Final answer:
The assertion that cool air rises is incorrect; it is warm air that rises due to being less dense. As the air parcel rises and expands due to lower pressure, its temperature drops due to a loss of internal energy, consistent with adiabatic cooling.
Step-by-step explanation:
The student's statement regarding a parcel of cool air rising in the atmosphere and encountering lower pressure is generally true; the behavior of the air can be explained by principles of physics related to temperature, pressure, and density. However, the assertion that a parcel of cool air is less dense and may rise is incorrect. Indeed, it is usually a parcel of warm air that is less dense and therefore rises in the atmosphere.
As a parcel of air ascends, the external pressure it encounters decreases, leading it to expand. Due to this expansion, the air does work on the surrounding environment, which results in a loss of internal energy. Consequently, the average kinetic energy of the gas particles decreases, and so does the temperature of the air parcel. This is consistent with the principle of adiabatic cooling in thermodynamics, which is relevant when the exchange of heat is minimal.
Contrary to the inquiry, the molecules don't slow down due to expansion; in fact, they spread apart doing work against the environmental pressure, leading to a reduction in temperature as per Charles's Law, which states that volume and temperature are directly proportional at constant pressure. So, the correct statement is that a parcel of air that contains warm air is less dense and thus may rise. When it rises and expands, the temperature will decrease.