September 14, 2015

Does water’s boiling point change with altitude? Americans aren’t sure

SPOILER ALERT: If you haven’t already done so, test yourself with our new Science Knowledge Quiz. We discuss one of the answers to the questions below.

elevation and boiling pointsIt seems like one of those basic science facts: Water boils at 212 degrees Fahrenheit (100 degrees Celsius), right? Well, not always. It depends on where you’re doing the boiling.

In fact, water will boil at about 202 degrees in Denver, due to the lower air pressure at such high elevations. In Pew Research Center’s recent survey on science knowledge, only 34% of Americans knew that water boils at a lower temperature in the Mile High City than in Los Angeles, which is close to sea level. This was the question in our quiz that the fewest people answered correctly: 26% said they thought water would boil at a higher temperature in Denver, while 39% said it would boil at the same temperature in both places.

The boiling point of water, or any liquid, varies according to the surrounding atmospheric pressure. A liquid boils, or begins turning to vapor, when its internal vapor pressure equals the atmospheric pressure. For instance, when you heat your tea kettle on a stovetop, you’re creating more water vapor; when the water’s vapor pressure rises enough to exceed the surrounding air pressure, bubbles start to form and the water boils.

But pressure drops as you gain elevation – say, by driving from Los Angeles to Denver – because there are fewer air molecules pressing on you. In Denver, the atmospheric pressure is only about 12 pounds per square inch, compared with 14.7 pounds per square inch in Los Angeles. With that much less pressure, you don’t need to apply as much heat to push vapor pressure beyond the surrounding atmospheric pressure – in other words, water boils at a lower temperature. Putting a liquid in a partial vacuum also will lower its boiling point. The reason is the same: By removing some of the air surrounding the liquid, you’re lowering the atmospheric pressure on it.

In La Rinconada, a mining town in the Peruvian Andes that, at more than 16,700 feet, is the highest permanently inhabited town in the world, water will boil at about 181 degrees. Were you of a mind to brew yourself a nice cup of tea at the peak of Mount Everest (29,029 feet), you’d only have to bring your water up to about 162 degrees for it to boil. At another extreme, in Death Valley, Calif. – the lowest point in the U.S., at 282 feet below sea level – water boils at slightly above 212 degrees.

The lower atmospheric pressures at high elevations affect cooking and baking, too, which is why many recipes and mixes come with special “high altitude” directions. Cooking generally takes longer at elevations above 3,000 feet or so, and foods tend to dry out faster. Doughs will rise faster (because gases expand more) and liquids in batters evaporate faster.

Topics: Public Knowledge, Science and Innovation

  1. Photo of Drew DeSilver

    is a senior writer at Pew Research Center.

49 Comments

  1. Becky8 months ago

    I live in Denver but got the boiling point question wrong in the science knowledge survey. I know from long waiting for water to boil (especially on cookstoves on camping trips) that it takes longer at high altitude for water to boil, but I always thought the actual boiling point temperature itself was the same regardless of altitude. Mortified to only learn the truth at my advanced age; “you learn something new every day.”

    1. Anonymous3 months ago

      I thought the same too, but finally I have learned that the truth in my old age

    2. Anonymous3 months ago

      Becky on you camping trips It actually would take less time to boil given the same amount of heat or energy as you would at sea level. But I think what your experiencing is the heating source for your camping trip either gas lit flame or fire burns cooler at altitude given less oxygen.

  2. EDC9 months ago

    One of the reasons for pulling a vacuum on an air conditioning system is to lower the boiling point of water to remove it from the system, and the other is check for leaks.

  3. Tanya S9 months ago

    I knew baking takes longer at higher elevations, so I thought water might be similar. Wrong! Thank you for explaining.

  4. Aline J Henderson10 months ago

    Absolutely love these quizzes! They test knowledge we really should have, instead of the useless fluff that most internet quizzes test. So I learn from the answers I miss, and I’m heartened by how well I normally do. The value of a good education really shows, at least on quizzes that test things you learn in school, like science and history. Hooray! Please post more quizzes!

  5. Me10 months ago

    The number of rude comments about this is astounding. I’m a 17 year old who has a 4.0 and a score of 33 on the ACT and I got it wrong. I just assumed that since there’s a boiling point that is widely accepted, the boiling point would actually serve it’s purpose everywhere. I’m not dumb or idiotic. I don’t “believe the sky is falling” And I do have common sense. I just didn’t know this one answer apparently like many others considering that they posted a link to it at the bottom of the page. There is no reason to get salty.

    1. Yacko10 months ago

      It isn’t just about boiling water. Almost all physics observations are idealized to a specific temperature or pressure and are different depending on the conditions. And it can affect everyday life, say as in the design of a bridge and its expansion and contraction during temperature swings.

    2. Mike9 months ago

      I wouldn’t worry. The same person claiming you aren’t entitled to your own scientific conclusions is basing their conclusion off zero observation.

  6. Steve10 months ago

    Careful, this issue is bordering on the true answer to football’s “deflate gate” issue.

    1. EDC9 months ago

      Your comment made me LOL.

  7. Matthew Andre10 months ago

    Common sense says that the air is thinner there so water would be slightly more dense and heat faster. At least that’s what I think I was thinking. But then I started to over think and just threw out an answer. It is after all internet.

  8. Phil10 months ago

    I was stunned that so few got this correct. We learned this in 5th grade.

    1. carol9 months ago

      Some of us may have learned this in 5th grade, but unfortunately life events and numbers of years since 5th grade have caused us to forget. Please forgive . . . . 🙂

  9. Christine10 months ago

    excuse me .. I am not even an american 🙂

  10. Conodo Mose10 months ago

    Those believing water boils at the same or higher temperature at altitude are the same people also convinced the climate change sky is falling. In short, only a fraction of the educated public qualify to make scientific judgments.

    1. Mike9 months ago

      I’d… like to see some kind of scientific data to support this correlation please. You really think you can make a comment like that on a science blog which provides zero evidence your conclusion?

      Anyway… a better explanation is we only retain certain information for a short time. Unless it is converted into long term memory by means of meaningful relation or repeated practical application (rehearsal). Even then, it can decay over time.

      Also, it makes no sense to make a statement in a single sentence and follow it up with “in short.” You’ve already written your misconstrued thought as concisely as is possible. Unnecessary redundancies hurt writing.

      1. May Duppname4 months ago

        …as do sentences beginning with conjunctions, like ‘also’, or ‘anyway’ (when the latter is used in that context, at least). :-B
        The sentence you dismissed as redundant was the one containing all the functional load. The initial sentence contained much more redundancy in terms of transmitting the point of the thought, no matter how erroneous you believe the thought to be.
        In principle, I agreed with your point. Scientifically speaking, you are correct that memories need to be recalled or used, to remain. Each recall can in itself theoretically ‘change’ the memory even then. An example of this is when we consistently struggle to retrieve a particular word (‘arbitrary’ always seems to elude me personally). Our brains store and subsequently retrieve the memories of us struggling to find the word, rather than the word itself. To recall a word, your brain uses the shortcut created on the last occasion you used it. It can still retrieve the word, but it will take a lot longer to find without using the shortcut. Insignificant’ or incidental memories will degrade more rapidly in terms of both quality and quantity over time unless refreshed.
        (Most) information can only be stored for approximately ten yrs without retrieval before the brain ‘overwrites’ it.
        But (you see what I did there? 😉 ) the old adage, “People in glass houses shouldn’t throw stones” seems apt. You had every right to request documentary evidence supporting or proving the point being made – on such, the foundations of science are built. You have no right, however, to criticise anyone else’s use of the English language whilst doing so, especially whilst using it so imprecisely and incorrectly yourself. Many scientists, technicians and engineers quickly lose the ability to simply explain what they do. As the complexity of their knowledge increases, their ability to dumb it down far enough to reach the man in the street diminishes ever further. That doesn’t mean they don’t know their stuff. You don’t need an English doctorate to be a damned fine architect, for example. Most people have no need to know either the exact boiling point of water at altitude, or the way to concisely express such an idea, in their daily lives. Be glad they are interested in the subject about which you are clearly so well versed.
        Did you really think you should make a comment in any public forum so prescriptively judgemental, criticising someone else’s thoughts and words, without regard for their rights to free expression, their feelings, your own linguistic shortcomings or the fact that such an opinion cannot actually be quantified by evidential means in any case?!
        You pointed out the splinter in the other person’s eye, without first removing the plank from your own. 🙂
        You ask for proof that the majority of people only have a limited knowledge of science, so cannot\ should not be relied upon to make scientific judgements. (I believe that was the point being made.) Any set of national exam results you may care to examine should offer as much proof as you could want. None of us are experts in all fields. Most of us are sadly ignorant of most of the basic principles underlying every complicated concept, unless we happen to be interested in it at the same time as having somebody more knowledgeable around to educate us further, or our interest in it is professional. Not everyone can be a Renaissance man. I recommend you research the principles of the ‘lie to child’ theory and then consider how many of these shortcuts we must blindly accept as fact before we can fully understand…well, anything at all…the way it really is. I don’t need to know how to build a pyramid to stand and stare in awestruck wonder at the pyramids of Giza. I may not be fully aware of the total cost, complexity and sacrifice involved in their erection, but a basic awareness of what went into building them is more than adequate.
        I mentioned your use of conjunctions to begin sentences. I could just as easily deconstruct your comment to find faults with grammar, sentence structure, typo’s etc. ‘But’ i won’t, as it helps no-one and would be as truly redundant as your earlier dismissive, disparaging, rather reductive reply. I’d advise you either to stick to Science, (where you clearly know your stuff) or to first become more informed about any other subjects in which you intend to criticise others. (English Language, for example.)
        Pot, kindly stop calling the kettle ‘black’.

  11. Victor10 months ago

    Graph 101:
    The point on the graph is not mt Everest, it is 162F, mt Everest. If you insist on calling it mt Everest, slide it to the vertical axis.
    A graph protocol is to plot the independent variable on the horizontal axis and the dependent on the vertical. The altitude is clearly the independent variable, not the boiling pt.
    Shame on you who make a living on numbers and displays.

  12. Catherine10 months ago

    A few years ago, I was working in La Paz, Bolivia for a few weeks. La Paz is roughly 4,000 m high or about 12,000 ft, or generally about 3x’s as high as Denver. I had some medical equipment with me that I needed to sterilize several times a day. The water I used boiled in a very short amount of time and also seemed to evaporate very rapidly. I would boil the items for much longer than I would at sea level b/c I was unsure if the items were still being adequately cleaned at the lower temperature. I am still unsure of this- does anyone know about sterilizing at the lower temperature and higher altitude? I am curious.

    1. Loes10 months ago

      Yes, this is an actual issue. It’s why medical equipment is often steam sterilized in an autoclave under high pressure, allowing the water to warm to 120 degrees Celsius before evaporating.

      It is not the boiling itself but the high temperature that is crucial for sterilization. Chances are that in La Paz, you didn’t reach temperatures high enough to kill of all microorganisms. Next time, I would suggest using a pressure cooker (light weight variants exist) or chemical sterilization.

  13. Jack10 months ago

    Yes, the trouble is, you talk about this as if were logical, Most people would know that the boiling point for water is 100c – they would not necessary know that this would vary depending on height, air pressure etc-so despite the tone of this article and the afore mentioned quiz-it is not an easy or obvious question to answer without a little research.

    1. mach3710 months ago

      More American know the boiling point of water as 212° F, rather than 100° C. There is no “logic” to knowing that the boiling point of water varies without knowing WHAT determines the boiling point. Altitude of locations around the Earth is directly related to air pressure. Knowing that air pressure is lower with increasing altitude is basic to understanding scientific principles involving liquids at various temperatures.

  14. Nelson10 months ago

    That’s why saliva in our mouth would evaporate if we were exposed to vacuum? Even without heat? Or why happen that?

  15. Joe Gems10 months ago

    Nope, sorry. As long as the water is boiling, it will remain at the boiling temperature, whatever that may be, and will get no hotter.

  16. ann smith10 months ago

    Except that you can’t brew a “nice cup of tea” at 162° F. The standard steep time at this temperature results in under extracted, weak tea, whereas extending steep time to counteract low temp results in over extracted, bitter tea. So, the ideal solution on Everest is to allow the water to boil longer to bring the temperature up to the norm for steeping black tea, which is at least 208° F.

    1. Leland10 months ago

      If the pressure remains at ambient atmospheric pressure, boiling the water longer won’t increase the liquid’s temperature. Everything above 162° is gas-phase, so you will just end up with less 162° water and more steam.

      Maybe you could work this with a pressure vessel, where the volume is fixed and the pressure goes up with temperature. I am unsure if a standard tea kettle qualifies.

      1. mach3710 months ago

        A tea kettle is not a pressure vessel. Based on many of the comments here, I am amazed at the lack of scientific knowledge shown.

    2. Ian Nicholson10 months ago

      It doesn’t matter how long you boil it for; it won’t get any hotter than boiling point which is a measly 72°C (why would anyone use Fahrenheit?) at 8,848m (again, why are we using feet?).
      It’s impossible to make a decent cup of tea at summit of Everest unless you do it in a pressurised container.

      1. Me10 months ago

        People use Fahrenheit because this is an article directed at Americans and Americans use Fahrenheit.

      2. Devon10 months ago

        I read an article that said for daily use degrees F is better for daily use by humans because it only requires 1 significant digit to indicate general temperature. Exp. The difference between 20C and 29C (68 & 84F) is the difference between wearing a long-sleeve shirt and pants or shorts and a t-shirt. With degrees F, you can derive a lot of useful information only from a single digits, i.e. 70s = t-shirt, 60s = pants, 50s = coat. Furthermore, Fahrenheit captures the temperatures of human life over a wider scale within 2 digits. So sub-zero and super-100 are considered extremes, while anything closer to the middle is considered average.

        I know that doesn’t mean much in terms of science, where 0 & 100 correspond to water freezing and boiling (at sea level), but if you’re really worried about science you should be using Kelvin anyway.

      3. Craig9 months ago

        If you ever visit the US, you better be prepared use Fahrenheit and miles!

    3. Slipstream Hansil10 months ago

      I don’t think you can raise water temp above boiling point. At that point all the temp stops rising as the energy is exerted changing the liquid to a gas. Unless you boil the tea under pressure.

    4. Slipstream Hansil10 months ago

      You can’t raise water temp beyond the boiling point (unless you make your tea under pressure).

    5. Orville10 months ago

      It seems to me that once the water is boiling at 162F, all you’ll be doing by boiling longer is creating more steam instead of raising the temp of the usable hot water. But I’m kinda hazy on that and could very possibly be wrong.

  17. Andrew Gallerani10 months ago

    BOO HOO! I know water takes LONGER to boil at higher altitudes. That’s where you got me! Interesting… but 11 out of 12! 🙂

    1. Laura10 months ago

      The same for me! I grew up in the mountains and was always how surprised at how fast water boiled when we went on vacation.

    2. K.10 months ago

      If water boils at a lower temperature, it does not take longer to boil (less time is required for the boiling temperature to be reached). Do you mean that it takes longer to cook a hard boiled egg or a box of pasta because the boiling water is not as hot as a pot of boiling water at sea level?

      1. Yacko10 months ago

        One trap for people getting this wrong are recipes. Whether you just boil an egg or make a meal in a pressure cooker, the cooking times gets longer because the water or steam has less heat content that at sea level. Perhaps they incorrectly equate this with water taking longer to boil.

  18. Joseph Santacecilia11 months ago

    Everyone knows that. Here is the formula – PV = nrT where P is pressure, V is volume, T is Temperature in Kelvin and n and r are constants. As pressure decreases, temperature decreases in direct proportion to the volume of the system.

    1. Dave10 months ago

      n is not a constant, but the quantity of gas in moles.

    2. Tom10 months ago

      Actually, that has nothing to do with this. The temperature will change, but this formumla says nothing about the boiling point of the liquid or gas.

    3. Leland10 months ago

      This is the ideal gas law. As you are probably aware, water is not a gas. Finding the phase boundary curve, which determines the temperature a liquid will boil at for a given pressure, is a more complicated matter.

  19. Steve Szilard11 months ago

    I found your question on tides misleading.
    While it is true, that the moon has more influence on the height of the tides than the sun because of its proximity to the earth, if the earth did not rotate, tides would happen once every 14 days, instead of twice a day.

    1. Fredrik H10 months ago

      And if there was no moon?

      1. Michel10 months ago

        Haha. Got him with that one.

      2. John10 months ago

        There would still be (and are) tides due to the gravitational effects of the sun and nearby planets. These tides would just not be as noticeable as those caused by the moon.

      3. mach3710 months ago

        But there IS a Moon!

      4. Sriprasad10 months ago

        (Big question!)
        Am not sure about the changes in tides, with no moon.
        But I saw in a video, which says, there will be no lives on earth if there is no moon! 😉 :O