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Name: Lee 
Status: student
Grade: 6-8
Country: China
Date: Summer 2012


Question:
I am currently wondering the effect of different type of salts on the melting rate of ice. I used fine, sea, rock, mineral and iodized salt mixed with 15ml of water and left it in the freezer to freeze. 1g and 2g of salt froze, while 3g partly froze and 4g and 5g did not freeze at all. I am wondering why this happen? I did not sprinkle the ice with water. Instead I mixed 15ml of water with the salt and left it to freeze. Also, I tested for the melting rate of each ice with the different type of salt but received inconsistent result. I would really like to find out which of the above salt at which mass(1g, 2g, 3g, 4g or 5g) melt the fastest.



Replies:
Hi Lee,

Think of water molecules as arranging themselves in a specific pattern in order to crystallize (or freeze). Any substance that gets in the way will inhibit or slow down the arrangement process. If there are other substances (such as the salt you used) mixed in the liquid, the water molecules will have to lose more heat (go to a lower temperature) in order to be able to arrange themselves into the crystal patterns necessary for freezing.

This is why you observed that the smaller amounts of salt still allowed the water to freeze, but the larger amounts of salt did not freeze at the particular temperature of your freezer. If you had a colder freezer (or much more water relative to the amount of salt you put in) then the 4 and 5g samples might have frozen as well.

Freezing rates are controlled by many factors. Cold spots in your freezer, the shape of the containers, the temperature of each solution as they entered the freezer, etc. So it is not easy to determine which one will freeze first. The amount of salt may not even be a strong factor in the speed of freezing. So this is why you had inconsistent results.

Greg (Roberto Gregorius) Canisius College


You have fallen into a common trap. It is possible to measure the solubility of various salts. However, measuring the RATE of freezing (or melting) is very difficult if not impossible to measure. HOW FAST is subject to many variables that are difficult to measure. For example, how do you take super-cooling into account? It is very common for salt solutions to be meta-stable below the “normal” freezing point. How do you take small particles that act as seeds for freezing? Freezing RATE also depends upon how fast the sample is cooled or warmed.

Any time you encounter a “how fast” question, be assured that the answer is difficult to determine. Rates of processes is a far more difficult to measure than whether the process occurs at all.

Vince Calder


Q1: I used fine, sea, rock, mineral and iodized salt mixed with 15ml of water and left it in the freezer to freeze. 1g and 2g of salt froze, while 3g partly froze and 4g and 5g did not freeze at all. I am wondering why this happen?

-> Before getting into the question, I want to clarify one thing: you started to mention different type of salt, then talked about various amount of salt. I do not see any relation between your question and the type of salt, so I will guess you are asking about the amount of salt, not the type.

A1: Basically, the decrease of freezing point is proportional to the solute (can be salt or other ions) concentration, and the origin of freezing point depression (is same as boiling point elevation) is entropic.

( http://en.wikipedia.org/wiki/Freezing_point_depression)

At melting point or freezing point, the vapor pressure of liquid and solid is equal (the solid and liquid phases are at equilibrium). Mathematically, the change of free energy deltaG (is composed of enthalpy deltaH and entropy deltaS) from solid to liquid is zero, deltaG= deltaH - TdeltaS = 0 (T is temperature) and T = deltaH/deltaS.

If you add salt into water, the change of enthalpy deltaH does not change much as it is the amount of heat released to form solids (ice, crystal) from water molecules: recall that salt does not go inside ice solids. Given that the change of entropy deltaS is the lost entropy from disordered liquid phase (water) to ordered solid phase (ice), the salt water increases the entropy of liquid. (deltaS of salt water is larger than deltaS of pure water) Overall, T is decreased which is freezing temperature depression.

http://en.wikipedia.org/wiki/Melting_point

http://en.wikipedia.org/wiki/File:Freezing_point_depression_and_boiling_point_elevation.png http://www.chemicalforums.com/index.php?topic=51012.0

Q2: Also, I tested for the melting rate of each ice with the different type of salt but received inconsistent result. I would really like to find out which of the above salt at which mass (1g, 2g, 3g, 4g or 5g) melt the fastest.

A2: In principle, freezing temperature depression should give the same result of melting temperature depression. One thing is if the salt is not mixed homogeneously, you might see hot spots which melt faster than surrounding – as they contain more salt and locally have lower melting point.

Weon-kyu Koh


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