Explain the relationship in between vapor pressure of water and the capacity of wait to hold water vapor. Describe the relationship in between relative humidity and also partial pressure of water vapor in the air. Calculate vapor thickness using vapor pressure. Calculation humidity and also dew point.

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The expression “it’s not the heat, it’s the humidity” makes a precious point. We keep cool in warm weather by evaporating sweat from our skin and water from our breathing passages. Since evaporation is inhibited through high humidity, we feel hotter at a given temperature once the humidity is high. Low humidity, ~ above the other hand, can reason discomfort from too much drying that mucous membranes and can cause an enhanced risk of respiratory infections.


Figure \(\PageIndex1\): Dew drops choose these, top top a banana sheet photographed just after sunrise, form when the waiting temperature drops to or listed below the dew point. At the dew point, the rate at i beg your pardon water molecules join together is greater than the rate at which lock separate, and also some of the water condenses to kind droplets. (credit: Aaron Escobar, Flickr)

When us say humidity, we really median relative humidity. Loved one humidity speak us just how much water vapor is in the air compared with the preferably possible. At its maximum, denoted together saturation, the loved one humidity is 100%, and also evaporation is inhibited. The quantity of water vapor in the air counts on temperature. Because that example, relative humidity rises in the evening, together air temperature declines, sometimes reaching the dew point. At the dew allude temperature, relative humidity is 100%, and fog may result from the condensation of water droplets if they are little enough to stay in suspension. Vice versa, if you wish to dry something (perhaps your hair), that is much more effective come blow warm air end it rather than cold air, because, amongst other things, the increase in temperature boosts the power of the molecules, for this reason the price of evaporation increases.

The lot of water vapor in the air depends on the vapor push of water. The liquid and solid phases are continuously providing off vapor since some of the molecules have actually high enough speeds to go into the gas phase; view Figure(a). If a lid is put over the container, together in Figure(b), evaporation continues, boosting the pressure, until sufficient vapor has built up for condensation come balance evaporation. Climate equilibrium has actually been achieved, and also the vapor push is equal to the partial pressure of water in the container. Vapor pressure boosts with temperature since molecular speed are higher as temperature increases. Table provides representative worths of water vapor pressure over a variety of temperatures.

Figure \(\PageIndex2\): (a) since of the distribution of speeds and also kinetic energies, some water molecules can break far to the vapor phase even at temperatures below the plain boiling point. (b) If the container is sealed, evaporation will continue until there is sufficient vapor density for the condensation price to equal the evaporation rate. This vapor density and also the partial press it creates are the saturation values. They increase with temperature and are elevation of the existence of various other gases, such as air. Lock depend just on the vapor push of water.

Relative humidity is pertained to the partial push of water vapor in the air. At 100% humidity, the partial pressure is same to the vapor pressure, and also no much more water can get in the vapor phase. If the partial press is less than the vapor pressure, then evaporation will take place, together humidity is much less than 100%. If the partial push is greater than the vapor pressure, condensation bring away place. In daily language, human being sometimes refer to the volume of air come “hold” water vapor, however this is not actually what happens. The water vapor is not organized by the air. The lot of water in wait is figured out by the vapor pressure of water and also has nothing to perform with the nature of air.

Temperature \(^oC\) Vapor press \((Pa)\) Saturation vapor density \(g/m^3\)
−50 4.0 0.039
-20 \(1.04 \times 10^2\) 0.89
-10 \(2.60 \times 10^2\) 2.36
0 \(6.10 \times 10^2\) 4.84
5 \(8.68 \times 10^2\) 6.80
10 \(1.19 \times 10^3\) 9.40
15 \(1.69 \times 10^3\) 12.8
20 \(2.33 \times 10^3\) 17.2
25 \(3.17 \times 10^3\) 23.0
30 \(4.24 \times 10^3\) 30.4
37 \(6.31 \times 10^3\) 44.0
40 \(7.34 \times 10^3\) 51.1
50 \(1.23 \times 10^4\) 82.4
60 \(1.99 \times 10^4\) 130
70 \(3.12 \times 10^4\) 197
80 \(4.73 \times 10^4\) 294
90 \(7.01 \times 10^4\) 418
95 \(8.59 \times 10^4\) 505
100 \(1.01 \times 10^5\) 598
120 \(1.99 \times 10^5\) 1095
150 \(4.76 \times 10^5\) 2430
200 \(1.55 \times 10^6\) 7090
220 \(2.32 \times 10^6\) 10,200

Saturation Vapor thickness of Water

Example \(\PageIndex1\): Calculating density Using Vapor Pressure

Table gives the vapor push of water at .\(20.0^oC\) together \(2.33 \times 10^3 \, Pa\). Usage the best gas regulation to calculate the density of water vapor in \(g/m^3\) that would produce a partial pressure equal to this vapor pressure. Compare the an outcome with the saturation vapor thickness given in the table.


To settle this problem, we need to break that down into a two steps. The partial pressure complies with the best gas law,


where \(n\) is the number of moles. If we fix this equation for \(n/V\)

to calculate the variety of moles per cubic meter, we can then convert this quantity to grams every cubic meter as requested. To carry out this, we should use the molecule mass that water, i beg your pardon is offered in the routine table.


1. Recognize the knowns and convert them to the appropriate units:

temperature \(T = 20^oC = 203 \, K\) vapor push \(P\) that water at \(20^oC\) is \(2.33 \times 10^3 \, Pa\) molecular mass of water is \(18.0 \, g/m\)

2. Settle the right gas regulation for \(n?V\).

\<\dfracnV = \dfracPRT\>

3. Substitute known values into the equation and solve because that \(n/V\).

\<\dfracnV = \dfracPRT = \dfrac2.33 \times 10^3 \, Pa(8.31 \, J/mol \cdot K)(293 \, K) = 0.957 \, mol/m^3\>

4. Transform the thickness in moles per cubic meter to grams per cubic meter.

\<\rho = \left(0.957\dfracmolm^3\right)\left(\dfrac18.0 \, gmol\right) = 17.2 \, g/m^3\>


The thickness is derived by suspect a press equal to the vapor push of water in ~ \(20.0^oC\). The thickness found is similar to the worth in Table, which means that a vapor thickness of \(17.2 \, g/m^3\) at \(20.9^oC\) create a partial push of \(2.33 \times 10^3 \, Pa\), same to the vapor pressure of water at that temperature. If the partial pressure is same to the vapor pressure, climate the liquid and vapor phases space in equilibrium, and the family member humidity is 100%. Thus, there have the right to be no an ext than 17.2 g of water vapor every \(m^3\) at \(20.0^oC\), so the this value is the saturation vapor thickness at that temperature. This instance illustrates just how water vapor behaves like an ideal gas: the pressure and also density are regular with the ideal gas regulation (assuming the thickness in the table is correct). The saturation vapor densities detailed in Table space the maximum quantities of water vapor that air can hold at miscellaneous temperatures.

Percent family member Humidity

We define percent family member humidity as the proportion of vapor thickness to saturation vapor density, or


We deserve to use this and the data in Table to perform a selection of exciting calculations, keeping in mind that family member humidity is based upon the compare of the partial push of water vapor in air and also ice.

We deserve to use this and also the data in Table to carry out a range of interesting calculations, maintaining in mental that relative humidity is based on the comparison of the partial push of water vapor in air and ice.

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Why go water boil at \(100^oC\)? girlfriend will keep in mind from Table the the vapor push of water at \(100^oC\) is \(1.01 \times 10^5 \, Pa\), or 1.00 atm. Thus, it have the right to evaporate without limit at this temperature and pressure. Yet why does it kind bubbles when it boils? This is due to the fact that water ordinarily contains far-reaching amounts of dissolved air and other impurities, which room observed as small bubbles of waiting in a glass of water. If a bubble starts the end at the bottom the the container at \(20^oC\), it consists of water vapor (about 2.30%). The pressure inside the bubble is resolved at 1.00 atm (we ignore the slight pressure exerted through the water around it). Together the temperature rises, the quantity of wait in the bubble remains the same, but the water vapor increases; the bubble increases to keep the push at 1.00 atm. In ~ \(100^oC\), water vapor start the balloon continuously due to the fact that the partial push of water is same to 1.00 atm in equilibrium. That cannot reach this pressure, however, because the bubble additionally contains air and also total push is 1.00 atm. The bubble grows in size and also thereby rises the buoyant force. The bubble division away and also rises rapidly to the surface—we call this boiling! (See Figure.)

A manufacturer of water gift heated over a flame. The beaker is displayed at three various times. In the first, in ~ twenty levels C, a small bubble sit on the bottom the the beaker. In the 2nd step, the water temperature is fifty degrees C and also the balloon is larger, though still sitting on the bottom of the beaker. In the 3rd step, the water temperature is one hundred degrees C. The balloon is larger and also is increasing toward the surface.what is the relationship between evaporation and relative humidity