|MadSci Network: Chemistry|
Salt water has a lower freezing temp and a higher boiling temp compared to pure water.
Try taking 2 glasses of water - each glass the same size - and place the same volume of water into each glass. Then heat one to boiling and leave leave the other at room temperature. Next, place both in the freezer. The glass with the boiled water will freeze first. Also, coffee with cold milk will cool slower then a black coffee.
Now back to the original question. The energy required to raise the salt water is more then the pure water. So, I believe that the salt water will behave like the black coffee.
It would be a good experiment for your class to try and see.
P.S. Here's why the hot water placed in a freezer freezes faster than cold water:
This question was raised in New Scientist in 1969, by a Tanzanian student named Erasto Mpemba. He discovered that ice cream mixture froze more quickly when put in the freezer hot than if allowed to cool to room temperature first. I got the same sceptical comments from my teachers as Mpemba did when I based my sixth form project on his question.
First, the project showed that water, either from the tap or distilled, behaved in the same way as ice cream mixture; the chemical composition is not important. Second, it demonstrated that a reduction in volume by evaporation from hot water was not the cause. Placing thermocouples into the water showed that water at about 10 °C, reached freezing point more quickly than water at about 30 °C as predicted by Newton's law of cooling, but that thereafter, water that started off warm solidified more quickly.
In fact, the maximum time taken for water to solidify in the freezer occurred with an initial temperature of about 5 °C, and the shortest time at about 35 °C. This paradoxical behaviour can be explained by a vertical temperature gradient in the water. The rate of heat loss from the upper surface is proportional to the temperature. If the surface can be kept at a higher temperature than the bulk of the liquid, then the rate of heat loss will be greater than from water with the same average temperature, uniformly distributed. If the water is in a tall metal can rather than in a flat dish, the paradoxical effect disappears. We argued that temperature gradients in the tall can were short-circuited by heat conduction through its metal walls.
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