Melting & Boiling: Heat Energy Explained

In summary, as heat energy is added to a substance like ice or water, most of it is used to weaken the intermolecular bonds rather than increase kinetic energy. This leads to an increase in potential energy as the particles drift apart. When the potential energy becomes too high, the intermolecular forces are overcome and the substance changes phases, resulting in a flat point on the temperature graph. This is due to the resistance of the intermolecular forces, which require a significant amount of heat energy to be overcome. At the boiling point, enough heat energy is added to allow large numbers of particles to escape from the liquid, resulting in the substance changing to a gas.
  • #1
Cheman
235
1
Boiling/ Melting...

I take it everyone knows the graph of temperature when you heat ice / water - with the flat bits when it melts and boils. My question is why is this the case? Obviously the heat energy you are putting in is going to kinetic energy of the particles when the graph is not flat, but when it is flat and the temperature is not changing what is the heat energy being converted into?
Thanks in advance. :wink:
 
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  • #2
The extra energy goes into overcoming the various forces that hold a solid (or liquid) together, that is, it contributes to the molecules' potential energy rather than its kinetic energy.

Claude.
 
  • #3
there is boiling point which the liquid will not go any higher then that temperture, extra heat energy put in only keeps at that level. other heat energy is just wasted.

if that heat energy keeps applying i guess the liquid will vaporize.

(note the process do not include any extra pressure to the liquid, so that the boiling point will not vary)
 
  • #4
Cheman said:
Obviously the heat energy you are putting in is going to kinetic energy of the particles when the graph is not flat...
Just to add to what Claude already explained: Realize that even when water is being heated, most of the energy goes into weakening the intermolecular bonds, not into increasing KE.
 
  • #5
re

You refer to the phase diagram or the T-S (temp - entropy) chart?

Either way, what is really happening is you put energy into the system, in your case liquid water. The energy is not absorbed unevenly, so you must raise the entropy of the entire system a certain degree before you can begin to change phases, i.e. from liquid to gas. This is commonly called the heat of transformation,

[tex]\triangle S=\triangle Q /T_{abs} [/tex]

Where S is the change in entropy (note you cannot calculate S, only a change in S), dQ is the heat added to the system, and Tabs is the absolute temp (in deg. R).

Entropy was really invented to describe the second law of thermodynamics, and it really tells you which direction a phase chance will proceed at a given set of parameters (will it go back to water, or continue to change to gas?).

Hope this helps.
 
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  • #6
I was referring to the phase diagram.

The one thing that bugs me is when you are heating the substance (in my example ice/ water), when you reach the changes in state is it that with heat energy you are adding no longer is used to increase kinetic but potential energy OR that the particles begain to gain potential energy as they drift apart (ie - ice to water), and the kinetic energy being lost is replenished by the heat energy you are adding? (thus flat points on graph)

Also, can anyone please explain why the potential energy of the particles increases at these points rather than kinetic? I've tried, but i find its rather hard to explain :wink:
Thanks.
 
  • #7
Electric attraction between hydrogen atoms in one water molecule and oxygen atoms in other water molecules have to be overcome if individual water molecules are to leave the surface of the liquid and become vapour.As more heat is added to the liquid water, the average distance between hydrogen atoms of one molecule and oxygen atoms of another molecule increases (and this amounts to an increase in potential energy) but there is resistance to this happening
(think of stretching a spring - it gets more difficult to do the more you stretch it)
and a lot of heat energy is needed.There comes a point at which the spring snaps -
the hydrogen-oxygen attraction between different molecules is overcome - and large numbers of water molecules can escape from the liquid.This is when the liquid boils.
 
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What is the difference between melting and boiling?

Melting is the process of changing a substance from a solid to a liquid state, while boiling is the process of changing a substance from a liquid to a gas state. Both processes involve the absorption of heat energy.

What causes a substance to melt or boil?

A substance will melt or boil when enough heat energy is applied to overcome the intermolecular forces between its particles. As the particles gain energy, they vibrate faster and eventually break apart, causing the substance to change states.

What factors affect the melting and boiling points of a substance?

The melting and boiling points of a substance are affected by the strength of its intermolecular forces, the nature of the substance's molecules, and the atmospheric pressure. Generally, substances with stronger intermolecular forces have higher melting and boiling points.

Can a substance skip the melting phase and go straight to boiling?

Yes, some substances have a melting point that is higher than the boiling point at standard atmospheric pressure. This means that they will go straight from a solid to a gas state without passing through a liquid phase. This process is called sublimation.

How does heat energy play a role in the melting and boiling process?

Heat energy is necessary for the melting and boiling process to occur. As heat is absorbed by a substance, the intermolecular forces between its particles weaken, allowing the substance to change states. The amount of heat energy required for melting and boiling varies for each substance and is known as its specific heat of fusion and specific heat of vaporization, respectively.

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