If an open flask of air is cooled, does the flask contain more gas molecules before or after cooling?
The correct answer and explanation is :
The correct answer is: After cooling, the flask contains fewer gas molecules.
When an open flask of air is cooled, the number of gas molecules inside the flask is not directly affected, because the flask is open to the environment. This means that air molecules can freely enter or exit the flask as it cools. However, what does change is the behavior of the gas molecules inside the flask, which leads to a decrease in the volume of air.
Here’s an explanation based on the behavior of gases:
Gases are composed of molecules that move randomly and have high kinetic energy. The temperature of a gas is directly related to the average kinetic energy of its molecules: higher temperatures correspond to faster-moving molecules, and lower temperatures correspond to slower-moving molecules. According to Charles’ Law, when the temperature of a gas decreases, the volume of the gas also decreases, provided that the pressure and the number of gas molecules remain constant. This relationship is given by the equation:
$V \propto T$
where:
- $V$ is the volume of the gas,
- $T$ is the temperature in Kelvin.
As the flask is cooled, the molecules in the air slow down, and the gas contracts. Since the flask is open, the cooler, denser air may cause some molecules to leave the flask due to the decreased pressure and temperature. This means that the gas density increases, but the number of gas molecules inside the flask decreases slightly because some molecules might exit.
Additionally, the cooling could cause condensation of water vapor from the air, which would further reduce the number of gas molecules present. Water vapor that condenses turns into liquid form, which effectively reduces the total number of gas molecules in the flask. Therefore, although the flask can still contain gas molecules after cooling, it will likely contain fewer molecules, primarily due to the decrease in temperature leading to condensation and possibly some loss of air molecules through the open flask.