If oil paint is mixed with turpentine or another solvent, it can be applied for thin films. When the turpentine evaporates and disappears into the air, the oil paste is left behind as a very thin film.
Evaporation is the process that changes a liquid into a gas. It can occur at any temperature, and is due to the movement of molecules in the liquid. For example, water left in a saucer will gradually evaporate. It should also be noted that evaporation is not necessarily always visible. Sometimes these molecules do not feature enough of a heat transfer to create a sufficient enough transition to vapor. However, evaporation is still taking place during this process but at a significantly slower rate than high energy molecule evaporation processes.
Effect of increasing the temperature on the rate of evaporation of a liquid At the higher temperature, the total fraction of molecules with enough kinetic energy to escape is larger, so the rate of evaporation is larger.
Evaporation produces a cooling effect:
The interesting thing about the process of evaporation is that it produces a cooling effect. When a person has a shower bath and been chilled by air, after some time, the molecules of water evaporate from the body to maintain a constant body temperature. This causes the cooling effect on the body of a person.
This picture illustrates the kinetic energy distribution in a liquid at a particular temperature. The energy required for a molecule to evaporate is marked as minimum kinetic energy. It is the energy needed by the molecule to escape from the attractions of its neighbors.
The molecules with kinetic energy equal to or greater than the minimum kinetic energy can leave the liquid. The minimum energy required to escape is much larger than the average kinetic energy. Hence, the molecules which have more kinetic energy will evaporate making the average Kelvin energy still less than the original. As the Kelvin temperature of liquid is directly proportional to the lower kinetic energy of the remaining molecules, the temperature becomes lower. Thus, process of evaporation produces the cooling effect.
The liquid has just begun to evaporate into the container. The rate of evaporation is greater than the rate of condensation. A dynamic equilibrium is reached when the rate of condensation is equal to rate of evaporation. In a given time period, the number of molecules entering the vapour equals the number that leave, so there is no net change in the number of gaseous molecules.
Rate of evaporation:
With the knowledge of the evaporation, one can also expect the rate of evaporation and also factors governing it.
The factors that govern the rate of evaporation are:
- Surface area
- Temperature
- Strengths of intermolecular attractions
Evaporation has a direct relationship within the surface area of liquid. As the liquid takes the shape of container, container with more surface area is favorable for more number of molecules to evaporate. Example: Tea in saucer cools faster than tea in a cup.
Surface area:
As evaporation is a surface phenomenon, it makes sense that when the surface area is increased, more molecules are able to escape and the liquid evaporates more quickly. For the liquids having same surface area, the rate of evaporation depends on two factors namely temperature and strength of intermolecular attractions.
Temperature:
The increase in the temperature increases the kinetic energy of the molecules. As soon as the molecule reaches the minimum kinetic energy, it will evaporate. Hence with increase in temperature, rate of evaporation also increases.
The same kinetic energy is needed for the escape of molecules at both the temperatures. Minimum kinetic energy required is determined by the kinds of attractions between the molecules and is independent of temperature.
At higher temperature, the total fraction of molecules having the kinetic energy equal to or greater than minimum kinetic energy is more. Hence with the increase in temperature, rate of evaporation per unit surface area of a given liquid is greater at a higher temperature.
Water colors are the very good application for the process of evaporation. When the water is mixed with the color and painted over the canvas, water evaporates into atmosphere, leaving the color on canvas.
Strengths of intermolecular attractions:
The stronger the intermolecular force is, the slower the rate
of evaporation will be. The intermolecular force is directly
related to the bonds between molecules and the molar mass.
The hydrogen bond is the strongest bond that molecules can
obtain and if the bonds between molecules are strong then
the attraction allows the solution to be held in its liquid
state longer. Since the bond is strong it brings difficultly
to break those bonds and when a liquid evaporates or becomes
gaseous bonds need to be broken to transition to a gaseous state,
it requires more energy to break those strong bonds which causes
the rate of evaporation to be slower. The strength of intermolecular
forces is directly related to the rate of evaporation,
if the evaporation rate is higher, intermolecular forces
are weaker. If the rate of evaporation is slower,
the intermolecular forces are stronger.