[Noobert]

We did a lab in chemistry this week and we used a different substance than we would have any other year (lauric acid). We were trying to find the freezing / melting point. But our data did not show much of a plataeu when graphing.  Lauric acid was not the best substance to use.  Why is it that Lauricacid does not show its freezing/melting point like other substances would have?  Does it go through these phases so fast that it can be seen in the 30 seceond intervals we recorded? I need to know this so I can explain why it did not really work in my conclusion. If you need more information about the lab and do not understand any thing I just wrote, here is my graph and procedures:


      Cool Heat

Objective Find the Freezing and melting point of Lauricacid.

Procedures:
Part 1
1.   Heat up water to about 70 degrees Celcius.
2.   Put testtube with Lauricacid in hot water.
3.   When testtube temperature reaches about 60 + degrees Celcius put the test tube in cold water.
4.   Take temperature every 30 seconds.
5.   When testtube temperature and water temperature are within 5 degrees of each other. Stop.
Part 2
1.   Heal water to 65 degrees Celcius.
2.   Record temperature every 30 seconds.
3.   Keep temperature at 65 degrees Celcius in hot water.
4.   Record until test tube temperature gets to 60 degrees celcius

here is the link to my chart:
(you can see that there is not a clear plateaufor the freezing pint)
http://www.chartgo.com/create.do?chart=line&dimension=2d&width=800&height=800&transparency=1&labels=1&legend=1&title=Cool+Heat+Results&xtitle=Time+%28minutes%29&ytitle=Degrees+Celcius&chrtbkgndcolor=gradientblue&roundedge=1&shadow=1&xaxis1=.5%0D%0A1.0%0D%0A1.5%0D%0A2.0%0D%0A2.5%0D%0A3.0%0D%0A3.5%0D%0A4.0%0D%0A4.5%0D%0A5.0%0D%0A5.5%0D%0A6.0%0D%0A6.5%0D%0A7.0%0D%0A7.5%0D%0A8.0%0D%0A8.5%0D%0A9.0%0D%0A9.5%0D%0A10.0%0D%0A10.5%0D%0A11.0%0D%0A11.5%0D%0A12.0%0D%0A12.5%0D%0A13.0%0D%0A13.5%0D%0A&yaxis1=60%0D%0A58%0D%0A52%0D%0A48%0D%0A46%0D%0A45%0D%0A44%0D%0A43%0D%0A43%0D%0A43%0D%0A42%0D%0A42%0D%0A42%0D%0A41%0D%0A41%0D%0A40%0D%0A40%0D%0A39%0D%0A38%0D%0A37%0D%0A36%0D%0A35%0D%0A34%0D%0A33%0D%0A31%0D%0A30%0D%0A29%0D%0A&group1=Part+1+Results&yaxis2=29%0D%0A32%0D%0A36%0D%0A40%0D%0A43%0D%0A45%0D%0A47%0D%0A49%0D%0A51%0D%0A52%0D%0A54%0D%0A58%0D%0A61&group2=Part+2+Results&&&from=generaljsp

[renge ishyo]

Well, all I can say is the literature melting point of Lauric Acid is 44⁰C which is very close to the intersection of your two lines...

[Noobert]

Thanks but I need to find is the melting point and freezing point. Which I believe is shown where each line "plateaus" on each line? I do not think that has anything to do with where they intersect.  Correct me if i am wrong..

We used a different substance last year that clearly showed the plateau in both lines... Why is it that lauric acid does not show any straightening or plateauing? Is it becuase it freezes much faster than it melts? I beieve I can see the freezing point because it stays at 43 for 3 times in a row. As far as the melting point goes I can not find it. Why is this?

Thanks
btw Great forum! Keep up the good work. I have been looking for a site like this in case I run into trouble on labs etc.

[renge ishyo]

The freezing point and melting point occur at the same temperature. You simply call this temperature the "freezing point" if you are going from a liquid to a solid (by lowering the temperature of the system) as you pass it, and you call this temperature the "melting point" if you are going from solid to a liquid (by raising the temperature of the system) as you pass it. So the point where the two curves intersect should be the answer, and it is.

My guess is that the experiment was designed this way on purpose because the plateau for lauric acid may not be as easy to discern (given your experimental setup) as it was in your previous experiments (as you say the change in phase might be too quick to notice clearly based on your experimental setup). Nevertheless, you do see a gradual flattening of the freezing point curve starting at about 43 right? Even if you did not have the second curve to help narrow things down you can still sort of see that the point of interest lies roughly near 43 from the first curve data alone. The second heating curve just helps you locate things more precisely.

[renge ishyo]

I should add that I think your experiment was rigged so that the intersection point would be close to the melting point. In general you can set up situations where the two curves would not intersect at the this point (by varying the quantity of water in the experiment for example). Just wanted to make that clear. It's hard to tell what the experiment was aiming for without seeing the accompanying text with it. it could be that the lab was meant to demonstrate Newton's Laws of heating and cooling for example with the melting point merely used as a marker.

[Noobert]

Actually now I am  not sure what we were supposed to do. We were suposed to find where it plateaud in both lines. This meant somthing. It only plateaud in the one line. Can anyone tell me what that is supposed to mean?

[renge ishyo]

I believe it was "supposed" to plateau at the same temp on both curves. My attempt at rationalizing the experiment aside, I think you just heated the sample too fast for the second curve.

[Noobert]

Ok thanks. I will reply back today when I figure out what the experiment was actually supposed to be.

[typhoon2028]

Did you use the same sample for both Part 1 & Part 2?

In part one the sample was melted completely than cooled.  Heat transfer should be better for a solid to liquid heat transfer.  Solid = glass test tube cooling liquid sample.

We do not know the appearance of the sample in Part 2.  Is it a powder?  Is it one solid mass?  Is it chunks?  All this affects heat transfer and may skew how the data looks.

Cooling is probably a better method for determining freezing/melting points, because it should be a slower process (in this case).

Was the water being mixed constantly? 

[Noobert]

Did you use the same sample for both Part 1 & Part 2?
Yes. Part 1 led right into part 2.

Was the water being mixed constantly? 
The hot water  bath was kept at the same temperature. It was not mixed.

Thanks.

[Noobert]

THe cool water was not mixed... I think that was what you were talking about correct?

Also at the time of melting the substance was a powder.