TERM: 2nd TERM
SUBJECT: BASIC SCIENCE
CLASS: JSS 2
WEEK EIGHT DATE: ………………..
TOPIC: EVAPORATION AND BOILING
Evaporation vs. Boiling
Evaporation occurs on the surface of liquid and it is a vaporization of liquid. It is a state of transition from liquid to gaseous state. The process occurs slowly and cannot be seen as well. It occurs when there is exposure of water to air and water molecules change into vapor and these vapors rise up and form clouds.
Boiling occurs on the entire mass of liquid and it is the vaporization of liquid. It occurs rapidly. It happens when the vapor pressure of the liquid is equal to the pressure exerted by the environmental pressure on liquid. It is a state of phase transition. The boiling occurs in three different stages: nucleate boiling, transition boiling and film boiling. There are no such stages for evaporation.
Boiling occurs when the temperature of the liquid is greater than the boiling point of the substance. Evaporation can occur at any temperature. It occurs as long as the substance Â remains Â liquid at a particular temperature. Evaporation occurs when there is an increased energy present and occurs rapidly. It occurs from the bottom of the container when allowed to boil. The bubbles form at the bottom of the container and then rise on top of the container. In boiling, bubbles do not form at the bottom and rise to the surface. Evaporation occurs at room temperature and therefore, occurs at a slower rate when compared to boiling. In boiling, there is formation of bubbles as it is a complex physical process and these bubbles are formed on a heated liquid. There is cavitation and acoustic effects seen in boiling. There is no such bubbles formed in evaporation and there is no cavitation and acoustic effect present in evaporation.
The microscopic difference between evaporation and boiling is as follows:
Factors influencing the rate of evaporation
Note: Air used here is a common example; however, the vapor phase can be other gases.
If the air already has a high concentration of the substance evaporating, then the given substance will evaporate more slowly.
If the air is already saturated with other substances, it can have a lower capacity for the substance evaporating.
This is in part related to the concentration points above. If "fresh" air (i.e., air which is neither already saturated with the substance nor with other substances) is moving over the substance all the time, then the concentration of the substance in the air is less likely to go up with time, thus encouraging faster evaporation. This is the result of the boundary layer at the evaporation surface decreasing with flow velocity, decreasing the diffusion distance in the stagnant layer.
The stronger the forces keeping the molecules together in the liquid state, the more energy one must get to escape. This is characterized by the enthalpy of vaporization.
Evaporation happens faster if there is less exertion on the surface keeping the molecules from launching themselves.
A substance that has a larger surface area will evaporate faster, as there are more surface molecules per unit of volume that are potentially able to escape.
the higher the temperature of the substance the greater the kinetic energy of the molecules at its surface and therefore the faster the rate of their evaporation.
In the US, the National Weather Service measures the actual rate of evaporation from a standardized "pan" open water surface outdoors, at various locations nationwide. Others do likewise around the world. The US data is collected and compiled into an annual evaporation map. The measurements range from under 30 to over 120 inches (3,000 mm) per year.
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