the definition of an acid.

Answer:

D. An acid has more hydrogen ions in solution.