# does it take energy to change the phase (not the temperature) of water?March 11, 2006 7:27 PM   Subscribe

Help me settle a physical sciences argument about boiling water.

Which is correct?

Position A:
It takes X amount of energy to raise a certain mass of water's temperature by Y degrees. If you have a gram of water and want to raise it from 97° to 98° C, you apply one calorie, for example.

Similarly, if you want to raise the temperature of liquid water from 99° to 100° C AND INDUCE A PHASE CHANGE from liquid to gas, you simply apply that same 1 calorie of heat energy.

Water is liquid at 99° C and gaseous at 100° C. Whether it appears in liquid or gaseous form is simply a function of its temperature.

Position B:
Water can exist in liquid or gaseous form at 100° C. When water boils, the liquid on the bottom of the pan is 100° C, and the steam bubbles rising out of it are also 100° C.

If you want to boil water, you need to apply heat energy until the water reaches 100° C, then you need to apply a certain amount of additional energy SIMPLY TO EFFECT THE PHASE CHANGE from liquid to gas.

This energy takes water that is 100° C and makes it into steam that is also 100° C. After that point, applying heat raises the temperature of the steam from 100° C upward.

But it takes energy to accomplish the phase change itself, energy which does not necessarily raise the temperature of the H20.

In essence: does it take energy to make water change phase? Or is phase simply a symptom of the water's temperature?
posted by scarabic to Science & Nature (21 answers total)

Position B is correct. I forget the name of that extra energy, but it indeed has a special name.
posted by ikkyu2 at 7:29 PM on March 11, 2006

B is correct.
posted by knave at 7:29 PM on March 11, 2006

Wow, I love Wikipedia. It's called Heat of vaporization. The article on boiling point is good too.
posted by ikkyu2 at 7:31 PM on March 11, 2006

Yep, B. There is energy absorbed which does not result in a change of temp.: this is called (I think) latent heat.
posted by pullayup at 7:32 PM on March 11, 2006

Right, that is, latent heat of vaporization.
posted by pullayup at 7:33 PM on March 11, 2006

Yup, position B. This is a thermodynamics problem. There's a huge difference between the internal energy of liquid water at 100C and steam at 100C (It's like an order of 10, if I remember correctly).
posted by onalark at 7:35 PM on March 11, 2006

Interestingly enough, steam burns can be profoundly bad because of that extra heat being released when it condenses back to water.
posted by 517 at 7:36 PM on March 11, 2006

Position B is more correct. While you are correct in that it takes one calorie to increase the temperature of liquid water by one degree centigrade it is irrelevant once you reach the boiling point. A good amount of energy must be added for the phase change to completely occur.

Say you have water in a closed container. You apply a constant amount of energy from a heating source for instance. Once the water hits 100 degrees it will stay at that temperature until it is all converted into vapor. Then if more energy is applied the vapor is heated above 100 degrees. The total energy it takes to complete the change is...i hope i get this right...the heat of formation.

Of course this is all taking place at 1 atmosphere of pressure, the pressure changes the boiling point, higher pressure makes it harder for the water to boil into the air while lower pressure makes it easier. This is why if you were boiling water in Denver it would be at a much lower temp. than at sea level.

I really hope I'm right on this, finishing an engineering degree this semester. If I didn't it's civil engineering, don't shoot! Thermodynamics was a while ago.
posted by crashlanding at 7:37 PM on March 11, 2006

Dangit, i forgot to preview, people are right about the heat of vaporization. Heat of formation is the phase change from liquid to solid.
posted by crashlanding at 7:38 PM on March 11, 2006

Heat of formation is the amount of energy it takes to create H2O out of atomic H and O. For the finished product, it represents energy stored in covalent bonds.
posted by Maxwell_Smart at 8:19 PM on March 11, 2006

And, the heat of vaporization represents the energetic cost of moving from a state where transient hydrogen bonds and van der waals interactions are readily formed, to a state where they are not.
posted by Maxwell_Smart at 8:21 PM on March 11, 2006

Response by poster: I win.

Strangley, this came up between me and my brother after I claimed that hydroelectric power was basically, at its root, solar-powered. He didn't follow my statement at first, and I explained that it takes the sun to keep lots of moisture in the atmosphere, and highland precipitation only occurs (and feeds tributaries that lead to dams) when wind drives that moisture colder climes where the ground elevation is higher (the wind, again, powered by the sun and differential temperatures).

He was of the mind that the amount of moisture in the air was simply an equilibirum type of situation, as with any surgace of liquid where some vapor escapes from the surface. And then we got into a protracted thing about whether it takes energy to actually change phase. This took place on an airplane. We probably annoyed the fuck out of the people next to us :)

It's a minor point and the larger conversation has a lot more nuances. I think I just took basic chemistry a lot more recently than him. He actually has a science degree, which I do not.
posted by scarabic at 9:04 PM on March 11, 2006

Water is liquid at 99° C and gaseous at 100° C

Water can also be gaseous at 0° C. At temperatures we are used to, water is constantly transforming between solid and gas phases by sublimation without any intervening liquid stage.

Boiling is our description of the physical process that occurs when the vapour pressure of the water equals the vapour pressure of the surrounding atmosphere. In this case, the gas pockets formed within a body of water will tend not be compressed back to zero diameter as the gases redissolve within the water liquid but instead will tend to rise due to their lighter density within a gravitational field and escape the surface of the liquid water. This is also why "boiling" points for water vary as you change altitude (which changes atmospheric pressure). Water "boils" at around 70° C on the highest mountain peaks. Go high enough or toa low enough atmosphere and the "boiling point" goes below zero, at which time it becomes impossible for liquid water to exist - it will all just sublimate away from solid to gas. That is the unfortunate state of the Martian atmosphere today.

"Temperature" is our description of an average kinetic energy of a bulk of matter. Temperature actually describes a probabilistic distribution of energies, the shape and extent of which changes depending on the Near the surface of the liquid water, some molecules of water have "temperatures" far in excess of the boiling point, while others have temperatures below this.

Only those water molecules with energies far in excess of that required to break free of the various intermolecular bonds between the water molecules will be able to leave as gas molecules. Molecules not possessing sufficient energy tend to kind of jiggle around and don't really go anyplace. Because this process is ongoing, the net effect is that the "temperature of the water remains macroscopically constant because the vast, vast majority of molecules left behind have energies at or below the critical level needed for release. The water bulk is also acting to redistribute the enegies continuously, resulting in a similar temeprature distribution during the phase change, as long as energy is being constantly transferred to the water to replace the kinetic energy carried away by the escaping water molecules.

A similar gthermodynamic redistribution process happens during the conversion of solid water to liquid or gas, ensuring the appearance of stable temperatures.

What's more interesting is the phenomenon of boiling point elevation, where the addition of an unreactive solute has the effect of raising the boiling point of water. What's interesting is that the amount of raise is in pretty direct proportion to the concentration of the solute added (assuming a non-dissociate, non-elctrolytic substance -- otherwise you have to get into molarities).

Most high school chem classes explain this in terms of fibs about solute particles somehow retarding the water molecules from leaving. Like they have little hooks on them or something. The real answer is that boiling is also entropically driven, and involes a large increase in entropy as a relatively well-ordered polar liquid transforms into a very disorganised gas. Adding solute just buggers this whole process up.

Water is a great little molecule - there's still a lot of stuff up in the air.
posted by meehawl at 9:05 PM on March 11, 2006

Response by poster: Yes, good call, meehawl, as I implied in my hydroelectrics context, this is all at 1 atmosphere. Sorry I didn't clarify that. Was trying to be all scientific, too! Stupid English Major! ;P
posted by scarabic at 9:15 PM on March 11, 2006

scarabic, if you want to prove this to yourself, it's easy to do. You need a pot and a thermometer.

Put a pot of water on the stove. Turn the stove on high. Watch the thermometer. Let's say it takes 75 seconds to go from 25C - room temp - to 100C. So 1 degree per second. So, in the next one second, the burner should apply enough heat to heat the water to 101C, it will all flash to steam instantly, and the pot will be empty. Doesn't happen? Sits at 100C for a long time while energy is still being applied at the same rate? (Don't touch the burner setting.) Why? Because the heat of vaporization is quite high compared to the energy required to just raise the temperature of water.

The energy necessary to change one gram of water at 100C from liquid to gas is equal to the energy required to change the temperature of that water by 543 degrees. Or, if it took 75 seconds to raise that water from 25C to 100C, you can expect it to take about 543 more seconds to boil all that water away.
posted by jellicle at 9:56 PM on March 11, 2006

You can set up quite a dramatic demo of meehawl's point about the effect of pressure on boiling point as follows:

Get a glass jam jar with a tight-sealing metal screwtop lid. Put on a decent pair of gardening gloves in case of unexpected shatterage. Fill the jar about two-thirds full of boiling hot water, then carefully screw down the lid.

You can now make the water inside the jar boil just by blowing on the lid, or make it boil absolutely furiously by running a thin stream of cold water over the lid.

You can keep this going even when the jar has cooled quite substantially. I don't think your brother will maintain his faith in position A in the face of this evidence.
posted by flabdablet at 10:45 PM on March 11, 2006

IT's not just water that has this property, everything does to a greater or lessor degree. People are experimenting with storing heat from solar collectors in phase changes of material. Water is bad for this because liquid water is denser than solid and it's melting point is so low (relative to solar collector and room temperatures). There has been some success with waxes that melt around 20-25 degrees.
posted by Mitheral at 12:22 AM on March 12, 2006

Here are a couple of real world examples, that might help convince him:

Sweating cools you down because the evaporation process pulls heat from your body. It doesn't work as well when the air is humid, because it can't evaporate as quickly.

Clouds of water vapor turning into snow will put heat into the atmosphere, which is why the temperature usually rises a few degrees when it is snowing.
posted by teg at 1:51 AM on March 12, 2006

posted by weapons-grade pandemonium at 9:50 AM on March 12, 2006

Strangley, this came up between me and my brother after I claimed that hydroelectric power was basically, at its root, solar-powered.

Most of our energy sources are solar-derived. The Sun is the original input for oil and coal (they are derived from plants an animals, which built up the chemical bonds with photosynthesis as the root form of energy). As animals, we eat either plants, or animals that ate plants, so our internal energy is ultimately solar, to a big degree.

Fusion, fission, and geothermal are not derived from solar, but they make up a small part of our energy use (of course, fission and geothermal are impossible were the compounds they rely upon not once in the core of a star -- so stars really are the energy source in this universe, and they do fusion).
posted by teece at 9:54 AM on March 12, 2006

Response by poster: I think fossil fuels also owe a lot pressure forces in the ground, but yeah, you're right about the solar input. However, a fossil fuel economy is not sustained or sustainable by the sun, unlike hydroelectric or wind, which are directly driven by the sun today. It's kind of a semantic point though.

Thanks for all the answers, yo.
posted by scarabic at 11:12 AM on March 12, 2006

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