The warm weather climates necessary for citrus orchards (like peaches) generally minimize the risk of frost conditions. Nevertheless, occasionally measures are needed to protect the trees and their fruit from freezing temperatures. Citrus crops become threatened when temperatures fall below 28 degrees F for 4 hours or more. Heating is the most effective protection against frost, and farmers often use heaters to warm the air temperature in the orchard. However, spraying the crops with water at conditions where freezing occurs also provides sufficient protection from the cold temperatures.
a. Using the following information, explain why spraying citrus crops with water at conditions where freezing occurs is successful for protecting the crops from cold temperatures.
The pressure in the orchard is constant (i.e., assume that atmospheric pressure does not change during the spraying of water and the formation of frost).
The enthalpy of fusion for water is equal to -6.01 kJ/mol.
The temperature of the water that is sprayed is about 32?F (i.e., Twater > 32degreesF).
b. Would spraying the crops with warm water (i.e., Twater > 32degreesF) be a more effective or less effective way to protect the crops from cold temperatures? Why? Hint: the heat capacity for liquid water at 20degreesC is 4.2 J g-1 K -1 .
a.First you need to define fusion.
Fusion is the physical change of a solid to a liquid which is absorbing heat. So if you are spraying the citrus crop with water at conditions where freezing occurs, the plant will absorb the heat from the change from solid to liquid to protect the crop.
b. The warm water will be more effective to protect the heat from the cold temperatures because the warm water has a greater initial heat rate and more heat will be given off to the surroundings to protect the plants from freezing. The colder water has a lower initial heat rate which wouldn't give off as much heat to the surroundings to protect the plants as well as the warmer water. The equation that relates to this is q = c x m x delta T
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