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Freezing Point Depression and Making Ice Cream

By: Lydia Chu and Laureen Liang

Laureen Liang

on 27 November 2010

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Transcript of Freezing Point Depression and Making Ice Cream

Freezing Point Depression and Making Ice Cream By: Lydia Chu & Laureen Liang No, it is not the freezing point undergoing a bout of depression! *laugh here* It actually describes the phenomenon where freezing point of a liquid is depressed/decreased when another compound is added, in this case it will be salt. Bodies of water filled with salt, like the ocean, will have a much lower freezing point than the regular 0°C. This makes it possible for oceans to be very very cold without freezing over, which is the reason why victims from the Titanic suffered hypothermia, The salt is also the reason why ocean water feels so buoyant! Now, something you should've heard before... Like we learned previously, when water is frozen at 0°C and turns into ice (solid), the molecules on the surface of the ice will constantly be melting and refreezing at the same time. NOM NOM NOM NOM munch! Water molecules would be changing into a liquid at the same rate as water molecules refreezing back into a solid. Therefore nothing will change and the ice would stay ICE. This process of melting and refreezing will continue to happen as long as the temperature is kept at 0°C. oops! there it goes again.. Do you guys remember how cold and numb our hands became when we made ice cream? It was around -10°C!!!!!!! That's extremely cold! Now exactly how does salt lower the freezing point? Firstly:
The chemical name for salt is NaCl.
Once salt dissolves in the water, it splits into Na+ and Cl- atoms. When water freezes, it has to be in a specific stabilized structure. What is
freezing point
depression? An ice cube's solid structure Therefore, the water molecules start moving slower and slower, the average kinetic energy (temperature) decreasing. Eventually becoming lower than 0°C, and gradually the very slow water molecules are able to stabilize and freeze. However, the salt particles get in the way of the water molecules forming a solid. The salt basically disrupts the process of melting and refreezing that usually happens on the surface of an ice cube. So which freezes at lower temperature, pure water or salt water? YOU TELL ME! CORRRRREEECCTTTTTTTTT!
Salt water is the right answer. This is why lakes and ponds will freeze over during the winter while ocean water will not, the North Pole being an exception! Speaking of nature...ever wonder why the freezing water of polar ice in the winter causes upwelling of nutrient rich water? Getting back to making ice cream, Why was salt added to the ice to make it? Ice cream can only be formed at around -10°C so the freezing point needs to be lowered. The salt does this by disrupting the process of melting and refreezing so that the molecules can't refreeze and will only melt. But in order to melt and turn into a liquid, it needs more energy, so it steals energy from itself and continues to go into the liquid state. This lowers its freezing point greatly and will make the ice cream cold enough to form. I SPY A... !!!!!!!!!!!!! !!!!!!!!!!!! !!!!!!!!!!!!!! !!!!!!!!!!!!!!!!! !!!! !!!!!!!!!!!!!!!!! !!!!! !!!!! Have you guys ever wondered how antifreeze for cars work? Probably not! But, we'll tell you guys anyways :) Antifreeze is basically a liquid added to the cooling system of a car to make sure the water within it doesn't freeze solid during very cold weather. It will lower the freezing point of the water enough to not freeze even at 0°C during the winter! Remembering to use antifreeze for youe car! It is very important. Otherwise, cold outside temperatures would freeze the water in the cooling system of the car.

Ice expands as it turns into a solid and that would ruin the car. Practical applications in everyday life where we make use of freezing point depression using salt to melt snow on the roads When salt is added to the snow, it decreases the freezing point, and since the snow is not the temperature of the “new” decreased freezing point, it will melt very fast. Why will soda pop begin to freeze when you open it if you remove it from the freezer before it has frozen? Remember that pure water freezes at 0°C. Whenever something is dissolved in a liquid, it will decrease the freezing point. In this case, the “something” that is dissolved is the carbon dioxide, rather than salt which we used in our previous example. Well, once the soda is opened, it will freeze instantly because the carbon dioxide escapes. It goes from a decreased freezing point (lets say in this case -10°C) to a higher freezing point (0°C, like that of a normal liquid at 1atm), causing it to freeze instantly. HOW TO CHILL A COKE IN JUST A SECOND INSTANTLY FREEZING WATER!!! Freezing point depression... Thanks for sharing the magic with us! SALT DECREASES THE FREEZING POINT
AND INCREASES THE BOILING POINT OF WATER Ice cream has to be made below 0C because it is not completely a liquid/water, it contains milk!

Therefore it has to be at a lower temperature for it to start freezing.
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