Salt Water and Melting Ice
Name: Frank V.
My 8 year old son has done a science fair project
entitled "Does an Ice Cube Melt Faster in Fresh Water or Salt Water?".We
have done the experiment several times and have concluded that an ice
cube melts faster in fresh water. Can you help me explain the
experiments results,so an 8 year old can better understand it.
NOTE: NOVEMBER 1 2014 - THERE WAS A REQUEST TO RE-EXAMINE THIS ARTICLE FOR ERRORS AND FAILURE TO ANSWER THE REQUEST. THE RESPONSES TO THIS POSSIBILITY ARE GIVEN BELOW:
So you have the correct answer, you did some experiments and
found correctly that an ice cube melts faster in fresh water...
Every pure substance has a definite temperature where it melts. It is
called melting point. (it is similar to the boiling point where
a pure substance boils). We can say that the temperatures in both
cases are related to the forces that hold the molecules together.
There is a physical constant called "molar heat of fusion" that
somehow controls the melting point temperature.
When you have salty water, the salts in the water make the molar
heat of fusion, (and the melting point) lower, and this means that
the ice inside this salted water will melt only at a lower temperature.
And the ice cubes will melt slowly...
That is the same principle one use when put some salt over
an ice pack to freeze some beer cans...The ice will stay
I hope, Dad, that the explanation will be satisfatory for your
8 year old son.
And tell others about NEWTON! It is a pleasure to
answer your questions!
(Dr. Mabel Rodrigues)
There is a bit of truth in the explanation, but ultimately I find in in error. This I believe is the true explanation:
Ice cubes float on water.
Cold water is denser than hot water.
Salt water is denser than than fresh water, whether cold or hot.
When an ice cube is placed in fresh water it begins to melt and
that cold water travels to the bottom of the container because it
is much denser. This encourages the heat transfer in the vicinity and
the ice cube melts fast.
On the other hand, when an ice cube is placed in salt water, it begins
to melt as well but that cold water still stays around the the ice cube
since the salt water is denser.
This does not fully encourage the heat transfer rate as compared to
the above scenario and the melting time increases as a result.
Hence, the ice cube melts faster in the fresh water than in the salt water.
The answers don't address the biggest reason for the difference.
The reason can be seen experimentally if you put a few drops of food coloring on the ice cube, and watch where it goes. In fresh water, the cold water that melts off the ice cube is denser than the surrounding water, and it sinks. The resulting circulation brings a steady supply of relatively warm water to the ice cube.
In salt water, the cold (fresh) melt water is not much denser than the surrounding salt water, and it doesn't sink, so it doesn't bring new warm water to the ice cube.
I don't think there is much wrong with the original answer. It is well
known that a salt solution freezes at a lower temperature than pure
water. Just as ice in pure water causes the water to be chilled to the
water's freezing point, it is also true that ice in a salt solution causes
the salt solution to cool to ITS (lower) freezing point.
When ice is immersed in a salt solution that has been allowed to reach
its (lower than 0°C) equilibrium temperature, the ice is (in a sense)
being "refrigerated" by the lower temperature salt solution. As a result,
less heat flows into the ice, and it therefore melts more slowly than if it
was immersed in (warmer) 0°C water.
The answers posted on the Newton website do not answer the question that you asked, and instead answers a question that you did not ask.
In your experiment, you used ordinary ice cubes that would be nominally pure water. You find that an ice cube melts faster when it is place into a fresh water bath than when placed into in a salt water bath. That is clear.
The quickness of the melting that you observe depends on many things. A full analysis of the problem is quite complicated: one must calculate the properties of the water bath and additionally the details of how the water flows and removes heat (and thereby melts the water) from the ice cube, and how much salt is put into the water.
But the factor that seems to me to be the biggest and MOST important is the “specific heat” of the water. If the water in the bath (salty or fresh) flows over the ice cube in nearly the same way then then the water that has a large “specific heat” will warm the ice cube faster and cause it to melt faster.
If we look up in a table the specific heat of saltwater and freshwater we find that specific heat of fresh water is 4207 J/kg K, and the specific heat of salty water is always smaller. For very salty water (120g/kg) the specific heat is only 3310 J/kg K!
Thus, we would predict that the that fresh water bath would melt the cube faster than a salt water bath based only the specific heat of the bath water alone. And that is the trend that you find when you do the experemnt. If this trend is true then water with the largest amounts of salt dissolved into it will produce the slowest melting because the specific heat is the lowest.
For the 8-year old the explanation could be “when you put the ice cube into the salt water bath the water melts the ice cube slowly. But when the ice cube is put into the fresh water bath the ice cube melts faster. This is because fresh water holds more heat and can draw heat away from the ice cube faster to make it melt. “
Argonne Natl Lab
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Update: June 2012