[Nora Goba]

Topic - Freezing point depresion

Hello! I'm doing my IA for IB Chem and I want to measure the melting points of frozen alkali chloride salt solutions (LiCl - CsCl). I know the freezing effect should be the effect of the coligative properties that are, as far as i understand the same for all of these, 2 ion salts. Should there be a difference between the freezing points and between the melting points? Can I even freeze such solutions or will the freezing point be to low (I'm using a household freezer)? What percentage solution should I even make and does that even matter since the pure water will freeze first and leave me with a saturated solution? Thanks!

[Arkcon]

Freezing point and melting point are exactly the same.

Why not use coligative properties to calculate the freezing point for some examples of salt concentrations.  The you'll know what to do, or what not to do.  Try going right the the very edge, and  see if the solution still freezes, then you'll know the process is non-ideal.

[Nora Goba]

I can't use coligative properties since all of my salts have the same molality. This is why this is a hard problem since I'm observing the differences within a single coligative condition of 2 ions with variations in only EN, size, charge density and so on. If you know how any of these conditions affect freezing point depression I would be grateful. 

[Borek]

Google for Debye-Huckle theory, ion association.

[Corribus]

Freezing point and melting point are exactly the same.

In the theoretical sense, this may be true. But practically speaking, this isn't usually the case. There's usually a hysteresis in the freezing point that is caused by, e.g., nucleation kinetics. The end result is that the apparent (measured) freezing point can be quite a bit different (lower) than the melting point. The degree of hysteresis can be affected by things such as the cooling rate.

This is why for sample characterization, the melting point is typically the property of interest.

[Nora Goba]

Is there supposed to be a straight horizontal line at the temperature of melting on the temp/time graph or should the slope just get smaller? I know in theory its supposed to be horizontal since there is the enthalpy of melting.

[Corribus]

What method are you using to determine melting point? DSC?

[Nora Goba]

A primitive design where I just let the ice gradually heat up through room temperature and observe the temperature with a probe in the core of the ice sample and put it on a graph with time. IN theory there should be a gradual increase in the core temperature until it reaches the melting point and then the temperature rise should slow down until the ice melts and then proceed to rise.