Behavior of Saturated Vapor | Triple Point | Humidity | Physics Grade XI

Hygrometry

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Behavior of Saturated Vapor | Triple Point | Humidity | Physics Grade XI

Behavior of Saturated Vapor | Triple Point | Humidity

Behavior of Saturated vapor

  1. The saturated vapor pressure depends upon nature of substance
  2. The saturated vapor pressure of a liquid depends upon its temperature. The saturated vapor pressure increases with rise in temperature and decreases with fall in temperature.
  3. The saturated vapor pressure is always greater than unsaturated vapor pressure.
  4. It doesn’t depend upon the volume occupied by the vapors.
  5. It is independent of the presence of vapors of other liquids present. The vapors should not have any chemical reaction
  6. The total pressure exerted by the vapors of all substances is equal to the sum of pressure exerted by the vapors of individual substance i.e. P = P1 + P2 + P3 + …………
  7. The saturated vapor doesn’t obey the gas laws whereas the unsaturated obeys the gas law
Triple point
It is a point in a phase diagram representing a particular value of pressure and temperature at which the solid, liquid, and gaseous state of the substance can co-exist. In other words, the triple point of a substance is the point in the phase diagram at which the vaporization curve, fusion curve and sublimation curve intersect as shown in the figure.

Triple PointVaporization curve: It is a graph between the pressure and boiling point of the substance in the liquid state.
Fusion curve: It is a graph between the pressure and melting point of the substance in solid state.
Sublimation curve: It is a graph between pressure and temperature at which a solid directly changes to a vapor state.
 
Humidity: Humidity is an indicator of the amount of water vapor in air.
Absolute Humidity: The mass of water vapor present in one cubic meter of air is called absolute humidity of the air. It is commonly expressed in g/m3 (gram per cubic meter).
Relative Humidity: It is defined as the ratio of the mass of water vapor, m present in certain volume of air at room temperature to the mass of water vapor, M required to saturate the same volume of air at the same temperature. i.e.
     Relative humidity = m/M * 100%
Relative humidity is generally expressed in percentage. If the ratio is 0.6, the relative humidity is 60% and if the air is already saturated then the humidity is 100%. As the pressure exerted by the vapor is directly proportional to the amount of vapor present in a given volume, the relative humidity may also be defined as
     Relative humidity = (vapor pressure of air / SVP at the same temperature) * 100
The vapor pressure of air at the actual temperature is equal to the saturated vapor pressure at the dew point. Thus, relative humidity can also be defined as
     Relative humidity =  (SVP at dew point / SVP at the air temperature) * 100
It is unitless quantity.
 
Dew Point: It is the temperature at which the water vapor present in air is just sufficient to saturate it. So, the vapor pressure at room temperature is equal to the saturated vapor pressure at dew point.
Hygrometer is a device that is used to measure the relative humidity of the air.

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