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Name: Gregory D.
Status: student
Age: 18
Location: N/A
Country: N/A
Date: Sunday, September 08, 2002


Question:
Hello, I was speaking with some classmates and the questions of how cold can water get came up. It is common folk lore in Florida that one should spray her orange trees with water before a freeze. The reason being that once the water freezes it will keep the plant warm, or only as cold as 32 degrees. This leads me to believe that water does not become colder once it freezes, if in fact the folk lore is true. Is it? Does water get colder as it its environment gets colder or does it simple lose volume or both? If it does not lose temperature, is this a property of crystals? Thank you for your time and effort.


Replies:
Water freezes at 32 Degrees, though this can be lowered if it were a mixture (like anti-freeze or salt solution). The reason one should spray the orange trees with water is to get a coating of ice on them to protect them from wind. This is the same "folk lore" why Eskimos build igloos out of snow/ice-because ice is an excellent insulator. The wind chill effect is what would actually damage the orange, bringing the temperature of the orange to freezing and rupturing the internal cells holding the juice thus killing the orange.

M. Baldwin


Water, like most things, has three distinct states: gaseous, liquid, and solid. In its liquid state, water has a minimum temperature of about 32 degrees Fahrenheit, and a maximum temperature of about 212 degrees Fahrenheit. Pressure and impurities (such as salt) affect these temperatures.

In order to heat or cool a mass of something, energy must be transferred. This is called the specific heat. For water, it takes 4.186 J/Kgm to change the temperature by one degree Celsius. This is one of the highest specific heats of any common mmaterial.

What this means is, it is very hard to change the temperature of water.

Additionally, when water goes through a phase change, going from water ton steam, or water to ice, it takes even more energy, and the temperature does not change. So, at 32 degrees, water can be ice or liquid water, and it will take a lot of energy transfer to change it from water to ice, or from ice to water. This is why a few cubes of ice can cool down a whole glass of water, as ice melts, it absorbs a lot of heat.

Once the phase change is complete, the temperature can change again. Ice can, (and almost always does) become colder than 32 degrees. Steam can also become hotter than 212 degrees (but the steam has to be trapped to do this).

So, the answer to your question is the folk lore has some merit, but will only offer a little protection for a very short, mild cold snap. The water will require it to be colder for longer in before the trees will freeze (not days, just a few hours longer). Also, when the water freezes, it will act like a very thin layer of insulation as well, and could prevent the tree from getting too much below freezing.

But, the ice can certainly get colder than 32 degrees, so it won't offer much protection, and, in the morning, it will take quite a bit longer for the tree to thaw out.

Eric Tolman
Computer Scientist


I am not sure that spraying fruit trees with water to protect the buds from freezing is folk lore. You could check with a county extension person in Florida to find that out. Water of course can get colder than 0 C. I can think of two reasons why coating the buds with ice prior to a freeze would have a beneficial effect: 1. Ice is not a very good conductor of heat. (In the arctic and Antarctic ice blocks are used as building material for some structures (e.g. igloos). 2. The coating of ice would protect the buds from wind. Even a relative wind speed lowers the "wind chill" factor. The buds exposed to cold air movement experiences the same "wind chill" effect humans do. The wind sweeps ground heat away, and has a dehydrating effect, both of which can damage the budding trees.

Vince Calder


As liquid water loses energy, it's temperature drops steadily until it gets to the freezing point. Here the temperature lingers for a while, as the liquid crystallizes (water must lose energy to crystallize). After all of the water has crystallized, the temperature again drops steadily as energy is lost.

So, if it is not going to get too cold, or if it's not going to stay cold too long, then the more water in the vicinity of the oranges the better. In fact, I think I would try putting a bunch of 55-gallon drums full of water all over the place to see if that would not help. (If the drums freeze, you want them out of there because they will do the same temperature-buffering job as the temperature rises.)

But if it is going to freeze hard, water is not going to help, and it might even make things worse because it will slow down the rate at which the oranges freeze, which will probably result in larger ice crystals that will do more damage to cell walls. On the other hand, if it is going to freeze hard, you have lost anyway.

Tim Mooney


Gregory,

Indeed, water can get colder than its freezing point. Consider the ice cubes in the freezer. If you put a thermometer inside with the cubes, you will discover the interior of the freezing compartment is almost always well below water's freezing point of 0 C (32 F). Think about it: I you were to drop an ice cube into liquid nitrogen at -195.5 C, the ice cube would cool to -195.5 C.

The operative principle in the orange grove scenario is related to the fact that in order for water to freeze, it must surrender whatever heat energy it holds. It is the release of that heat which keeps the oranges no colder than 0 C. The various substances dissolved in the orange juice and tree sap offer some "antifreeze" protection. Thus, their "freezing point" is somewhat lower than ) 0 C.

Bear in mind, the orange trees are not simply sprayed once with water until a coating of ice forms. If that were the only thing done, the ice film (and the oranges inside) would continue to cool to whatever the surrounding temperature is.

So long as the trees are misted with liquid water that subsequently freezes, the oranges get no colder than about 0 C. Of course there is a downside to prolonged spraying. Too much ice and the trees will break from the added weight.

Regards,
ProfHoff 459


Spraying orange trees, or any other plants, for protection when the temperature is expected to briefly drop below freezing is a valid concept. It works because of a thermodynamic property called the "latent heat of fusion". What it boils down to (sorry) is that water can only exist in both the solid and liquid phases at 32 degrees. So, if you start chilling liquid water, it will keep dropping in temperature until it hits 32 degrees. Then, it will stay at 32 degrees as it begins freezing and will not drop below that temperature until all the water has turned solid. After that, the temperature will keep dropping until it reaches the ambient temperature, which may be substantially below freezing.

So, if the temperature is expected to dip below freezing for a short time, then you can use this property to sacrificially freeze the sprayed-on water on the exterior of the plant, which will keep the fruit at 32 degrees F and unfrozen in the center. The time of protection can be extended if you keep spraying the fruit with water using a sprinkler, forcing the exterior of the fruit to stay liquid and solid at the same time. However, if the temperature is going to stay below the freezing mark for an extended period of time, then this strategy will not work. Eventually, all the water will freeze and the rest of the plant will also freeze, then drop to the ambient temperature, ruining the fruit and possibly killing the tree. The water in the plant's cells expands when it freezes, rupturing the cell walls. That's why fruit that has been frozen, then thawed, appears to be bruised and has a mushy texture.

Andy Johnson


Well in this case the folklore is lore...Water (as a gas, a liquid or a solid) will continue to lose heat to a surrounding environment as long as the environment is colder than the water. I suggest you stick a thermometer into a bowl of ice that was just removed from the freezer...

Peter Faletra Ph.D.
Assistant Director
Science Education
Office of Science
Department of Energy


To a very good approximation, the temperature of water does not change as the water freezes. As the water goes from being partly frozen to mostly frozen to totally frozen, it loses energy (heat). However, the temperature does not drop, because the energy comes from the water molecules occupying a lower-energy structure, the ice crystal (ice has a lower potential energy than liquid water).

After all the water freezes, it is not possible to extract any more energy from the liquid-solid transition. Then, the ice simply gets colder, and can no longer protect the orange tree from freezing. Only liquid water can do that.

Richard E. Barrans Jr., Ph.D.
Director of Academic Programs
PG Research Foundation, Darien, Illinois


Once water freezes and becomes ice, it WILL continue to drop in temp as it gets colder.The ice in your freezer at, say, 0 degees F, is much colder than the ice that has just frozen at 32 degrees F. Similarly the food in your freezer, which is largely composed of water, will get colder and colder as the temperature drops. The frozen vegetables in the back of the freezer case at the supermarket are colder than those in the front. The bag of ice in the deepest recesses of the supermarket freezer will melt more slowly in your car than the bag of ice from the front.

Sarina Kopinsky, MSc



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