I'm having trouble with a question on my Science assignment...

Question

The question is:

"A substance freezes at -58 degrees celsius. Therefore, the substance melts at ______."

I've looked all over my book but have not found a clear answer. Any help and explanation is appreciated!

Answer 1

  -57 degrees celsius?

Answer 2

the melting point of a pure substance is the same as the freezing point

so the answer is -58 C

think about water

it melts at 0 C (32 F) and of course it freezes at the same temperature

if you think about it you will see that it makes sense that the freezing point and the melting point are the same

Answer 3

The substance freezes at -58 degrees celsius, so that it melts at -58 degrees celsius.

For example, water freezes at 0 degree celsius and it is also melts at 0 degrees celsius.

If the substance is special that do not melts at -58 degrees celsius, pls state what the substance is.

Answer 4

The typical values for MP and FP are usually obtained from "pure" substances.  And, typically, the time allowed for the phase change is not the thermodynamically "required" "infinite" time.  That is, to a greater or lesser extent, the phase change temperature is "rushed" along.  So, wouldn't it be nice if the temperature was approached from both above it, and below it?  That way if the two measurements matched we would be relatively certain that we got the "right" temperature.  (Again assuming ultra purity <= which is not very common, actually).  The expectation for equality of FP and MP gets weaker as we move from crystalline to amorphous solids and from low molecular weight to polymeric (hi MW) compounds.  This is because small crystalline molecules can be "neatly" and easily packed while bulky amorphous molecules have to move around a litle bit to settle into the lowest energy position - which takes time.  And since we only allow a certain amount of time to change phase, the freezing especially may be delayed giving artificially low temperature results.  Melting tends to be more clear cut, unless the molecules are tangled (polymers) up.

It has been calculated (and I believe experimentally confirmed) that water actually will spontaneously freeze at -14°C if ultrapure and without foreign surfaces (dust or just the container walls).  But for everyday purposes saying that the MP/FP of water is -14 C would be useless because the special conditions required are not even remotely practical.

Anyway, to answer your question, without more information, we'd assume MP=FP.  This is much more likely to be true for a substance melting at -58 C than at +58C or +258C. (you figure out why)

Answer 5

The melting and freezing points are actually at the same inflection point on the phase diagram.  So, if it freezes (becomes a solid) at -58 degrees, then it melts (becomes a liquid) at the same point.  It really just depends on if you are cooling it or heating it.  -58 is the temp at which the phase changes

Answer 6

for most substances freezing and melting points are essentially the same

Answer 7

I would say they are trying to get you thinking like with water. Water freezes at 0, water melts at 0.

So I would guess -58