What Is Water Vapor?

Water vapor is a gaseous form of water (H ₂ O). When the water reaches the boiling point, the water becomes water vapor. At a standard atmospheric pressure at sea level, the boiling point of water is 99.974 ° C or 212 ° F or 373.15 ° K. When the water is below the boiling point, the water can also slowly evaporate into water vapor. In extremely low pressure environments (less than 0.006 atmospheres), ice can directly sublimate into water vapor. Water vapor can cause a greenhouse effect and is a greenhouse gas.

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At normal temperature, water will slowly become water vapor and fly into the air.

Water (H2O) is caused by

Occurs only on liquid surfaces

If two liquid substances dissolve each other to such an extent that they are so negligible, they can be regarded as immiscible mixtures. In a mixture containing several immiscible volatile substances, the partial pressure pi of each component i at a certain temperature is equal to the vapor pressure pi0 of the compound at the same temperature when it exists alone: pi = pi0. It does not depend on the mole fraction of each compound in the mixture. This means that each component of the mixture evaporates independently. This property is completely different from the mixture of the inter-solution (ie, the solution). The partial pressure of each component in the inter-solution is equal to the product of Raoult's Law, the product of the vapor pressure of the compound when it exists alone and its molar fraction in solution.

According to Dalton's law, the total gas phase pressure p corresponding to an immiscible mixture liquid is always equal to the sum of the partial pressures of the constituent gases, so the total vapor pressure of a mixture of immiscible volatile substances is as shown in the equation: ptotal = p1 + p2 + ... + pi.

It can be known from the above formula that the total vapor pressure of the mixture is always greater than the vapor pressure of any component at any temperature, because it includes the vapor pressure of the other components of the mixture. It can be seen that under the same external pressure, the boiling point of the mixture of immiscible substances is lower than the boiling temperature of the component with the lowest boiling point.

Represents the relationship between the vapor pressure and temperature of two immiscible mixtures of water (bp100 ° C) and bromobenzene (bp156 ° C). The mixture boils at about 95 ° C, that is, the total vapor pressure at this temperature is equal to atmospheric pressure. As theoretically foreseen, this temperature is below the boiling point of the lowest boiling component of this mixture, water. Since steam distillation can be performed at 100 ° C or lower, it is an effective method for the separation of compounds with poor thermal stability and decomposition at high temperatures.

The composition of the condensate in steam distillation is determined by the molecular weight of the compounds being distilled and their corresponding vapor pressures at this distillation temperature. For the mixture of two immiscible components A and B, the ideal gas law can be applied if the vapor of A and B is regarded as the ideal gas.

For a mixture of water and bromobenzene, the vapor pressure of the mixture of _bromobenzene and water at 95 ° C is p _bromobenzene = 16kpa and p _water = 85.3kpa, and the relative molecular weights are M _bromobenzene = 157, M _water = 18. The composition of the distillate can be calculated from equation (1): W _bromobenzene : W _water = 16 × 157 ÷ (85.3 × 18) = 1.635 Therefore, in the distillate, the mass fraction of bromobenzene is : 1.635 ÷ (1 + 1.635) × 100% = 62%.

As a result, although the vapor pressure of bromobenzene is small at the distillation temperature, due to its large relative molecular mass, bromobenzene is more than water in the water vapor distilled liquid by mass.

In view of the fact that the molecular weight of organic compounds is usually much larger than that of water, a compound has an appropriate vapor pressure near 100 ° C, and even with only 1kpa, good results can be obtained by steam distillation (comparing quality with quality). Even solid materials can sometimes be extracted by steam distillation. ^[2]