Phase Change Diagram: Solved & Explained

[Kater]

[SOLVED] Phase change diagram

Now that I think about it, this might be more of a chemistry question, but it's for the class "General Physics 1" so go figure :)

Homework Statement
A container holds 0.550 kg of ice at -15.0 *C. The mass of the container can be ignored. Heat is supplied to the container at the constant rate of 800.0 J/min for 500.0 min.

After how many minutes does the ice start to melt? (Already solved, 20.7 min)
After how many minutes, from the time when the heating is first started, does the temperature begin to rise above 0.00 *C? (Already solved, 251 min)

Plot a curve showing the temperature as a function of the time elapsed. (HALP ME)

The attempt at a solution
Well this part has stumped me, as I could only figure out how to get the first three points, not sure how to find a fourth. I went to my prof to ask, and he told me that it was not possible to find a fourth. Happily, I went back home to submit the 3 points on my online homework. Homework says I'm wrong. This either means my professor is wrong, or my 3 points are wrong. My points are the initial point (0,-15), the melting point (21,0), and the rise-above-0 point (251,0).

This is what I have, which is wrong. (remove the spaces, I'm not spam-advertising I swear lol)
ht tp://i85.photobucket.com/albums/k80/katie9492804/1434. gif


My book doesn't seem to have anything about the diagrams except some unhelpful equations that do not include time.

Help/hints appreciated! The homework is due tonight at 10pm CST. I know, pretty last minute, but I thought my prof had told me exactly how to solve it so that's why I waited. This is my last problem of my last assignment of the year, I want this over with! :)

 

[tiny-tim]

Welcome to PF!

Hi Kater! Welcome to PF!

Chemistry's not really my thing, but it seems to me the graph needs to slant up again after 251 minutes … can't you work out the temperature rise in the same way as for the ice?

If that doesn't help, I suggest you do repost this in Other Sciences.

 

[Kater]

Thanks for the welcome! :)

Yeah it should definitely slant up, I'm just not sure where to put the point. When I use the same method to find a random point, say an increase of 10 degrees, I get it should be at 265min, which is also wrong. I'm wondering if the problem is that I put one point at 21, when it should really be 20.7 and it only let's me put whole numbers.

I think I'm going to click the "show answer" button just so that I can submit my parts a) and b) to get credit for those. I'll ask the prof for clarification on Monday and hopefully he can explain again, as well as hopefully manually give me credit since he told me wrong info, hehe. :)

I guess I should label this "solved"?

 

[tiny-tim]

Hi Kater!

I'm about 6 hours ahead of you here in London, so I had to go to sleep and miss your 10pm deadline. :zzz:

In future, feel free to show us your working, and maybe someone will see where you went wrong.