Boiling water: A moutain Climbers perspective and question on a old Cecil answer.

Cecil's Columns/Staff Reports
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A mountain climbers perspective, any suggestions on this?

This was sent to me from the Straight Dope archives (I have posted the original question and answer below my question, please read.)

Very interesting question and answer… but… one part of Cecil’s answer leaves me with a question. Here’s the problem I have with part Cecil’s answer:

“… a vacuum never causes water to freeze; it causes water to boil. As air pressure decreases, so does boiling point. That’s why water boils much faster on a mountaintop than it does at sea level.” (removed from original, see blow for full content).

Now, one of my “other” lives is as a mountain climber. A few years ago I was in Nepal climbing a very large mountain call Mara Peak. It has always been common knowledge for multi day climbing trips, that the higher you go, the more fuel you will need to boil your water. This is based upon the fact the air is “thinner” the higher you get, thus less oxygen for the flame to heat the pot to heat the water etc. So to compensate, you must heat the water for a lot longer. Eventually it will come to a boil. Based up my experience at high elevation, water does not “boil much faster on a mountain top than it does at sea level”.

Far be it that I would ever think Cecil is wrong. So I must be missing something here. Maybe the answer lies in the method of boiling water?? But if so, then what method is Cecil using to get water to boil faster at higher elevations??? Tempted to send this one to Cecil myself, but he is a busy man and all.
Thanks for any suggestions.

Shane Somebody.

Orignial Question from SD Archives:

Would a glass of water in space freeze or boil?

30-Mar-1984

Dear Cecil:

If cold is simply the absence of heat, i.e., the absence of rapidly moving molecules of water or air, then how come vacuum-packed canned food doesn’t come out frozen, or at least very cold? And then if you walked a hundred feet out of your spaceship with a glass of water, would the water freeze because of the vacuum, or would it boil since there’s no air pressure or barometric pressure to overcome? --Barry H., Chicago

Cecil replies:

Christ Almighty, Barry, you’re asking for a short course in thermodynamics. Don’t you guys want to know about Neil Sedaka anymore?

Let’s clear up a couple misconceptions to start with. First, your idea that cold is “the absence of rapidly moving molecules of water or air” is a bit confused. Cold refers to very slow-moving molecules of anything, whether water, air, or Eskimo Pies. If you have no molecules at all, the concept of temperature is meaningless. That’s why it’s technically incorrect to speak of the “cold of outer space”–strictly speaking, space has no temperature, period. (On the other hand, space will make objects that are floating around in it cold–in some cases, very cold. Space is what’s known as a “temperature sink,” meaning it sucks heat out of things. But we’ll get back to this in a minute.)

Second, a vacuum never causes water to freeze; it causes water to boil. As air pressure decreases, so does boiling point. That’s why water boils much faster on a mountaintop than it does at sea level. By the same token, you can make water boil at room temperature in the laboratory by applying a partial vacuum.

Now then. The contents of an earthbound vacuum-packed can do not freeze because they’re in contact with the sides thereof–they absorb room heat by conduction. There is no room heat in space, though, so the temperature of a solid object floating in the void consists of the difference between the heat the object absorbs from the sun and the internal heat it radiates away. This temperature is dependent on such things as the reflectance of the object’s surface, its shape, mass, orientation toward the sun, and so on.

Polished aluminum will absorb sufficient heat to raise its temperature as high as 850 degrees Fahrenheit; certain types of white paint, on the other hand, absorb so little heat that their temperature may not get much above -40 Fahrenheit, even in full sunlight. Parts of the space shuttle get down to -180 to -250 degrees Fahrenheit.

Theoretically, the temperature of an object in deep space could get down pretty close to absolute zero, -460 degrees Fahrenheit. But even in the middle of nowhere there’s enough in the way of stray particles and radiation to heat thing up to 3 degrees Kelvin–that is, the equivalent of 3 degrees Celsius (5 degrees F) above absolute zero.

Finally, we have the question of liquids in space. In a vacuum most liquids have such a low boiling point that they vaporize almost instantly. For that reason, most substances exist in space in either the gaseous or the solid state. When the astronauts take a leak while on a mission and expel the result into space, it boils violently. The vapor then passes immediately into the solid state (a process known as desublimation), and you end up with a cloud of very fine crystals of frozen tinkle. It is by such humble demonstrations that great scientific truths are conveyed.

–CECIL ADAMS

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I believe you’ve got it. I would guess that the method of boiling mentioned briefly in the article would involve something a little less variable than a built fire. Probably a an electrically-powered stovetop.

And for future reference, comments on Cecil’s columns belong in the forum titled “Comments on Cecil’s Columns.”

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Water will boil at high altitudes, but it isn’t as hot as
boiling water at sea level. This is because the air pressure is lower at high elevations. Boiling occurs when the water is hot enough to have the same pressure as the surrounding air, so that it can form bubbles. As water is heated, its steam pressure rises, until it reaches the pressure of the surrounding air. At high altitudes, this air pressure is lower than at sea level, so the water doesn’t have to get so hot to get to boiling. Because
the temperature of the boiling water is lower at high elevations than at sea level, it takes longer to cook things at high altitudes than at sea level.

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one associates boiling water with sea level activities
how does the mountain top boiling point reconcile to eggs or tea
i understand the fuel side of it but it still takes so many btu’s (i am very old) to raise the temp of so much water to such and such a temp.
notwithstanding its specific heat remaining constant.?
btu’s:rolleyes: God knows what they are using nowadays:cool:

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Since this is a question about one of Cecil’s columns, I’ll move this thread to the Comments on Cecil’s Columns forum. A link to the column is appreciated. Providing one can be as simple as pasting the URL into a post, making sure to leave a blank space on either side of it. Like so: http://www.straightdope.com/classics/a1_127.html

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True, but Cecil was referring to the temperature needed to cause boiling. He did not consider the other factors.

I have another question in that same column. He mentions that any object in space would be at least 3 degrees Kelvin, but isn’t that also the temperature of the background radiation? If so, all objects in space would be at least that since there is at least 3 degrees radiation in space.

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To make that a little clearer, Cecil’s argument involves the total amount of heat required to boil water, which is much less at high altitude; all other things being equal, water will indeed boil faster on a high mountain.

Your objection merely indicates that “all other things” are not, in fact, equal.

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Since air is thinner at higher altitudes, perhaps fuel burns less efficiently resulting in longer time requiredto boil water even though water boils at a lower temperature.

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Two questions:-

  1. Cecil says in this article that water vapour will desublimate in a vacuum to become ice crystals.

How? Water vapour is a gas, so the molecules are separated by a quite some way. If they were to desublimate, wouldn’t you end up with a cloud of individual ice molecules, rather than crystals? I thought that crystal formation had to result from the cooling of a blob of liquid whose molecules were already bound together by intermolecular forces. Or does an individual molecule count as a very, very, VERY small crystal? :slight_smile:

  1. Cecil says that “a vacuum never causes water to freeze; it causes water to boil.”

At what temperature does water vapour desublimate in a vacuum? If water is ejected from a spacecraft at this temperature, wouldn’t it just freeze (contradicting Cecil’s assertion that this could never happen), rather than boiling and then immediately desublimating?

Thanks for any clarification anyone can give on this!

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The molecules do not need to be bound before freezing; as long as they approach each other and lack the kinetic energy to escape, they will form crystals (very tiny crystals, as Cecil said).

Water will always boil in a vacuum, until the vapor pressure reaches equilibrium. In space, this is never reached. However, the mere act of boiling does not make the molecules “hot” (quite the opposite), and once the physical act of equalizing pressure has occurred, they are quite free to reform as ice crystals (provided they get close enough). Temperature is, of course, a nearly meaningless concept in a vacuum.

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With less oxygen, the fire doesn’t get as hot, so it takes longer to build up the heat required to boil. Boiling occurs at a lower temperature, but it still requires heat.

I suppose “water boils faster on a mountaintop” could refer to on a stove, where the heat is more consistent. Or he could have just meant “boils at a lower temperature”.

mrcrow said:

Um, what? Yes, a certain amount of heat is required to cook an egg, or make a pot of tea warm, or boil off water to vapor, or melt ice. But the temperature at which it occurs is lower, due to the lower vapor pressure. Specific heat does not change, but latent heat (heat required for phase change) varies by temperature and pressure.

[quote]
btu’s God knows what they are using nowadays**
Calories. (But the Calories on food packages are kilocalories.) Also, Joules, Ergs, or Watts can be used for energy, depending upon the specific needs. Joules and Ergs are energy, Watts being power (energy per unit time).

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I’m going to go out on a limb and dispute that. If the fuel is converted to flame, there must be enough oxygen for it to burn. Perhaps the water takes longer to boil because it is colder? Not just the water colder–the most heat-expensive part of boiling water is the jump from not-quite-boiling to boiling–but the ambient air.

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You need more fuel because you need to cook longer. If water boils at 170F, its going to take longer for your boiled potatoes to get done.

Likewise, people use pressure cookers to decrease cooking times by increasing the boiling point. I imagine that pressure cookers are popular at high altitudes. Is that true?

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Yes, pressure cookers are used at High Altitudes. Of course the weight of pressure cookers does not allow them to be used all the time. You will see them used a lot by the Nepalese people on the trekking routes in Nepal. But, a key to a successful summit is minimizing the weight on your back. Today’s technology has lead to the creation of some amazingly light weight stoves that are very efficient at cooking at high altitudes. Note, these are not pressure cookers. They are open flame stoves with a support structure to rest your pot on. All in all excellent pieces of equipment.
shane Somebody

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Actually, both specific heat and latent heat do change with temperature and pressure. Specific heat is not a strong function of either over the temperature ranges we are discussing. Latent heat changes some, but actually, the latent heat of vaporization increases with decreasing pressure.

What causes the water to start boiling sooner at higher altitudes is the fact that it boils at a lower temperature. This is an effect of specific heat. You only need to heat the water to 162 degrees F at 5 p.s.i. as opposed to the 213 degrees F at 15 p.s.i. Once boiling, you actually have to apply more heat to evaporate a given mass of water at the lower pressure.

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Quoth barbitu8:

Isn’t that exactly what Cecil said? OK, so he said “stray particles and radiation”, rather than “Cosmic microwave background radiation”, but that’s a reasonable rephrasing for a lay audience.

Incidentally, though, it’s not quite true that everything in space will be at least 2.7 Kelvins. That’s only true if the object has had sufficient time to equilibriate, and it has a positive specific heat. Now, granted, almost all objects have a positive specific heat, but not black holes. The more heat you put into a black hole, the colder it’ll get. So black holes can be much colder than the background radiation, in the picoKelvin range or colder.

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I saw a demonstration in an 8th grade science class that seems to prove and disprove much of what has been said so far. Mr. King filled a 2-inch deep glass dish with water from the faucet. Then he put it in a bell jar and hooked up a vacuum pump. As the pressure went down the jar, the water began a rolling, jumping boil. Then, to the astonishment of all of us, ice formed on the surface of the water. Soon, a sizable lump of ice was there, with water boiling all around it! Mr. King shut off the pump and explained that the partial vacuum had lowered the boiling point of the water. With all that furious boiling (evaporation) in the absence of any additional heat, the tempurature of the water dropped to the point of freezing.

The question in Cecil’s column was about the complete vacuum of space, and this display was in a partial vacuum, so I’m not sure it really shows anybody up. However, a partial vacuum will cause water to freeze.

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It’s not the vacuum causing the water to be cold, though; it’s the boiling. If you kept that dish of water under the vacuum jar for a while, eventually it’d warm up again and melt, even with the pressure still (almost) vacuum.

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Chronos, this is just a semantic dance. Of course the boiling caused the freezing, but the partial vacuum caused the boiling. Now, boiling usually follows the application of heat, so we seldom think about the heat loss from evaporation. When boiling happens in the absence of additional heat, the chilling effect becomes obvious. That’s why the ice was so surprising to us 8th graders long ago.

–Nott

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I agree with you, of course, AskNott, but I thought that there was potential for confusion on the part of other folks reading your post. If you just say that “vacuum causes freezing”, then some folks (like the fellow who originally wrote in to Cecil) might get the impression that the water would stay permanently frozen so long as it stayed in vacuum, which would not happen.