Vapour Pressure

• Saturated and unsaturated vapor pressure and its relation to boiling
• Humidity
• Relative Humidity
• Dew point and its relationship weather

Vapor Pressure

When a liquid evaporates in a closed container, the vapor formed above the liquid exerts a pressure. According to kinetic molecular theory, the molecules of the vapor are in constant motion and will hence exert a pressure just like the molecules of a gas. This pressure is called the vapor pressure of the liquid.

Saturated and unsaturated vapor pressure and its relation to boiling

A saturated vapor is a vapor that is in contact with its own liquid within a confined space. When the enclosed space above a liquid is saturated with vapor molecules and can hold no more molecules, the pressure exerted by this saturated vapor is said to be the saturated vapor pressure (s. v. p) of the liquid. The vapor is said to be saturated when the number of molecules escaping from the liquid per unit is equal to the number returning to the liquid per unit time. The saturated vapor is thus said to be in a state of dynamic equilibrium with its own liquid. Saturated vapor pressure increases with temperature.

On the other hand, the unsaturated vapor is the vapor which is not in contact with its own liquid in a confine space. It is not in dynamic equilibrium with its own liquid. The rate at which the liquid evaporates is greater than the rate at which the liquid condenses. Thus, the pressure exerted by a vapor which is not in contact with its own liquid in a confined space is called unsaturated vapor pressure.

Humidity

Humidity is the measure of wetness of the atmosphere. The exact amount of water vapor in the atmosphere at a given temperature is called absolute humidity. At higher temperature, the atmosphere contains more water vapor compared to water vapor present at low temperature.

Relative Humidity Relative humidity is defined as the ratio of the mass of water vapor per unit volume present in the atmosphere to the mass per unit volume of the water vapor needed to saturate the atmosphere.

An instrument known as the hygrometer is used to measure relative humidity of air.

Dew point and its relationship to weather

Dew point is the temperature at which the water vapors present in the air just sufficient to saturate it. It is also defined as the temperature at which the pressure of the water is vapor is equal to the saturation vapor pressure.

Mist – mist occur in wet air with high relative humidity above 75  when water vapor in the air is cooled below its dew point. Mist limits visibility to about 1000m or less.

Fog – fog is formed when water vapor in the air is cooled down to its dew point. Fog is of more effect than the mist as it can reduce visibility to less than 200m.

CLASSWORK 6

1. Define vapor pressure
2. Differentiate between saturated and unsaturated vapor pressure

ASSIGNMENT 6

SECTION A

1. The temperature at which the saturated vapor pressure of a liquid is equal to the atmospheric pressure gives rise to a phenomenon called (a) boiling point (b) melting point (c) evaporation (d) freezing point (e) no answer
2. The instrument used to measure relative humidity of an environment is (a) hydrometer (b) hygrometer (c) humid-meter (d) hygroscope (e) none of the above
3. Which of these statements is/are correct? (i) the atmosphere above is cooler than the ones below (ii) for unsaturated vapor, the rate at which the liquid evaporates is greater than the rate at which the liquid condenses (iii) mist is of lesser weather effect than fog (a) all of the above (b) (i) and (ii) only (c) (i) and (iii) only (d) (ii) and (iii) only (e) none of the above
4. Which of these is true of saturated vapor pressure? (i) saturated vapor pressure of a liquid increases with temperature (ii) saturated vapor pressure of a liquid does not have contact with the liquid (iii) saturated vapor pressure is in a state of dynamic equilibrium with its own liquid(a) all of the above (b) (i) and (ii) only (c) (i) and (iii) only (d) (ii) and (iii) only (e) none of the above
5. The temperature at which the water vapors present in the air just sufficient to saturate it is referred to as (a) relative humidity (b) vaporization (c) dew point (d) condensation point (e) no answer

SECTION B

1. Explain the following terms: (i) Dew point (ii) Fog (iii) Mist
2. Write short note on these: (i) humidity (ii) pressure
3. Differentiate between mist and fog