Question

The temperature at which a vapor-gas mixture becomes saturated when cooled at constant total pressure out of contact with a liquid is called:

• A. Bubble point
• B. Dew point
• C. Wet bulb temperature
• D. Dry bulb temperature

Hint:

For any unsaturated vapor-gas mixture, the temperatures always follow the relationship:
\[ T_{\text{dry-bulb}} \gt T_{\text{wet-bulb}} \gt T_{\text{dew-point}} \]
At \( 100\% \) saturation, all three temperatures are equal.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question asks for the term that describes the temperature at which a vapor-gas mixture reaches saturation when cooled at a constant total pressure and without any contact with liquid.

Step 2: Key Formula or Approach:
As a vapor-gas mixture is cooled at constant pressure, the partial pressure of the vapor remains constant, but the saturation vapor pressure of the component decreases with decreasing temperature.
Saturation occurs when the partial pressure of the vapor equals its saturation vapor pressure:
\[ p_v = p^*_v(T_d) \]
This temperature \( T_d \) is the dew point.

Step 3: Detailed Explanation:

• Dew Point: If we cool a warm, unsaturated humid air stream at constant pressure, the air's capacity to hold water vapor decreases.
The temperature at which the air is cooled to \( 100\% \) relative humidity (saturation) is the dew point.
At this temperature, the first droplets of liquid condensate (dew) begin to form.

• Bubble Point: This is the temperature at which the first bubble of vapor forms when heating a liquid mixture at constant pressure. It applies to liquid mixtures, not gas-vapor mixtures.

• Dry Bulb Temperature: This is the actual physical temperature of the gas-vapor mixture measured by a standard thermometer.

• Wet Bulb Temperature: This is the steady-state temperature reached by a small amount of liquid evaporating into a large, unsaturated gas-vapor stream under adiabatic conditions.


Step 4: Final Answer:
The temperature described in the question is the dew point.