Posted by **Jon** on Thursday, April 3, 2008 at 5:14pm.

The melting point of a solid is 90.0 d/C. What is the heat required to change 2.5 kg of this solid at 40.0 d/C to a liquid? The specific heat of the solid is 390 J/kg-K and its heat of fusion is 4000 J/kg.

I got 2 different answers 2 different ways:

2.5(50)(4000) which is 5.0 x 10^5 J or, 2.5(4000) which is 1.0 x 10^4 J

- physics -
**DrBob222**, Thursday, April 3, 2008 at 5:23pm
This is two problems rolled into one.

q1 = heat required to raise T of solid from 40.0 C to 90.0 C.

q1 = mass x specific heat x delta T.

q2 = heat required to melt the solid once the T has reached 90.0 C.

q2 = mass x heat of fusion.

Total heat = q1 + q2.

- physics -
**Jon**, Thursday, April 3, 2008 at 6:45pm
I read what you said but could it be this also?

390*2.5*50

specific heat x mass x change in temperature.

answer: 4.9 x 10^4 J

- physics -
**DrBob222**, Thursday, April 3, 2008 at 7:39pm
No, it can't be that.

390*2.5*50 is q1. That's the amount of heat necessary to raise the temperature of the solid from 40 C to its melting point of 90 C. But the total heat is that amount PLUS the amount necessary to melt the solid. That is q2 which is 2.5 kg x 4000 J/kg.

Therefore, the total is

(390*2.5*50) + (2.5*4000) = ??

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