ANS:A - +ve

The difference between the wet bulb temperature and the adiabatic saturation temperature of an unsaturated mixture can provide insights into the state of the mixture and the potential for evaporative cooling. Let's explore each term and their relationship:

1. Wet Bulb Temperature:
   • The wet bulb temperature is the lowest temperature that can be reached by evaporating water into the air at constant pressure and with no heat transfer to the surrounding environment.
   • It is typically measured using a thermometer with its bulb covered in a wet fabric exposed to moving air.
   • The wet bulb temperature represents the lowest temperature achievable through evaporative cooling, as evaporation of water from the wet fabric cools the thermometer bulb due to the latent heat of vaporization.
2. Adiabatic Saturation Temperature:
   • The adiabatic saturation temperature is the temperature that a parcel of air would reach through adiabatic expansion if it were saturated with water vapor.
   • Adiabatic expansion refers to the process where a gas expands without any heat exchange with the surroundings.
   • When a parcel of air is saturated (contains the maximum amount of water vapor it can hold at a given temperature and pressure), its temperature decreases as it expands adiabatically due to the cooling effect of water vapor condensing into liquid water.

Now, let's consider the difference between these temperatures:

• If the wet bulb temperature is lower than the adiabatic saturation temperature, it suggests that the air is not saturated with water vapor.
• This could occur when the relative humidity of the air is less than 100%, meaning it can hold more water vapor before reaching saturation.
• In this case, the difference between the wet bulb temperature and the adiabatic saturation temperature would be positive.
• Conversely, if the wet bulb temperature is equal to or higher than the adiabatic saturation temperature, it suggests that the air is saturated or supersaturated with water vapor.
• In such conditions, there would be no evaporative cooling potential, and the difference between the wet bulb temperature and the adiabatic saturation temperature would be zero or negative.

In summary, the difference between the wet bulb temperature and the adiabatic saturation temperature of an unsaturated mixture can be positive, indicating the potential for evaporative cooling, or zero/negative, indicating saturation or supersaturation with water vapor and no further potential for evaporative cooling. Therefore, the correct answer is: +ve.