A substance with a pH of 7 is
A) a weak base.
B) a strong acid.
C) neutral.
D) a weak acid.
E) a strong base.
The correct answer and explanation is:
The correct answer is C) neutral.
A pH of 7 is considered neutral because it represents a balanced concentration of hydrogen ions (H⁺) and hydroxide ions (OH⁻) in a solution. The pH scale ranges from 0 to 14, where a pH of 7 is in the middle. A solution with a pH lower than 7 is acidic, while a solution with a pH higher than 7 is alkaline (or basic).
At pH 7, the concentration of hydrogen ions is equal to the concentration of hydroxide ions. This balance is characteristic of pure water, which is the standard example of a neutral substance. Water dissociates into hydrogen ions and hydroxide ions in equal amounts, maintaining this neutrality. The concentration of both ions in pure water at 25°C is 1 × 10⁻⁷ moles per liter, which gives a pH of 7.
A substance with a pH lower than 7, such as lemon juice or vinegar, is acidic because it has a higher concentration of hydrogen ions. On the other hand, a substance with a pH greater than 7, like ammonia or sodium hydroxide, is considered alkaline or a base, as it has a higher concentration of hydroxide ions.
Therefore, a substance with a pH of 7 is neither acidic nor basic, but neutral. This neutrality is important in many biological and chemical processes, as it provides a stable environment for reactions and organisms.