# Why is steam hotter than boiling water?

1. Feb 4, 2009

### toqp

I know I should get this, but I don't... So, I've been told that whenever I put some water into a kettle and then heat it to the boiling point, the steam coming from the kettle is hotter than the boiling water. Why is that?

And why is the temperature graph usually depicted as it is at:
http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/phase.html

In that graph the temperature of the steam starts of at the boiling point 100 *C (I hope you can cope with *C, I'm lousy with *F). But if the steam is hotter than 100 *C, then there should be a gap in the graph, shouldn't there? The graph states that heating the steam continues from 100 *C. But how can we heat steam from 100 *C if it's hotter than that?

The temperature of the water doesn't rise above the boiling point. I get that. Wikipedia: "Temperature can be defined as the average energy in each degree of freedom in the particles in a system." Therefore, when the water at 100 *C, it can't receiver neither translational, vibrational or rotational kinetic energy, if it's to remain at 100 *C.

However, Hyperphysics groups vibrational and rotational kinetic energy with potential energy. And according to the same picture, rotational and vibrational energy are negative energies, as they are grouped together with negative potential energy. So what does this mean?

If, in heating the water, the total negative sum of potential, rotational and vibrational energy increases and approaches zero, but the translational kinetic energy doesn't change (in the picture the height of the translational energy bar remains the same), then how come doesn't temperature or the total kinetic energy change?

Besides... what even happens when a body of water uniformly heated to 100 *C is being heated more?

There's negative potential energy and positive translational kinetic energy? So the potential energy approaches zero, and when it reaches zero, the water molecule is freed and leaves the boiling water and turns into steam. But, if the translational kinetic energy isn't changed when the water is warmed, then how can steam be hotter than the boiling water? They have the same kinetic energy, but steam doesn't have the potential energy of liquid because the heat lifted it up from the potential well.

(And folks not even studying physics seem to get this better than me...)

2. Feb 4, 2009

### stewartcs

It isn't hotter. The point at which the saturated water changes to saturated vapor is dependent on the pressure. Since it is a kettle on your stove the pressure is atmospheric. The water transitions to saturated vapor (steam) at 100 C, so unless the pressure changes, the steam will also be 100 C. If the pressure is increased, you can get superheated vapor (i.e. superheated steam) that is greater than 100 C. However, this will not happen at atmospheric pressure.

http://engr.bd.psu.edu/davej/classes/thermo/chapter2.html

CS

3. Feb 4, 2009

### minger

The most simplest explanation is to study that graph you provided. Temperature is kind of a measure of internal energy. As the internal energy of matter increases to does it temperature....normally.

In thermodynamics we can loosely relate the change of energy in a mass to its temperature by:
$$\Delta H = mc\Delta T$$
Where m = mass, and c = specific heat. It basically says that if we add heat to a mass, the temperature increases linearly as a function of its own mass, and material properties (wood needs more heat to increase its temperature than aluminum does).

However, there are plateaus in the graph you provided. That's because nature is a *****, and if we want to change phases, then theres a cost. So, if I want to convert water into steam, there are two steps. I must first add heat to get the water to 100°C. Once there, I need to add more heat to convert the liquid from 100°C water to 100°C steam. This is called heat of vaporization. Likewise, if I want to go from steam to water, there needs to be a mass that can absorb that heat.

4. Feb 4, 2009

### toqp

What still puzzles me is the way those different kinetic energies build up the statistical phenomenom of temperature. I mean why are rotational and vibrational energies grouped together with negative potential energy.

Ok. In that case, wouldn't the water too be at a greater temperature, because it wouldn't start boiling at 100 *C? So, even in this case the superheated vapour wouldn't be hotter than water, because the water would be superheated also?

Well that's what I thought, but as stated in the original post, according to some people the steam is supposed to be hotter than 100 *C when it leaves the kettle.

But according to you and stewartcs, the steam is actually at 100 *C, and therefore it is not hotter than the boiling water.

5. Feb 4, 2009

### turbo

To the OP, look at the first graph in your link. Compare the heat that is added to water (100 cal/g) to get it to 100 deg C, to the heat that is added (540 cal/g) to turn the water into steam at the same temperature. The steam is no hotter than the water but it contains more usable heat energy per gram, and it can release that heat as it encounters a cooler medium and makes the phase-change back to water.

6. Feb 4, 2009

### toqp

This I can understand. I wonder if this is what someone means when he tells that steam is hotter.

Last edited: Feb 4, 2009
7. Feb 4, 2009

### turbo

Exactly! The steam can be at exactly the same temperature as the water, yet contain 540 cal/g more usable heat. Saying that the steam is "hotter" is imprecise enough to cause confusion when people equate temperature and heat.

8. Feb 4, 2009

### stewartcs

Correct. Take for example a pressure cooker. That water will not boil at 100 C since the pressure is greater. The water is allowed to be heated to a higher temperature (which in this case cooks your food faster). It boils once the relief valve allows the pressure to be vented at some predetermined limit.

If it is superheated vapor, then no liquid or saturated water is left...only vapor (the water is in the superheated vapor phase only). So continuing to add heat will "superheat" the vapor.

CS

9. Feb 4, 2009

### toqp

I'd still like to ask about one thing though (it was in an earlier post, but I'll move it here and add some thoughts.)

What causes the fact that a substance can't be heated above it's (pressure dependent) boiling point? What causes it that the kinetic energy of the particles that make up the substance can't be increased after that point?

At a microscopic level, a molecule in the liquid, receives kinetic energy from the heat. But depending on the atmospheric pressure, there is more or less collision between the molecules in the liquid and in the air. So the temperature of the liquid can't increase after some certain point, because whatever energy is given to the molecules in the liquid in heating, is then translated to the air molecules in the collisions? Therefore the kinetic energy and the temperature doesn't increase, because it is all convected into the surrounding air?

That's the only reason I can come up with, that somehow relates to the fact that boiling point depends on the atmospheric pressure.

But suppose that we have a perfectly isolated system where there's only some water at 100 *C and some air at an atmospheric pressure. The water is then heated, but the kinetic energy doesn't increase. If my assumption above is correct, then the air gets warmer and its pressure increases. Therefore the boiling point of the water decreases.

At some point the temperature of the air and the water are equal, and water is at its boiling point. Suppose that there's is still some water left in liquid form, and not all is vapourized.

If we then continue heating the water, what happens?