Freezing Point Depression

[Rolling]

I have a question for you.

So I was trying to understand freezing point depression, so I created dissolved a BUNCH of salt in water and put it into a beaker. I then took another beaker and put a bunch of ice cubes in it and then I poured the salt solution. I put a thermometer in to measure the temperature change, and I noticed that I was able to get the temperature to about -5 degrees Celsius and the water did not freeze. However, I have a question for you. Why is it then, when I simply add a bunch of ice to my glass of water, and the temperature of the water likely gets below 0 degrees celsius, it doesn't freeze? Is it because I'm adding too a lot of liquid water and that water and so it doesn't lose enough heat to freeze? If I only added a little liquid water would it start to freeze? I'm so confused!

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[UpQuark]

I have a question for you.

So I was trying to understand freezing point depression, so I created dissolved a BUNCH of salt in water and put it into a beaker. I then took another beaker and put a bunch of ice cubes in it and then I poured the salt solution. I put a thermometer in to measure the temperature change, and I noticed that I was able to get the temperature to about -5 degrees Celsius and the water did not freeze. However, I have a question for you. Why is it then, when I simply add a bunch of ice to my glass of water, and the temperature of the water likely gets below 0 degrees celsius, it doesn't freeze? Is it because I'm adding too a lot of liquid water and that water and so it doesn't lose enough heat to freeze? If I only added a little liquid water would it start to freeze? I'm so confused!

It's because the freezing point with that much salt is way lower than -5 degrees. Try calculate it out using deltT = kf x m x i (kf = 1.86 I believe). I guarantee it will be much lower (more negative) than -5. Also, once you have your solution, put it in a tub of dry ice or something way colder than ice and observe the point when it will freeze with thermometer in the solution.

In your situation, you're simply adding salt to disrupt the intermolecular bonding b/w the individual water molecules. You CANNOT determine the new freezing point from just taking before and after temperature measurements - you have to actually make the solution freeze to find it's new freezing point.

Kinda confusing, but hope that helps.

 

[Rolling]

Well thank you, but that's not exactly what I was asking. I was just asking why this doesn't happen when I just put ice in a cup of water?

 

[SN2ed]

Thread moved. This type of thread belongs in the Study Q&A forum.

 

[Rabolisk]

It takes a lot of energy to freeze water. For comparison, it takes less energy to cool water from 50 to 0 degrees than it does to freeze at 0 degrees. Boiling is even worse. Think about how easy it is to heat water to boiling, but how hard it is to actually boil an entire pot of water away. Also, when the ambient temperature is greater than 0 degrees, heat will constantly enter the solution warming both water and ice up.

 

[movax]

I would suggest reviewing delta H fusion and delta H vaporization. (EK Lecture 5 is excellent for this). When melting ice into water, at some point the energy you're adding will stop simply increasing vibrational/etc energy of the solid, but now go into breaking bonds. Once this is done, your temperature will continue to rise again. This is why temperature will appear to stay constant at 0C until the ice is melted and then it will continue to heat.

The figure in 5.4 of EK Chemistry explains this perfectly.