Why must an experiment have a control group?

Explanation:

The control group provides a baseline in the experiment. The variable that is being studied in the experiment is not changed or is limited to zero in the control group.

1. dermis

2. chemical

3. urine

4. viruses

5. antibodies

before, trains worked using the steam engine. thus, it uses heat so it is crucial to apply heat balance. the steam engine absorbs energy during the stage of boiling water. so, this is the endothermic reaction. when the steam is used to work the piston, it has to release this energy through running the train. the excess energy is released as an exhaust to the chimney. this is the exothermic process because it releases energy to the surroundings.