Question 7

A substance with a pH of 3 is 10 × more acidic than a substance with a pH of 4.

The pH scale is logarithmic, meaning that each whole number change on the scale represents a tenfold change in acidity or alkalinity. Therefore, a substance with a pH of 3 is indeed 10 times more acidic than one with a pH of 4.

A) A substance with a pH of 3 is 10 × more alkaline than a substance with a pH of 4.

This statement is incorrect because it misrepresents the relationship between pH values. A lower pH indicates higher acidity, not alkalinity. Thus, a substance with a pH of 3 cannot be more alkaline than one with a pH of 4.

B) A substance with a pH of 3 is two × more alkaline than a substance with a pH of 4.

This option is also incorrect. It fails to recognize that a decrease in pH indicates an increase in acidity, not alkalinity. Therefore, a pH of 3 cannot be described as more alkaline than a pH of 4.

C) A substance with a pH of 3 is two × more acidic than a substance with a pH of 4.

This statement is misleading. While a pH of 3 is indeed more acidic than a pH of 4, the relationship is not a simple factor of two. The correct relationship indicates that the pH of 3 is ten times more acidic than a pH of 4.

D) A substance with a pH of 3 is 10 × more acidic than a substance with a pH of 4.

This option is correct because it accurately reflects the logarithmic nature of the pH scale. A decrease of one pH unit corresponds to a tenfold increase in acidity, making this statement true.

Conclusion

Option D correctly captures the logarithmic relationship inherent in the pH scale, affirming that a substance with a pH of 3 is 10 times more acidic than one with a pH of 4. All other options misunderstand or misinterpret this relationship, leading to incorrect assertions about acidity and alkalinity. Understanding the pH scale is crucial for accurately assessing the properties of substances in chemistry.