Water Condensation, Boiling, and Evaporation

WATER CONDENSATION, BOILING, AND EVAPORATION

Condensation is the transformation of the vapor form to the liquid form at a temperature that, in water in nature, occurs below the boiling temperature. This is why dew forms at lower temperatures. The boiling point can be described as the “normal boiling point”, which is the transfer of water as liquid to H2O gas at a temperature of 100 degrees and at 1 atmosphere or 760 torr. This “normal boiling point” is not the boiling point at lower pressures and at higher pressures. As mentioned, the boiling point of water occurs at a lower pressure, such as at higher altitudes.

Steam droplets will form above a pot of boiling water because the temperature will be lower above the pot of water so that some of the gas turns back into droplets that are seen as steam. What you need to know, though, is that, at the boiling point, vapor will not usually condense into a liquid. In addition, the liquid will not boil at its boiling point but needs a temperature above that in order to boil. We’ll talk more about that later. Finally, you need to know that the dew point is equivalent to the condensation point and both are equivalent to the boiling point at 100 degrees Celsius at 760 torr.

If water is boiling and becomes steam, if the droplets are very small and the partial pressure of water is less than the vapor pressure, the tendency will be for the droplets to evaporate. When in droplet form, the surface area to volume ratio is reduced and the curvature means that each molecule faces fewer attraction forces from its nearest neighbor. This also supports evaporation.

In the situation of a bubble inside a liquid such as water, the tendency is for the bubble to collapse. This is a situation where a bubble, a hole in a liquid, has molecules just on the outer surface (in the water) that curves inward, making an increase in nearestneighbor attractions. The temperature must rise within the liquid above the boiling point in order for the bubble to expand and not to collapse.

There is also the hydrostatic pressure of the weight of the water above the bubble that has formed. This too needs to be overcome in order to have the bubble expand and not collapse. Water must be superheated in order for it to actually “boil” and have expansion

of the bubbles forming at the bottom of a pot of water. Superheating involves the heating of water above its boiling point (under higher pressure circumstances).

So, if tiny drops have forces that tend to evaporate them, how do they condense and form rain, for example? In the example of a vapor cooled in a container, the condensation happens on the inner surface of the container because it is in that location that there are greater intermolecular attractions between water molecules that will stabilize droplets. This also explains why condensate appears on the outside of an icy glass or on grass. Droplets of steam above a pot of boiling water happen because of dust particles in the air.

Precipitation in the environment happens because of particles in the air that “attract” water in liquid form. This can be dust, volcanic ash, meteoric dust, and smoke. This will stabilize the small droplets until they grow to a size that causes rain, snow, or fog to develop. These droplets are microscopic but have a macroscopic impact on the development of rain and other precipitation. Fog itself can be irritating—not because of the moisture—but because of the dust particles that underlie the moisture.

What is the difference between the boiling point of a liquid and the evaporation of a liquid? When a liquid evaporates, it does so from the surface of the liquid. When liquid boils, it boils from the interior of the liquid and the bubbles are propelled to the surface by the fact that they have a lower density than liquid water. These bubbles expand as they rise because they aren’t affected by the hydrostatic pressure of being deeply under the water, which makes them even more buoyant.

Because boiling involves the superheating of liquids, there can be an explosion of the liquid if it is disturbed as the liquid is superheated in a container. When water is microwaved, superheating can occur throughout the liquid so that when a powder is added to it, such as hot cocoa powder, the liquid can suddenly boil and will potentially burn the individual. This doesn’t happen when water is boiled on the stovetop, where it boils from the bottom up.

Sublimation involves the direct vaporization of a solid to its gaseous form. This happens with any solid, including ice that will evaporate in cold, dry weather. It will involve a measurable number less than its melting point. This will be a different number

depending on the vapor pressure. It is seen when dry ice or CO2 solid sublimes at 1 atmosphere pressure without going through the melting phase.

The temperatures and pressures at which a given phase of a substance is considered stable is considered a property of the substance (with temperature and pressure being important factors). A phase diagram can be written of any element or molecule. Figure 36 showed a phase diagram for water and figure 36 shows a phase diagram for a hypothetical substance:

Figure 36.

A phase diagram is somewhat distorted because of the wide variations in temperature and pressure and the need to use one diagram to describe some crucial features of phase changes. The phase diagram goes from low temperature to high temperature and from low pressure to high pressure. There are several lines noted, including the melting line that depends on the temperature primarily. The sublimation curve involves the curve at which solids go to gaseous form. The vapor pressure curve involves liquids going to gases.

There are two points to note on the phase diagram. The first is the triple point. This is the equilibrium point of a certain temperature and pressure, when the vapor pressures

of solids, gases, and liquids will be identical. The second point is the critical point as mentioned already. This is the endpoint of the vapor pressure curve with no separate liquid phase present. Above this point, the substance is referred to as a fluid or a “supercritical fluid” involving high temperatures and pressures.

There are several crucial things to know on the phase diagram. These are the normal melting temperatures and the normal boiling temperatures of a substance. These exist as the pressure is 1 atmosphere. Water is unique in that the slope of the melting curve is dependent on pressure, meaning that the melting point of ice decreases with increasing pressure. If subjected to high pressure, ice at zero degrees will melt and ice floats on liquid water, which is an unusual property compared to other substances.

In addition, water can be supercooled, which is the phenomenon of “freezing rain”. This is a nonstable state of water in which it can be cooled below its freezing point for a period of time. The triple point of water is just 0.0075 degrees above the freezing point, allowing for all three phases to coexist at this temperature. Above the critical point of water, which is 374 degrees Celsius, no separate liquid phase can exist in water.

Interestingly, dry ice is dry because its triple point is at 5.11 atmospheres so under normal pressures, there can be no liquid CO2; it all goes to the gaseous phase under the pressures seen in normal use circumstances. The critical point of CO2 is only 31 degrees Celsius. This is why CO2 extinguishers must be very strong to withstand the pressure of 73 atmospheres in which the CO2 in the fire extinguisher would be entirely vaporized. Supercritical CO2 fluid is used as a solvent to remove the caffeine from coffee beans.

Every substance has a phase diagram that will follow similar rules but that are unique to the substance. Helium, for example, has a very unique phase diagram in which sublimation is not possible. It can only be frozen at very high temperatures and there are two liquid phases (helium I and helium II). Helium II is essentially a superfluid, which behaves differently from a quantum perspective when compared to ordinary atoms.