Ask A Scientist General Science Archive

name         Gregory D.
status       student
age          18
 
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.
----------------------------------------
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
==============================================================