Humidity, Evaporation, and Boiling

The expression “it’s not the heat, it’s the humidity” makes a valid point. We keep cool in hot weather by evaporating sweat from our skin and water from our breathing passages. Because evaporation is inhibited by high humidity, we feel hotter at a given temperature when the humidity is high. Low humidity, on the other hand, can cause discomfort from excessive drying of mucous membranes and can lead to an increased risk of respiratory infections.

When we say humidity, we really mean relative humidity. Relative humidity tells us how much water vapor is in the air compared with the maximum possible. At its maximum, denoted as saturation, the relative humidity is 100%, and evaporation is inhibited. The amount of water vapor in the air depends on temperature. For example, relative humidity rises in the evening, as air temperature declines, sometimes reaching the dew point. At the dew point temperature, relative humidity is 100%, and fog may result from the condensation of water droplets if they are small enough to stay in suspension. Conversely, if you wish to dry something (perhaps your hair), it is more effective to blow hot air over it rather than cold air, because, among other things, the increase in temperature increases the energy of the molecules, so the rate of evaporation increases.

The amount of water vapor in the air depends on the vapor pressure of water. The liquid and solid phases are continuously giving off vapor because some of the molecules have high enough speeds to enter the gas phase; see [link](a). If a lid is placed over the container, as in [link](b), evaporation continues, increasing the pressure, until sufficient vapor has built up for condensation to balance evaporation. Then equilibrium has been achieved, and the vapor pressure is equal to the partial pressure of water in the container. Vapor pressure increases with temperature because molecular speeds are higher as temperature increases. [link] gives representative values of water vapor pressure over a range of temperatures.

Two containers, each filled two-thirds with water. One is open to the atmosphere and the other is sealed at the top. The water molecules are depicted as circles with vector arrows of different lengths and directions to indicate velocity. In the sealed container the density of molecules in the air above the water is greater than in the unsealed container. In the sealed container, water is condensing along the walls and top of the upper part of the container.

Relative humidity is related to the partial pressure of water vapor in the air. At 100% humidity, the partial pressure is equal to the vapor pressure, and no more water can enter the vapor phase. If the partial pressure is less than the vapor pressure, then evaporation will take place, as humidity is less than 100%. If the partial pressure is greater than the vapor pressure, condensation takes place. In everyday language, people sometimes refer to the capacity of air to “hold” water vapor, but this is not actually what happens. The water vapor is not held by the air. The amount of water in air is determined by the vapor pressure of water and has nothing to do with the properties of air.

Saturation Vapor Density of Water
Temperature (ºC) size 12{ \( °C \) } {} Vapor pressure (Pa) Saturation vapor density (g/m3)
−50 4.0 0.039
−20 1 . 04 × 10 2 size 12{1 "." "04" times "10" rSup { size 8{2} } } {} 0.89
−10 2 . 60 × 10 2 size 12{2 "." "60"´"10" rSup { size 8{2} } } {} 2.36
0 6 . 10 × 10 2 size 12{6 "." "10"´"10" rSup { size 8{2} } } {} 4.84
5 8 . 68 × 10 2 size 12{8 "." "68"´"10" rSup { size 8{2} } } {} 6.80
10 1 . 19 × 10 3 size 12{1 "." "19"´"10" rSup { size 8{3} } } {} 9.40
15 1 . 69 × 10 3 size 12{1 "." "69"´"10" rSup { size 8{3} } } {} 12.8
20 2 . 33 × 10 3 size 12{2 "." "33"´"10" rSup { size 8{3} } } {} 17.2
25 3 . 17 × 10 3 size 12{3 "." "17"´"10" rSup { size 8{3} } } {} 23.0
30 4 . 24 × 10 3 size 12{4 "." "24"´"10" rSup { size 8{3} } } {} 30.4
37 6 . 31 × 10 3 size 12{6 "." "31"´"10" rSup { size 8{3} } } {} 44.0
40 7 . 34 × 10 3 size 12{7 "." "34"´"10" rSup { size 8{3} } } {} 51.1
50 1 . 23 × 10 4 size 12{1 "." "23" times "10" rSup { size 8{4} } } {} 82.4
60 1 . 99 × 10 4 size 12{1 "." "99"´"10" rSup { size 8{4} } } {} 130
70 3 . 12 × 10 4 size 12{3 "." "12"´"10" rSup { size 8{4} } } {} 197
80 4 . 73 × 10 4 size 12{4 "." "73"´"10" rSup { size 8{4} } } {} 294
90 7 . 01 × 10 4 size 12{7 "." "01"´"10" rSup { size 8{4} } } {} 418
95 8 . 59 × 10 4 size 12{8 "." "59"´"10" rSup { size 8{4} } } {} 505
100 1 . 01 × 10 5 size 12{1 "." "99"´"10" rSup { size 8{5} } } {} 598
120 1 . 99 × 10 5 size 12{1 "." "99"´"10" rSup { size 8{5} } } {} 1095
150 4 . 76 × 10 5 size 12{4 "." "76"´"10" rSup { size 8{5} } } {} 2430
200 1 . 55 × 10 6 size 12{1 "." "55"´"10" rSup { size 8{6} } } {} 7090
220 2 . 32 × 10 6 size 12{2 "." "32"´"10" rSup { size 8{6} } } {} 10,200
Calculating Density Using Vapor Pressure

[link] gives the vapor pressure of water at 20.0ºC size 12{"20" "." 0°C} {}

as 2.33×103 Pa. size 12{2 "." "33"´"10" rSup { size 8{3} } " Pa" "." } {}

Use the ideal gas law to calculate the density of water vapor in g/m3 size 12{g/m rSup { size 8{3} } } {}

that would create a partial pressure equal to this vapor pressure. Compare the result with the saturation vapor density given in the table.

Strategy

To solve this problem, we need to break it down into a two steps. The partial pressure follows the ideal gas law,

PV=nRT, size 12{ size 11{ ital "PV"= ital "nRT"}} {}

where n size 12{n} {}

is the number of moles. If we solve this equation for n/V size 12{n/V} {}

to calculate the number of moles per cubic meter, we can then convert this quantity to grams per cubic meter as requested. To do this, we need to use the molecular mass of water, which is given in the periodic table.

Solution

1. Identify the knowns and convert them to the proper units:

  1. temperature T=20ºC=293 K size 12{T="20"°"C=293 K"} {}
  2. vapor pressure P size 12{P} {}

    of water at

    20ºC size 12{"20"°C} {}

    is

    2.33×103 Pa size 12{2 "." "33" times "10" rSup { size 8{3} } " Pa"} {}
  3. molecular mass of water is 18.0 g/mol size 12{"18" "." 0" g/mol"} {}

2. Solve the ideal gas law for n/V size 12{n/V} {}

.

n V = P RT size 12{ { { size 11{n}} over { size 11{V}} } = { { size 11{P}} over { size 11{ ital "RT"}} } } {}

3. Substitute known values into the equation and solve for n/V size 12{n/V} {}

.

n V = P RT = 2 . 33 × 10 3 Pa 8 . 31 J/mol ⋅ K 293 K = 0 . 957 mol/m 3 size 12{ { { size 11{n}} over { size 11{V}} } = { { size 11{P}} over { size 11{ ital "RT"}} } = { { size 11{2 "." "33" times "10" rSup { size 8{3} } `"Pa"}} over { size 12{ left (8 "." "31"`"J/mol" cdot K right ) left ("293"`K right )} } } =0 "." "957"`"mol/m" rSup { size 8{3} } } {}

4. Convert the density in moles per cubic meter to grams per cubic meter.

ρ = 0 . 957 mol m 3 18 . 0 g mol = 17 . 2 g/m 3 size 12{ size 11{ρ= left ( size 11{0 "." "957" { { size 11{"mol"}} over { size 11{m rSup { size 8{3} } }} } } right ) left ( size 12{ { {"18" "." "0 g"} over { size 12{"mol"} } } } right )="17" "." 2" g/m" rSup { size 8{3} } }} {}

Discussion

The density is obtained by assuming a pressure equal to the vapor pressure of water at 20.0ºC size 12{"20" "." 0°C} {}

. The density found is identical to the value in [link], which means that a vapor density of 17.2 g/m3 size 12{"17" "." 2" g/m" rSup { size 8{3} } } {}

at 20.0ºC size 12{"20" "." 0°C} {}

creates a partial pressure of 2.33×103 Pa, size 12{2 "." "33"´"10" rSup { size 8{3} } " Pa,"} {}

equal to the vapor pressure of water at that temperature. If the partial pressure is equal to the vapor pressure, then the liquid and vapor phases are in equilibrium, and the relative humidity is 100%. Thus, there can be no more than 17.2 g of water vapor per m3 size 12{m rSup { size 8{3} } } {}

at 20.0ºC size 12{"20" "." 0°C} {}

, so that this value is the saturation vapor density at that temperature. This example illustrates how water vapor behaves like an ideal gas: the pressure and density are consistent with the ideal gas law (assuming the density in the table is correct). The saturation vapor densities listed in [link] are the maximum amounts of water vapor that air can hold at various temperatures.

Percent Relative Humidity

We define percent relative humidity as the ratio of vapor density to saturation vapor density, or

percent relative humidity = vapor density saturation vapor density × 100 size 12{ size 11{"percent relative humidity"= { { size 11{"vapor density"}} over { size 11{"saturation vapor density"}} } times "100"}} {}

We can use this and the data in [link] to do a variety of interesting calculations, keeping in mind that relative humidity is based on the comparison of the partial pressure of water vapor in air and ice.

Calculating Humidity and Dew Point

(a) Calculate the percent relative humidity on a day when the temperature is 25.0ºC size 12{"25" "." 0°C} {}

and the air contains 9.40 g of water vapor per m3 size 12{m rSup { size 8{3} } } {}

. (b) At what temperature will this air reach 100% relative humidity (the saturation density)? This temperature is the dew point. (c) What is the humidity when the air temperature is 25.0ºC size 12{"25" "." 0°C} {}

and the dew point is –10.0ºC size 12{ +- "10" "." 0°C} {}

?

Strategy and Solution

(a) Percent relative humidity is defined as the ratio of vapor density to saturation vapor density.

percent relative humidity = vapor density saturation vapor density × 100 size 12{ size 11{"percent relative humidity"= { { size 11{"vapor density"}} over { size 11{"saturation vapor density"}} } times "100"}} {}

The first is given to be 9.40 g/m3 size 12{9 "." "40 g/m" rSup { size 8{3} } } {}

, and the second is found in [link] to be 23.0 g/m3 size 12{"23" "." "0 g/m" rSup { size 8{3} } } {}

. Thus,

percent relative humidity = 9 . 40 g/m 3 23 . 0 g/m 3 × 100 = 40 . 9 .% size 12{ size 11{"percent relative humidity"= { { size 11{9 "." "40 g/m" rSup { size 8{3} } }} over { size 12{"23" "." "0 g/m" rSup { size 8{3} } } } } ´"100"="40" "." 9% "." }} {}

(b) The air contains 9.40 g/m3 size 12{9 "." "40 g/m" rSup { size 8{3} } } {}

of water vapor. The relative humidity will be 100% at a temperature where 9.40 g/m3 size 12{9 "." "40 g/m" rSup { size 8{3} } } {}

is the saturation density. Inspection of [link] reveals this to be the case at 10.0ºC size 12{"10" "." 0°C} {}

, where the relative humidity will be 100%. That temperature is called the dew point for air with this concentration of water vapor.

(c) Here, the dew point temperature is given to be –10.0ºC size 12{ +- "10" "." 0°C} {}

. Using [link], we see that the vapor density is 2.36 g/m3 size 12{2 "." "36 g/m" rSup { size 8{3} } } {}

, because this value is the saturation vapor density at –10.0ºC size 12{ +- "10" "." 0°C} {}

. The saturation vapor density at 25.0ºC size 12{"25" "." 0°C} {}

is seen to be 23.0 g/m3 size 12{"23" "." "0 g/m" rSup { size 8{3} } } {}

. Thus, the relative humidity at 25.0ºC size 12{"25" "." 0°C} {}

is

percent relative humidity = 2 . 36 g/m 3 23 . 0 g/m 3 × 100 = 10 . 3 % . size 12{ size 11{"percent relative humidity"= { { size 11{2 "." "36 g/m" rSup { size 8{3} } }} over { size 12{"23" "." "0 g/m" rSup { size 8{3} } } } } ×"100"="10" "." 3% "." }} {}

Discussion

The importance of dew point is that air temperature cannot drop below 10.0ºC size 12{"10" "." 0°C} {}

in part (b), or –10.0ºC size 12{ +- "10" "." 0°C} {}

in part (c), without water vapor condensing out of the air. If condensation occurs, considerable transfer of heat occurs (discussed in Heat and Heat Transfer Methods), which prevents the temperature from further dropping. When dew points are below 0ºC size 12{0°C} {}

, freezing temperatures are a greater possibility, which explains why farmers keep track of the dew point. Low humidity in deserts means low dew-point temperatures. Thus condensation is unlikely. If the temperature drops, vapor does not condense in liquid drops. Because no heat is released into the air, the air temperature drops more rapidly compared to air with higher humidity. Likewise, at high temperatures, liquid droplets do not evaporate, so that no heat is removed from the gas to the liquid phase. This explains the large range of temperature in arid regions.

Why does water boil at 100ºC size 12{"100"°C} {}

? You will note from [link] that the vapor pressure of water at 100ºC size 12{"100"°C} {}

is 1.01×105 Pa size 12{1 "." "01"´"10" rSup { size 8{5} } " Pa"} {}

, or 1.00 atm. Thus, it can evaporate without limit at this temperature and pressure. But why does it form bubbles when it boils? This is because water ordinarily contains significant amounts of dissolved air and other impurities, which are observed as small bubbles of air in a glass of water. If a bubble starts out at the bottom of the container at 20ºC size 12{"20"°C} {}

, it contains water vapor (about 2.30%). The pressure inside the bubble is fixed at 1.00 atm (we ignore the slight pressure exerted by the water around it). As the temperature rises, the amount of air in the bubble stays the same, but the water vapor increases; the bubble expands to keep the pressure at 1.00 atm. At 100ºC size 12{"100"°C} {}

, water vapor enters the bubble continuously since the partial pressure of water is equal to 1.00 atm in equilibrium. It cannot reach this pressure, however, since the bubble also contains air and total pressure is 1.00 atm. The bubble grows in size and thereby increases the buoyant force. The bubble breaks away and rises rapidly to the surface—we call this boiling! (See [link].)

A beaker of water being heated over a flame. The beaker is shown at three different times. In the first, at twenty degrees C, a small bubble sits on the bottom of the beaker. In the second step, the water temperature is fifty degrees C and the bubble is larger, though still sitting on the bottom of the beaker. In the third step, the water temperature is one hundred degrees C. The bubble is larger and is rising toward the surface.

Check Your Understanding

Freeze drying is a process in which substances, such as foods, are dried by placing them in a vacuum chamber and lowering the atmospheric pressure around them. How does the lowered atmospheric pressure speed the drying process, and why does it cause the temperature of the food to drop?

Decreased the atmospheric pressure results in decreased partial pressure of water, hence a lower humidity. So evaporation of water from food, for example, will be enhanced. The molecules of water most likely to break away from the food will be those with the greatest velocities. Those remaining thus have a lower average velocity and a lower temperature. This can (and does) result in the freezing and drying of the food; hence the process is aptly named freeze drying.

PhET Explorations: States of Matter

Watch different types of molecules form a solid, liquid, or gas. Add or remove heat and watch the phase change. Change the temperature or volume of a container and see a pressure-temperature diagram respond in real time. Relate the interaction potential to the forces between molecules.* * *

Section Summary

Conceptual Questions

Because humidity depends only on water’s vapor pressure and temperature, are the saturation vapor densities listed in [link] valid in an atmosphere of helium at a pressure of 1.01×105 N/m2 size 12{1 "." "01"´"10" rSup { size 8{5} } " N/m" rSup { size 8{2} } } {}

, rather than air? Are those values affected by altitude on Earth?

Why does a beaker of 40.0ºC size 12{"40" "." 0°C} {}

water placed in a vacuum chamber start to boil as the chamber is evacuated (air is pumped out of the chamber)? At what pressure does the boiling begin? Would food cook any faster in such a beaker?

Why does rubbing alcohol evaporate much more rapidly than water at STP (standard temperature and pressure)?

Problems & Exercises

Dry air is 78.1% nitrogen. What is the partial pressure of nitrogen when the atmospheric pressure is 1.01×105 N/m2 size 12{1 "." "01"´"10" rSup { size 8{5} } " N/m" rSup { size 8{2} } } {}

?

7 . 89 × 10 4 Pa size 12{ size 11{7 "." "89" times "10" rSup { size 8{4} } " Pa"}} {}

(a) What is the vapor pressure of water at 20.0ºC size 12{"20" "." 0°C} {}

? (b) What percentage of atmospheric pressure does this correspond to? (c) What percent of 20.0ºC size 12{"20" "." 0°C} {}

air is water vapor if it has 100% relative humidity? (The density of dry air at 20.0ºC size 12{"20" "." 0°C} {}

is 1.20 kg/m3 size 12{1 "." "20"" kg/m" rSup { size 8{3} } } {}

.)

Pressure cookers increase cooking speed by raising the boiling temperature of water above its value at atmospheric pressure. (a) What pressure is necessary to raise the boiling point to 120.0ºC size 12{"120" "." 0°C} {}

? (b) What gauge pressure does this correspond to?

(a) 1.99×105 Pa size 12{ size 11{1 "." "99" times "10" rSup { size 8{5} } " Pa"}} {}

(b) 0.97 atm

(a) At what temperature does water boil at an altitude of 1500 m (about 5000 ft) on a day when atmospheric pressure is 8.59×104 N/m2? size 12{8 "." "59" times "10" rSup { size 8{4} } " N/m" rSup { size 8{2} } ?} {}

(b) What about at an altitude of 3000 m (about 10,000 ft) when atmospheric pressure is 7.00×104 N/m2? size 12{7 "." "00" times "10" rSup { size 8{4} } " N/m" rSup { size 8{2} } ?} {}

What is the atmospheric pressure on top of Mt. Everest on a day when water boils there at a temperature of 70.0ºC? size 12{"70" "." 0°"C?"} {}

3 . 12 × 10 4 Pa size 12{ size 11{3 "." "12" times "10" rSup { size 8{4} } " Pa"}} {}

At a spot in the high Andes, water boils at 80.0ºC size 12{"80" "." 0°C} {}

, greatly reducing the cooking speed of potatoes, for example. What is atmospheric pressure at this location?

What is the relative humidity on a 25.0ºC size 12{"25" "." 0°C} {}

day when the air contains 18.0 g/m3 size 12{"18" "." 0" g/m" rSup { size 8{3} } } {}

of water vapor?

78.3%

What is the density of water vapor in g/m3 size 12{"g/m" rSup { size 8{3} } } {}

on a hot dry day in the desert when the temperature is 40.0ºC size 12{"40" "." 0°C} {}

and the relative humidity is 6.00%?

A deep-sea diver should breathe a gas mixture that has the same oxygen partial pressure as at sea level, where dry air contains 20.9% oxygen and has a total pressure of 1.01×105 N/m2 size 12{1 "." "01"´"10" rSup { size 8{5} } " N/m" rSup { size 8{2} } } {}

. (a) What is the partial pressure of oxygen at sea level? (b) If the diver breathes a gas mixture at a pressure of 2.00×106 N/m2 size 12{2 "." "00"´"10" rSup { size 8{6} } " N/m" rSup { size 8{2} } } {}

, what percent oxygen should it be to have the same oxygen partial pressure as at sea level?

(a) 2.12×104 Pa size 12{ size 11{2 "." "12" times "10" rSup { size 8{4} } " Pa"}} {}

(b) 1.06% size 12{ size 11{1 "." "06"%}} {}

The vapor pressure of water at 40.0ºC size 12{"40" "." 0°C} {}

is 7.34×103 N/m2 size 12{7 "." "34"´"10" rSup { size 8{3} } " N/m" rSup { size 8{2} } } {}

. Using the ideal gas law, calculate the density of water vapor in g/m3 size 12{"g/m" rSup { size 8{3} } } {}

that creates a partial pressure equal to this vapor pressure. The result should be the same as the saturation vapor density at that temperature (51.1 g/m3). size 12{ \( "51" "." "1 g/m" rSup { size 8{3} } \) "." } {}

Air in human lungs has a temperature of 37.0ºC size 12{"37" "." 0°C} {}

and a saturation vapor density of 44.0 g/m3 size 12{"44" "." "0 g/m" rSup { size 8{3} } } {}

. (a) If 2.00 L of air is exhaled and very dry air inhaled, what is the maximum loss of water vapor by the person? (b) Calculate the partial pressure of water vapor having this density, and compare it with the vapor pressure of 6.31×103 N/m2 size 12{6 "." "31"´"10" rSup { size 8{3} } " N/m" rSup { size 8{2} } } {}

.

(a) 8.80×10−2 g size 12{ size 11{8 "." "80" times "10" rSup { size 8{ - 2} } " g"}} {}

(b) 6.30×103 Pa size 12{ size 11{6 "." "30" times "10" rSup { size 8{3} } " Pa"}} {}

; the two values are nearly identical.

If the relative humidity is 90.0% on a muggy summer morning when the temperature is 20.0ºC size 12{"20" "." 0°C} {}

, what will it be later in the day when the temperature is 30.0ºC size 12{"30" "." 0°C} {}

, assuming the water vapor density remains constant?

Late on an autumn day, the relative humidity is 45.0% and the temperature is 20.0ºC size 12{"20" "." 0°C} {}

. What will the relative humidity be that evening when the temperature has dropped to 10.0ºC size 12{"10" "." 0°C} {}

, assuming constant water vapor density?

82.3%

Atmospheric pressure atop Mt. Everest is 3.30 × 104 N/m2 size 12{3 "." "30"´"10" rSup { size 8{4} } " N/m" rSup { size 8{2} } } {}

. (a) What is the partial pressure of oxygen there if it is 20.9% of the air? (b) What percent oxygen should a mountain climber breathe so that its partial pressure is the same as at sea level, where atmospheric pressure is 1.01×105 N/m2? size 12{1 "." "01" times "10" rSup { size 8{5} } " N/m" rSup { size 8{2} } ?} {}

(c) One of the most severe problems for those climbing very high mountains is the extreme drying of breathing passages. Why does this drying occur?

What is the dew point (the temperature at which 100% relative humidity would occur) on a day when relative humidity is 39.0% at a temperature of 20.0ºC size 12{"20" "." 0°C} {}

?

4 . 77 º C size 12{ size 11{4 "." "77"°C}} {}

On a certain day, the temperature is 25.0ºC size 12{"25" "." 0°C} {}

and the relative humidity is 90.0%. How many grams of water must condense out of each cubic meter of air if the temperature falls to 15.0ºC size 12{"15" "." 0°C} {}

? Such a drop in temperature can, thus, produce heavy dew or fog.

Integrated Concepts

The boiling point of water increases with depth because pressure increases with depth. At what depth will fresh water have a boiling point of 150ºC size 12{"150"°C} {}

, if the surface of the water is at sea level?

38 . 3 m size 12{ size 11{"38" "." 3``m}} {}

Integrated Concepts

(a) At what depth in fresh water is the critical pressure of water reached, given that the surface is at sea level? (b) At what temperature will this water boil? (c) Is a significantly higher temperature needed to boil water at a greater depth?

Integrated Concepts

To get an idea of the small effect that temperature has on Archimedes’ principle, calculate the fraction of a copper block’s weight that is supported by the buoyant force in 0ºC size 12{0°C} {}

water and compare this fraction with the fraction supported in 95.0ºC size 12{"95" "." 0°C} {}

water.

FB/wCuFB/wCu′=1.02 size 12{ { { left ( {F rSub { size 8{B} } } slash {w rSub { size 8{"Cu"} } } right )} over { left ( {F rSub { size 8{B} } } slash {w rSub { size 8{"Cu"} } } right ) rSup { size 8{′} } } } =1 "." "02"} {}

. The buoyant force supports nearly the exact same amount of force on the copper block in both circumstances.

Integrated Concepts

If you want to cook in water at 150ºC size 12{"150"°C} {}

, you need a pressure cooker that can withstand the necessary pressure. (a) What pressure is required for the boiling point of water to be this high? (b) If the lid of the pressure cooker is a disk 25.0 cm in diameter, what force must it be able to withstand at this pressure?

Unreasonable Results

(a) How many moles per cubic meter of an ideal gas are there at a pressure of 1.00×1014 N/m2 size 12{1 "." "00"´"10" rSup { size 8{"14"} } " N/m" rSup { size 8{2} } } {}

and at 0ºC size 12{0°C} {}

? (b) What is unreasonable about this result? (c) Which premise or assumption is responsible?

(a) 4.41×1010mol/m3 size 12{ size 11{4 "." "41" times "10" rSup { size 8{"10"} } `"mol/m" rSup { size 8{3} } }} {}

(b) It’s unreasonably large.

(c) At high pressures such as these, the ideal gas law can no longer be applied. As a result, unreasonable answers come up when it is used.

Unreasonable Results

(a) An automobile mechanic claims that an aluminum rod fits loosely into its hole on an aluminum engine block because the engine is hot and the rod is cold. If the hole is 10.0% bigger in diameter than the 22.0ºC size 12{"22" "." 0°C} {}

rod, at what temperature will the rod be the same size as the hole? (b) What is unreasonable about this temperature? (c) Which premise is responsible?

Unreasonable Results

The temperature inside a supernova explosion is said to be 2.00×1013 K size 12{2 "." "00"´"10" rSup { size 8{"13"} } " K"} {}

. (a) What would the average velocity vrms size 12{v rSub { size 8{"rms"} } } {}

of hydrogen atoms be? (b) What is unreasonable about this velocity? (c) Which premise or assumption is responsible?

(a) 7.03×108m/s size 12{ size 11{7 "." "03" times "10" rSup { size 8{8} } `"m/s"}} {}

(b) The velocity is too high—it’s greater than the speed of light.

(c) The assumption that hydrogen inside a supernova behaves as an idea gas is responsible, because of the great temperature and density in the core of a star. Furthermore, when a velocity greater than the speed of light is obtained, classical physics must be replaced by relativity, a subject not yet covered.

Unreasonable Results

Suppose the relative humidity is 80% on a day when the temperature is 30.0ºC size 12{"30" "." 0°C} {}

. (a) What will the relative humidity be if the air cools to 25.0ºC size 12{"25" "." 0°C} {}

and the vapor density remains constant? (b) What is unreasonable about this result? (c) Which premise is responsible?

Glossary

dew point
the temperature at which relative humidity is 100%; the temperature at which water starts to condense out of the air
saturation
the condition of 100% relative humidity
percent relative humidity
the ratio of vapor density to saturation vapor density
relative humidity
the amount of water in the air relative to the maximum amount the air can hold
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