 |
 |
Salted Ice and Temperature Drop
Name: Blair F.
Status: student
Age: 12
Location: N/A
Country: N/A
Date: 1999-2001
Question:
We have been doing some expeiments with salt and ice. We
know that salt will melt ice and that the saltwater solution then goes
below 32 degrees F. I understand why the ice melts but I do not
understand why the temperature goes down. Where does the cold come from?
Replies:
Blair,
Cold does not "come from" anywhere. Cold is the absence of heat. As the salt
dissolves in the meltwater, intermolecular bonds between water molecules
in the
ice lattice are broken. Bond breaking processes always require energy. The
energy flow to enable the breaking process comes from the surroundings. If you
were to be holding the glass containing the ice-salt-water mixture, heat
energy
would flow out of your hand into the glass. Heat flow away from you is
perceived as cold.
Regards,
ProfHoff 290
Blair F.,
In general, as salt (the solute) is added to the water
(the solvent), the resulting solution will have a lower
freezing point than the pure solvent, in this case-
water. In chemistry, the properties that vary according
to the ratio of the weights of the solute and solvent
are known as colligative properties. I hope that this
answers your question.
Sincerely,
Bob Trach
It takes energy to change a solid to a liquid. That energy has to come from
somewhere, right?
Tim Mooney
To "explain" the lowering of the freezing point of ice "properly" requires
knowledge of an area of physical chemistry called thermodynamics, which I am
assuming you are not ready for, so I will try to put the concepts across as
well as I can using as little of the jargon as possible.
Pure water and ice at 0 C. are said to be "in equilibrium" which means
simply that they will co-exist indefinitely so long as the temperature
remains at 0 C. This equilibrium is written as if it were a chemical
reaction:
ice(pure) = water(pure)
at 0 C.
If the temperature is raised above 0 C. the ice will eventually melt, and
the ice and water are no longer "in equilibrium". If the temperature is
lowered below 0 C. (Ignore the fact that water can sometimes be super-cooled
below 0 C. and remain a liquid, at least for a while because that is an
unstable condition.) If the temperature of the ice/water mixture is lowered
below 0 C. the water will eventually all freeze leaving only ice, and the
ice and water are no longer "in equilibrium."
There is a quantity called the "chemical potential" that is a property of
the pure ice and the pure water. At 0 C. this "chemical potential" of the
ice and the water are equal to each other. (Don't read too much into the
term "chemical potential" literally because it is a jargon word. Think of it
more as some measurable quantity that depends on temperature, pressure, and
the composition of the things involved.The only thing you have to know about
this "chemical potential" is that it increases with temperature, and it
decreases as some substance is dissolved in the water. For solids and
liquids like ice and water, this quantity, this "chemical potential", is
independent of the pressure unless the pressure is very high. For our
purposes you can say pressure has no effect. That leaves only the
temperature and the composition of the things involved (ice and water).
In place of the pure water, suppose we have some salt dissolved in the
water.
ice(pure solid) =
salt water (liquid solution)
The presence of the salt lowers the "chemical potential" of the water. But
the "chemical potential" of the ice remains the same. If we were to hold the
temperature of the ice and salt solution at 0 C., the ice would all melt
because it has a higher "chemical potential" than the salt solution.
Remember I said that the addition of salt reduces the "chemical potential"
of the liquid water. If the pure ice is to remain in equilibrium with the
salt solution, it must lose some of its "chemical potential". Since pressure
has no effect (you have to trust me on that), and no salt can dissolve in
the solid ice, the only way for the ice to lose "chemical potential" is for
the temperature to be lowered. Remember, I said that "chemical potential"
increases with increasing temperature. And that is what happens. The
temperature of the ice = salt solution decreases until the "chemical
potential" of the ice and NOW the salt solution become equal.
For water, this requires that the temperature drop of (delta T)= -1.86 * m,
where m is the number of moles of ions (both Na+ and Cl- for salt) per 1000
gm. water.
I know this sounds pretty complicated, but your question is really not as
simple as you might think and your asking it shows your perception and
curiosity about what is happening. That is why I tried to give you more than
just the "standard answer" which really isn't true.
Vince Calder
Good question!
When salt is added to ice, nothing happens at first. You need for a little
of the ice to melt, so that some salt can dissolve in the meltwater to make
saltwater.
When ice melts and no salt is around, ice and its meltwater are constantly
exchanging. All the time, water molecules from the ice are detaching from
the surface and diffusing into the liquid water, and water molecules from
the liquid are attaching to the solid ice surface. If the ice is melting,
the molecules detaching from the ice slightly outnumber the molecules
attaching to it. If the water is freezing, the balance is slightly in the
other direction.
Things get interesting when the liquid water is impure, for instance when it
is saltwater. Then, the number of water molecules in a given volume of
liquid is a little less than it would be in pure water. This means that
fewer water molecules from the liquid phase will attach to the solid,
because there are fewer of them around. Since the solid ice hasn't changed
any, however, the rate of molecules detaching from the surface and going
into the liquid phase also doesn't change. So, the rate of detachment falls
below the rate of attachment, and overall the solid melts.
When water molecules attach to or detach from the ice surface, something
else happens. It turns out that a water molecule in ice has a lower
potential energy than a water molecule in liquid water. This is because it
is surrounded by other water molecules in a very precise arrangement. When
a water molecule leaves the ice surface, it has to pick up some energy to
break out of its potential energy "hole." Going the other way, when a water
molecule attaches to the ice surface from the liquid, it must give up some
energy to allow it to stay in its new potential energy "hole."
Pure ice (no salt around) melts at a very specific temperature because at
that temperature the kinetic energy of the molecules is great enough that
they won't stay in the precise ice arrangement. When salt is added to the
liquid phase, making it more difficult for water molecules to attach onto
the solid ice, the water molecules that detach from the ice still need to
pick up energy. They get this energy from their surroundings, which are the
rest of the liquid and solid water. In other words, as the water molecules
leave the ice and go into the water, they absorb energy from the ice and
water, making it colder.
The ice will continue to melt, that is, the water moecules' detachment rate
will be larger than the attachment rate, until the temperature is low enough
that it's harder and harder for the water molecules in the ice to pick up
the energy they need to detach. Finally, the attachment and detachment
rates will be the same, so the energy flow will again be in balance. Then
the temperature won't drop any further.
Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois
Click here to return to the General Topics Archives
| |
Update: June 2012
|
|
