Final answer:
A control in an experiment eliminates alternative explanations and allows for comparison with the treatment group, validating the hypothesis and ensuring changes are due to the independent variable.
Step-by-step explanation:
In scientific experiments, a control serves as a benchmark to compare results against. Its purpose is twofold: firstly, to ensure that any changes in the experiment are solely due to the independent variable; and secondly, to provide baseline data for interpreting results. This helps eliminate alternative explanations for the outcome, allowing for a clearer understanding of the effect of the independent variable. By comparing the experimental groups to the control group, discrepancies that arise can be attributed to the changes in the independent variable, provided that all other conditions have been kept constant. For instance, in the algae growth experiment, if the ponds treated with phosphate exhibit more growth than the control group, one can infer that phosphate promotes algae growth. Therefore, controls are vital for validating the hypothesis and increasing the reliability of the experiment.