Relative Humidity - Dew Point, Vapor & Partial Pressure, Evaporation, Condensation - Physics

Summary

The video explains humidity, relative humidity, vapor pressure, partial pressure, evaporation, condensation, and dew point.

Humidity and Relative Humidity

• Humidity refers to the amount of water vapor in the air. In humid conditions, sweat evaporates less readily, making it harder for the body to cool and making the weather feel hotter.
• Relative humidity (RH) compares the amount of water vapor actually in the air with the amount the air can hold at that temperature:

RH = (partial pressure of water vapor ÷ vapor pressure of water) × 100%

• Example: At 30°C, if water vapor’s partial pressure is 18 Torr and its vapor pressure is 31.8 Torr, then RH is approximately 56.6%.

Partial Pressure and Vapor Pressure

• Partial pressure is the pressure exerted by the water-vapor molecules actually present in the air. It varies with the amount of water vapor.
• Vapor pressure is the maximum equilibrium partial pressure of water vapor at a particular temperature. It depends on temperature and increases as temperature rises.
• At 30°C, the video gives water’s vapor pressure as 31.8 Torr.

Evaporation, Condensation, and Saturation

• Evaporation changes liquid water into vapor; condensation changes water vapor into liquid.
• When water is present, evaporation can add vapor to the air. As the partial pressure rises, condensation also begins.
• If partial pressure is below vapor pressure, evaporation exceeds condensation. The air is unsaturated, and RH is below 100%.
• If partial pressure equals vapor pressure, evaporation and condensation occur at equal rates. The air is saturated, and RH is 100%.
• If partial pressure exceeds vapor pressure, condensation exceeds evaporation. The air is supersaturated, and RH is above 100% at that point; excess vapor tends to condense.

Dew Point

• The dew point is the temperature at which the air becomes saturated: the water vapor’s partial pressure equals water’s vapor pressure, and RH is 100%.
• If the air cools below the dew point, condensation begins, potentially appearing as fog, clouds, precipitation, or liquid on a surface.
• Example: The video considers air at 30°C with a water-vapor partial pressure of 27 Torr. Cooling it lowers water’s vapor pressure. If the partial pressure remains 27 Torr while cooling, condensation begins when the vapor pressure falls to that value. The video suggests this might occur around 27°C, but presents that temperature as an assumption rather than an exact calculation.

Calculating Relative Humidity or Identifying the Dew Point

• To calculate RH, divide the water vapor’s partial pressure by its vapor pressure at the current temperature, then multiply by 100%.
• To identify the dew point, find the temperature at which the vapor pressure equals the water vapor’s partial pressure. Equivalently, cool the air until condensation first appears.

Speakers and Sources

• Presenter/narrator: The Organic Chemistry Tutor.
• No other speakers or sources are identified in the subtitles.