Never having studied thermodynamics much, I’m relying on you others who have to give me the dope on steam. We all know that the boiling point of water is 100 degrees, but what exactly is the point at which steam is possible, assuming room temperature and sea level pressure? I can drink a steaming cup of coffee, but have no plans to attempt the trick with one that’s boiling. Exactly how and why does steam form?
I’m not entirely sure what you are asking. I think you are asking about the “steam” that you see comming off of a hot cup of coffee. If that is the case, what you are actually seeing is water vapor that has condensed into very small water droplets, and while it may feel hot, it is not steam. Steam, on the other hand, is not visible. It consists of individual water molecules that have much more energy in them than the water vapor you can see. I hope that helps. If it doesn’t, say so. I may have misunderstood what you were asking.
Sorry, I missed something in your post. Steam is possible under the conditions you specified only at 100 degrees C. Like I said above… the “steam” you see comming off of a cooler body of water is not actually steam.
I am talking about the vapor that you see rising from hot water, not true steam. Still, what is it exactly that gives those particular droplets of water enough energy to separate from the larger body of liquid? What is the escape mechanism, and how hot does liquid need to be under ordinary conditions to go from just plain hot to what we normally refer to as “steaming hot”?
The escape mechanism is this:
The temperature of a substance is a measure of the average kinetic (vibrational) energy of the atoms/molecules of the substance. At any given time, there are molecules that have a much greater and a much smaller kinetic energy than the average. Those water molecules with the greatest kinetic energy in the sample will escape into the atmosphere if they happen to be at the surface of the liquid/atmosphere boundary. This escape of some of the molecules is always happening. The hotter the sample of water, the more often one of the molecules will escape, but like I said… it is always happening. A glass of room temperature water evaporates through this process, although it happens more slowly than if the water were heated. Likewise, the room temperature water will evaporate more quickly than 1 degree C water. For that matter, ice has molecules with enough kinetic energy to escape the surface of the ice. The process of a solid evaporating is called “sublimation”. When any of these escaped molecules comes in contact with another escaped water molecule, they will “stick” together. If another bumps into the first two, it will stick. At some point, if there are enough of these small drops of water above the cup, they will become concentrated enough to become visible.
As stated above, the hotter the water, the faster the evaporation will occur. At some point, the evaporation will occur so quickly that the water vapor becomes visible. The temperature at which the vapor becomes visible will be dependent on two factors: atmospheric temperature, and humidity. The lower the atmospheric temperature, the lower the temperature of the water needs to be before you will see “steam”. The higher the humidity, the lower the temperature of the water needs to be before you will see “steam” (although the effect of humidity will not be as strong a factor as atmospheric temperature). On a cold, dry day, the water vapor in your breath will condense into these little visible droplets of water that we are all familiar with seeing. So, to answer your question about what temperature the water needs to be before something becomes “steaming hot”… it depends. So, all of that for “it depends”… sucks huh? 
Actually, the “it depends” answer is really enough now that I’ve got the background information as to exactly what is going on here. Now I don’t have to wonder what the heck is going on every time I see a cup of coffee. It never bothered me before, but once the question occurred to me, it kind of stuck there in my head and I knew it wouldn’t let go until it was satisfied.
Actually it depends on three things: atmospheric temperature, pressure and humidity.
The greater the difference between water and atmospheric temperature, the greater the heat loss through evaporation. That’s because the atmosphere isn’t “pumping back” some heat into the water. Even when the water is the same temperature as the atmosphere, some water will still evaporate.
The lower the atmospheric pressure, the greater the heat loss. That’s because the atmosphere isn’t “pushing back” as hard, and molecules with a lower average kinetic energy can escape.
I’m not really sure what effect ambient humidity will have on the process of visible or invisible evaporation.
SingleDad, I didn’t mention pressure because the OP already specified a specific pressure - “assuming room temperature and sea level pressure”. You are correct about your explanation though.
As for humidity… It is going to be a very small effect, and I was hesitant to mention it, but it will have a tiny effect. More ambient moisture in the air will give the escaping water molecules more condensation points. Like I said… a TINY effect. Actually, I just thought of something that I left out that would have a greater effect on the “steam” being visible than the humidity. That would be atmospheric circulation. If there were a decent wind going, it could disperse the escaped water, thus making it more difficult to see the “steam”. So, a stagnant atmosphere would allow the “steam” to be seen at a lower pot temperature.
To pick nits, the stuff coming off the top of the coffee surface is steam, even though the coffee is at less than its boiling point. You’ll notice that it doesn’t become visible until it rises a little bit, and cools off enough to condense into water droplets.
What I think is most insteresting about steam is the property you should have learned about in ninth grade called latent heat. If you have a 100 gram cup of coffee at 40 C, and you add 100 calories of heat, it will heat up to 41 C. Each 100 calories you add will increase its temperature by 1 C. Until you get to the boiling point, where it takes an astounding 54000 calories to convert all 100 g to steam, without even raising its temperature.
The reverse of this explains why very hot water cools down so rapidly. For every 0.1 grams of water that evaporates, it takes 54 calories of heat away with it, enough to cool the 100 g coffee down by half a degree C! If only 1 g evaporates, the coffee will have cooled down by 5.4 C.
So if you keep the cup covered so that it doesn’t evaporate (as quickly), the coffee stays hot much longer.
Don’t forget that steam is technicnally water vapor, regardless of how hot or cool it is. When water vapor at standard (sea level) pressure is cooler than 100 C, it will condense into water.
Conversely, water at standard pressure will boil into vapor (steam) at 100 C.
At lower pressure, water will boil into vapor at less than 100 C. And consequently, at higher pressure, such as in a pressure cooker, water won’t boil until it is much hotter than 100 C.
At standard pressure, if you see liquid water, you know that it is 100 C or less. Water can’t get hotter than that – no, really, I’m serious – water can’t get hotter than 100 C at standard pressure. That’s the importance of a boiling point. When it gets hotter than 100 C, it will vaporize (boil).
Steam, however, can get much, much, much, much hotter than water. That’s why circulating steam in a room radiator makes a better heating agent than water. And that’s why steam burns are much, much, much, much, much worse than scalding water burns.
(Incidentally, steam occupies a much, much, much, much larger volume than the same amount of water, which is why steam engines and turbines work.)
A hot cup of coffee, which is less than 100 C will produce steam because, as has been stated, some of the water molecules within the coffee have a higher than average kinetic energy (heat) and will escape from the surface. However, what you see is a cloud of condensed steam (i.e., liquid water) into tiny water droplets (which can be pretty hot, but not hotter that 100 C). But even though the droplets are not more than 100 C, they are surrounded by water vapor that can be much hotter, so watch out.
Peace.
Water is also wet.
I think the reason steam burns you so easily is what I mentioned before about latent heat. If you get one gram of water at 100 C on you, it transfers about 65 calories to your skin (100-35).
If you get one gram of steam on your skin, it transfers 605 calories (540 for latent heat to condense it, plus the 65 to cool it down) - a factor of nearly 10x!
Most of the time when we come into contact with steam, it will be not much hotter than 100 C, because steam is usually produced simply by boiling water, not by trapping steam and heating it up to godawful temperatures.
Some of the posters to this thread are confused between steam and water vapor. You cannot see steam. What you are seeing is water vapor - the condensation of the steam into a mist. Hanging in the air, it takes on a cloud like appearance. When the water droplets collect on a surface, only then do we tend to think as it NOW being condensation. However, this is a common misconception.
Actually, water vapor is steam. Vapor=gas; steam=water gas etc. etc.
That doesn’t mean what you * see * is water vapor or steam. It is neither. It’s liquid water suspended in air. The term for a suspension of material of one phase in a material of another is called a colloid. In this case, a liquid suspended in the air is something called an aerosol (a sol is a liquid suspended in another liquid). That fog you see over a hot cup of coffee is simply an water aerosol. Any good introductory high school chemistry text book has a chart of colloids in it.
Oh, and WRT steam burning you more than water, this looks true at first, but your experience tells you something different. You can easily hold your hand over a boiling pot of water but would get a nasty burn if you put your hand in the water. This is because steam is about 1000 times less dense than water. This means that one gram of water takes up about 1 milliliter of space. One gram of steam takes up about 1 L of space… Remember, it’s not the mass of something that concerns you when you are going to be burned, it’s the surface area of your hand that will come in contact with it. Lets actually do the math.
PV=nRT
P=1 atm
V= unknown
n=1 gram/18.02 grams per mole (for water)
R=.0821 L atm/mol K (universal gas constant)
T=373 K (boiling point of water).
solving the above, the volume of 1 gram of steam is 1.69 liters, or 1.69 cubic decimeters or .0169 cubic meters (converting to cubic meters for a reason soon to be shown).
Liquid water, being a condensed, incompressible phase has only a slightly temperature dependant density, which is about 1 g/mL, so 1 gram of water takes up 1 mL of space or .000001 cubic meters.
Let’s assume the average is aproximately a flat rectangle that is, hypothetically (to make the calculations easier) 12 cm by 6 cm. That’s 72 square centimeters or .0072 square meters.
lets assume the water and the steam each to occupy a space of a prism whose surface area is equal to that of you hand. (which is important, because the water or steam will only come into contact with teh surface of your hand). Lets arbitrarily say that that prism has a height of 1 meter (to make our calculations easier). That would mean our hand would be absorbing the heat from .0072 cubic meters of substance. For liquid water, it would absorb the heat of 7200 grams of water. For steam, it would absorb the heat of .426 grams of water. Thus, our hand immersed in steam is in contact with .426/7200 times less mass of steam, or .0000592 times less mass of substance. Since our figures above for the surface area and volume of the object above were arbitrarily assigned, we could just have easliy made them variables. This number, .0000592 is the ratio of surface area of 1 gram of steam to 1 gram of water.
Now, if steam has 605 calories per gram, and water has 65 calories per gram, then steam has 605/65 or 9.32 times as much energy per gram.
However, surface area is the important stat here. Since 1 gram of steam will come in contact with .0000592 time more surface area than water, it will take .0000592 x 9.32 .000551 times smaller mass of water to burn you the same amount as steam, or to put it more conveniently, it will take 1814 grams of steam to burn you as efficiently as 1 gram of water. The difference here is we’ve taken into account the efficiency of transfering the energy to your hand required to actually burn you, not just the total energy contained in the sample. Since steam is VASTLY less efficient in transfering energy than water is, any latent heat held by the steam is more than overcome by the fact that steam is VASTLY less dense than water.
If you can easily hold your hand over a boiling pot of water, you are holding your hand above the point at which the steam is condensing into water droplets. Since your hand is only coming into contact with water droplets, most of the energy has already been released from the water, and it does not burn you. I assure you that if you actually held your hand in the region just above surface of the boiling water, you would experience a much different sensation, and it would not be easy to keep your hand there, as you would most assuredly receive a “nasty burn”.
Let’s not give the impression that steam is not very harmful. Sure, you are probably more likely to come into contact with significant amounts of 100 C water, but steam is definitely not something to be trifled with.
Call me crazy, but I would rather have someone pour 1 mL of 100 C water in my hand than to sit with my hand exposed to 1814 g (3083 L) of 100 C steam. I don’t fault the numbers… it’s the method or the concept that I have a problem with.
Your calculations are assuming that the steam is stagnant, and unaffected by our hypothetical hand. Our hypothetical hand would act as a heat sink, causing the steam to condense (which is how it would transfer its energy to the hand). What this all boils down to is that you are not taking into account that when the steam condenses on our hypothetical hand, a localized vacuum is created, thus drawing in more steam. Our hypothetical hand is, in effect, acting as a steam magnet by giving the steam a place to condense. There’s also the issue of flow. Steam is almost never stagnant. It is usually moving due to either pressure or convection currents. The flow of the steam will cause even more steam to contact the hypothetical hand. If these processes happen over a period of time, it will not take very long for the condensing steam to release gobs and gobs of energy into your hand.
Very true, given sufficient time, 100 C steam will burn you more than 100 C water. But for instantaneous contact, 100 C water is much more of burn hazard, since at the point of contact, much more energy will be transfered to your hand. Look at it this way: A kilogram of water will take up a liter of space. A kilogram of steam will take up 1690 liters of space, assuming, of course, it was pure steam.
Two problems:
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1690 liters of steam would be about 16.9 cubic meters, or a space about as big as a person. I think we can all visualize what a liter of water would look like.
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It’s possible to have a liter of water, sitting out on the counter, and to comeback in a few minutes and still have a liter of water (minus a few microliters evaporation) If I had a liter of pure steam, and opened it in a space the size of your kitchen, in a few minutes the steam would have spread throughout the room.
Lets see what this means for our steam.
If we have 1690 liters of pure steam at standard pressure and allow it to equilibrate in a standard size room (lets say a room 12 meters long by 12 meters wide by 3 meters tall, a volume of 432 cubic meters) also at standard pressure, that means that at equilibrium, the steam will have a partial pressure of 16.9/432 * 760 torr = 29 torr. Assuming the rest of the room is also at 100 C, will this burn you? Not likely. Think about opening the oven. You can reach into an oven at 375 F, about 190 C, which has actively boiling water in it, and not be immediately burned. Reaching into the pot will definately scald you, but you’d have to leave your hand in the oven a minute or two to get an equally bad burn. 1 seconds of exposure to 100 C steam will burn you far less than 10 seconds of exposure to 100 C water. Now, if you sat around in 100 C steam for a few minutes waiting for it to condense upon your body, I would predict a pretty nasty burn. But for contact exposure, the water will burn you far worse than the steam will.
I’m sure that’s just a typo, but I thought I’d point out that 1690 liters of steam is actually 1.69 cubic meters. Even so, 1.69 cubic meters is quite a bit larger than a person.
I think here is where we are ultimately having our main disagreement. You seem to have been thinking of large dilutions of the steam (mostly air), while I have been thinking of concentrated steam (all steam). I agree that once the steam has been severely diluted in air, it will not do any damage to you. I have been thinking more of a situation where you have a busted pipe of nothing but steam coming out, and hitting you before it has a chance to cool or dilute. After all, our 100 C water has not been diluted in air (whatever that would mean ), so I don’t see any reason to dilute the steam. Even in the oven scenario, the vast majority of what contacts your skin will be air, which does not impart its heat to your flesh very well. Now, if steam were all that were in the oven (no air), and you were able to thrust your hand in before air got in, or heat escaped, we’d have some really bad burns on our hands 
, and they would occur almost instantly.
My “busted pipe” scenario would also involve a flowing steam rather than a stagnant steam that would require you to sit around and wait for the steam to come to you to release its energy.
Like I stated before… I think… well… I just noticed that your sig line sums it up quite nicely “What we’ve got here is a failure to communicate”. We have been comparing two unlike sets of conditions without realizing it. Using the scenario you’ve painted, I agree that the water would burn you worse. Using the scenario I’ve painted, you are screwed regardless of which water phase hits you .
Quite true… I was talking about where a person might, in their everyday lives, run into both steam and water in the same situation. In a vigorously boiling pot of water, you stand a much greater burn hazard from the liquid water than from the steam over the water.
A busted steam pipe contains steam at temperatures greater than 100C, so i have no doubt that it will burn you quite badly.