Why is it important to use a lid on the polystyrene cup during the experiment?

Using a lid on the polystyrene cup during the experiment is important because it helps to minimize heat loss. In many chemical experiments, particularly those involving reactions that produce or consume heat (such as exothermic or endothermic reactions), maintaining a stable temperature is crucial for accurate results. A polystyrene cup is chosen for its insulation properties, and covering it with a lid further reduces the transfer of heat to the surrounding air.

By minimizing heat loss, the measured temperature change during the reaction reflects the actual thermal changes occurring within the system, allowing for a more accurate determination of reaction enthalpy and ensuring that the results are valid. This careful control of the thermal environment directly impacts the experiment's reliability and the interpretation of the data collected.