Heat of fusion and vaporization

If attracting bodies collide and stick together, energy must be given off, and normally it would be heat. To stick, they must attract, but there also could be a repulsive force. In the case of both, the collision could give off energy or absorb energy, depending on which effect is greater. Nuclear fusion is an example of this. The H bomb and the Sun give off energy, and in a supernova, the fusion of the big nuclei absorb energy. Your body, like the universe as a whole, contains a lot of hydrogen atoms; most of the other atoms have been in a supernova.

Atoms attract each other. The largest attraction is in a solid, less in a liquid and almost not at all in a gas. Therefore if a gas condenses to a liquid or a liquid solidifies, heat energy is given off by the material. Conversely if a solid melts or a liquid evaporates or boils, heat must be added to it or it will get colder. The cooling effect is obvious when you wet your hand with a volatile liquid. Evaporation of sweat is your body's cooling system. Another example: put salt on ice which is at 0^o C and it will get colder because salt causes melting of some of the ice.

Here is something you don't need to know about vaporization, so skip this paragraph. The paragraph above is correct as far as it goes, but I should mention that most of the energy needed to vaporize something is the energy required for the expansion to take place. A lot of work ( = PD V) must be done against the atmosphere while turning a liquid into a vapor. Another related thing: If a gas is dissolved in a liquid and it is released in gaseous form, the same reasoning says that this requires energy. I have not noticed my beer getting colder when I open it, but it must. A student told me that he had some beer outdoors on a cold day, and when he opened it, it suddenly froze.

The amount of heat energy involved is proportional to the number of atoms in the phase change, which is proportional to the mass that changes. So we write Q = Lm, where Q is the quantity of heat energy and L is a constant called the latent heat. The L is either L_f or L_v for fusion or vaporization. (To fuse used to mean to melt. Thus the fuse in an electrical circuit melts to cut off the current if it is too large. I think that because melting provides a way of making things stick together, the meaning evolved into putting together. In nuclear fusion, for example the nuclei stick together. The Sun's energy is from fusion of hydrogen into helium, so shouldn't the Sun's heat be called heat of fusion?)

H₂O: L_f = 80 calories/gram and L_v = 540 cal/g. Oddly, water becomes less dense when it freezes. This is a good thing for the fish in lakes that freeze over.

Say you put a 0^o C. ice cube into a styrofoam cup containing 100 grams of water at 20^o C, then cover it up. After the ice melted, the final temperature was12^o. What was the mass of the ice cube? Assume negligible heat exchange between the cup and the environment and between water and cup. You could fall into a common trap and calculate 10 grams, but the correct answer is 8.7 grams.

Hit this if you want to mosey back to the main page on fluids, heat, and electricity.

More about heat energy is in thermodynamics.

Other main physics pages:

mechanics
vibrations and waves
quantum

Comments, questions: fredrick gram