Final answer:
The independent variable in Heidi's experiment is the consumption of chocolate, and the dependent variable is the number of pimples. A control would be maintaining other factors that could influence acne constant.
Step-by-step explanation:
The scenario described involves Heidi testing whether eating chocolate causes pimples. For her informal experiment, the independent variable is the consumption of chocolate, because it is the factor that she changes to observe any effects. The dependent variable is the number of pimples on Heidi's face, as this could potentially change in response to the independent variable (chocolate consumption).
A control in this context could be a period where Heidi's diet and other factors affecting acne, like hygiene or stress levels, are kept constant so that the only change would be the chocolate consumption.