Phases of matter

Can u help me with these two questions? I'm not really sure how to go about them properly.

State and explain whether the internal energy of the following increases, decreases or doesn't change.

i) Solid gold is melting to its liquid form at the same temperature

ii) Water, to which ice has been added

Reply 1

**mentalheroin**

9

andrewlee89

Can u help me with these two questions? I'm not really sure how to go about them properly.

State and explain whether the internal energy of the following increases, decreases or doesn't change.

i) Solid gold is melting to its liquid form at the same temperature

ii) Water, to which ice has been added

for part 1, the internal energy, which is just the measure of the sum of the average kinetic and potential energies of its molecules, remains constant because the temp, ie the avg kinetic energy remains constant.

for the next one, i think it would go down, coz ice has a lower internal energy then the water so when you add them it averages to asmaller value.

Reply 2

teachercol

9

a)The potential energy will change as the gold melts because bonds break. You have to put energy in to melt the gold so the liquid gold has more internal energy than the solid gold

(thats why steam at 100C burns more than water at 100C)

b)If you arent putting any energy in or doing any work then the internal energy stays the same.

Reply 3

F1 fanatic

16

teachercol

a)The potential energy will change as the gold melts because bonds break. You have to put energy in to melt the gold so the liquid gold has more internal energy than the solid gold

(thats why steam at 100C burns more than water at 100C)

b)If you arent putting any energy in or doing any work then the internal energy stays the same.

surely for b) it depends what you are defining to be the system?

Reply 4

InAbsentia

Indeed. A definite answer cannot be given until the OP clarifies the question.

If the system is defined as being only the water, then energy will be thermodynamically lost to the ice as the wider system strives for thermal equilibrium.

If the system includes the ice, then no energy is lost or gained, and all energy transfer takes place within the system.

Reply 5

teachercol

9

True

Clearly energy can flow either way to/from the surroundings depending on the external temperature

Reply 6

Kyle_S-C

12

ii) is asking about the internal energy of the water, to which ice has been added. Hence the internal energy goes down, as the water has lost energy to the ice assuming that the ice doesn't melt (since the ice would then become part of the water).

Reply 7

**mentalheroin**

9

If you add ice to water, surely you are adding energy to the system because the ice has an internal energy. And when you add the ice, the new system has a lower internal energy because the average energy per molecule in the new system, ie the water and ice combined, is smaller than it was in the water and greater than it was in the ice.

Reply 8

F1 fanatic

16

Ok, so the cases are these, depending on how you define your system:

1)If the water is defined as the system, throughout the problem, then the internal energy of the water will go down, as heat will be lost from the hot body (water) to cold surroundings (ice).

2) If the water and ice are communally always considered the system then the energy does nothing, since

initial water energy + initial ice energy = final water energy + final ice energy.

In other words it is a closed system, assuming no interaction with the surroundings.

3) If the water is the initial system, and the ice & water the final system, then the internal energy will go up as you are adding to what you define to be the system.

Of these I favour taking case 1) in this simple(ish) problem.

Phases of matter

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