A question asks how the amount of salt affects the boiling point of water. What is the independent variable?

Correct answer: Amount of salt

The direct answer is A: the amount of salt. The independent variable is the factor that the investigator deliberately changes to study its effect. Here the question asks how changing salt amount affects the boiling point, so salt amount is changed in planned levels, while boiling point is measured. Option A is correct. Option B, water’s boiling point, is the dependent variable because it responds to the salt amount; it is measured, not deliberately selected as the cause. Option C, container size, is not the factor named in the question and should normally be kept constant to avoid confounding. Option D, thermometer type, is equipment and should be kept suitable and consistent; it is not the investigated cause. A fair investigation changes one intended factor, measures the response, and controls other relevant conditions. Exam cue: ask, “What did the scientist change on purpose?” That is usually the independent variable.