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**[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! :)