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A rather simple analogy/explanation would be appreciated.

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3 Answers 3

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A very simple analogy would be: The melting point of copper is at 1085°C. Is a block of copper always 1085°C or can it be colder than that?


Your two questions are not really about the same thing. At atmospheric pressure, water is liquid from 0 to 100°C. Any colder than that, and it will freeze to become ice, any hotter and it will evaporate to become steam. Nothing prevents us from cooling ice to temperatures lower than 0°C.

This misconception might come from the fact that in ice-water, i.e. a mixture of ice and water, the water will always be at 0°C. The transformation from solid to liquid takes some amout of energy, which we usually call latent heat. Let's look at what happens to ice as we add energy to it. If it is colder than 0°C, it will start heating up, until it reaches 0°C. At that point, it will start melting. But, because melting takes energy, we must continue to add this energy to the system. Instead of increasing the temperature further, all the energy we add now goes into melting the ice, and the temperature stays unchanged. Both the ice and the liquid water (the part that has already melted) are now at 0°C. As long as there is ice present, adding energy to the system will only melt the ice and not increase the temperature. Only when all the ice has turned liquid, will adding energy increase the temperature again above 0°C.

The same argument as in the beginning goes for steam. Once the water is gaseous, nothing prevents us from heating it further. But, as water is only liquid from 0 to 100°C, we cannot cool it below 0 or heat it above 100°C without it transitioning to a different phase (again assuming atmospheric pressure).

There are some exceptions to this. Supercooled or superheated water are below 0 and above 100°C, respectively. But they are not in thermodynamic equilibrium, and any sufficient disturbance will cause them to freeze or evaporate almost instantly. In order to start freezing, the water needs some imperfections at which the crystallization can begin. These can be little particles of dust or whatever else floats around in the water. If the water and container are really pure, and there are no such places at which the crystallization process can begin, the water will stay liquid even below 0°C.

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    $\begingroup$ A recent question on supercooled water: Why was water freezing almost instantaneously when shaking a bottle that spent the night outside during a frosty night? $\endgroup$
    – PM 2Ring
    Commented May 7, 2021 at 12:01
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    $\begingroup$ "[...] how can it be "from" 0 degrees Celsius?" - at 0°C it can be liquid or solid, so from that point onwards (to higher temperatures) it is liquid. To explain this in detail would probably not qualify as a "simple response". What part of the answer is not clear to you? "Nothing prevents us from cooling ice to temperatures lower than 0°C" is the simplest I can formulate this. $\endgroup$
    – noah
    Commented May 7, 2021 at 13:25
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    $\begingroup$ +1 for the final analogy, which I think would have made a fine answer all by itself. $\endgroup$
    – WillO
    Commented May 7, 2021 at 13:46
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    $\begingroup$ @ShahzadRahim I have edited the answer to address your points. $\endgroup$
    – noah
    Commented May 7, 2021 at 14:20
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    $\begingroup$ @ShahzadRahim Let's ignore supercooled water for now. At 0°C, water can be liquid or solid. If you have some water and start extracting energy from it, it will cool down to 0°C, and upon extracting more energy, will stay at 0°C but start freezing to ice. In this state, the ice is also at 0°C. Only when all the water is frozen, and you remove more energy, the ice gets colder than 0°C. So you see, if I tell you I have a bowl of H2O at 0°C, you cannot tell if it is completely solid, or liquid, or partly frozen, because these are all possible at 0°C. $\endgroup$
    – noah
    Commented May 8, 2021 at 11:49
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The temperature is really just the inner energy, or the mean velocity of particles. When we do work to the material, its inner energy changes. In most cases, the temperature changes linearily with the inner energy. But when we want water to evaporate, we need to do even more work to go across the boiling point because it needs to escape gravity. So, we need to give even more energy for water to evaporate. Same is with the freezing point. It looks like this on graph: latent heat

And here comes the answer to your question: ice can be colder than 0 °C. It just so happens, that the water molecules get packed together tightly at 0 °C (we call this ice), but this doesn't prohibite it from colding even more because the molecules in ice still vibrate with some velocity. We said that temperature is just inner energy, or the mean velocity of the particle. At lower temperature the particles move slower and they can (theoretically) reach the velocity of 0. This happens at around –273.15 °C, so the ice can get cold to this temperature.

About second question: it depends on definition of water. My definition is: water is ice, liquid water and steam. So water can get hotter than 100 °C. But this isn't liquid water anymore, it is steam. When liquid water reaches 100 °C, we "rename" it to steam. So, you are partially right, because liquid water needs to wait to achieve enough heat (graph above) to go over the boiling point to steam. And yes, steam can be hotter than 100°C.

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  • $\begingroup$ Could you also please explain, whether the temperature of water can increase more than its boiling point? (Which is the second part of my question) can water be over 100 degrees Celsius? I most sincerely appreciate your response $\endgroup$ Commented May 7, 2021 at 11:50
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    $\begingroup$ @ShahzadRahim Normal pure liquid water in a cup at normal pressure cant be heated over 100°C since it will go through a phase transition at that point. But the water vapor after the phase transition can be heated over 100°C. This is shown in the graphic as well. But this figure is just a slice of a higher dimensional graph since pressure is another variable on which the properties like the boiling point, depends. At higher pressure your water will start to boil at a higher temperature and the liquid phase can be hotter than 100°C before it starts to boil. $\endgroup$
    – Hans Wurst
    Commented May 7, 2021 at 13:25
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You can make ice as cold as you want as long as you have a sufficiently good freezer. If you set your freezer to -1 C and put water in it, you will (after enough time) have ice that is also -1 C. If you set your freezer to -10 C and put water in it you will (after enough time) have ice that is also -10 C.

The thing that is special about 0 C is that it is the point (under normal-on-most-of-Earth-conditions) at which water can be solid (ice) or liquid. Once the water gets colder, it will definitely be ice and once it gets warmer it will definitely - up to the boiling point when it turns to gas - be liquid. This is the answer for most "usual" situations on Earth under normal conditions. As other answers and comments have indicated, there are other possibilities like supercooling and the atmospheric pressure also matters. But as you seem to be interested in the simplest everyday cases, I think this is your simple answer.

On the other end, 100 C is the point where the water can be gas (steam) or liquid. Warmer and it will definitely be gas. Colder and it will - down to the freezing point - be liquid. There's nothing to prevent the water from getting hotter than 100 C in theory, although in practice you'd have to trap the steam to keep it near enough your source of heat for this to happen. Again this is under the most normal everyday conditions. There are other possible effects like superheating and the atmospheric pressure also matters for boiling.

Of the two, by the way, the atmospheric pressure has some consequences the everyday person living at high altitudes at the boiling end. You will occasionally see cooking instructions for foods that give different directions for people living at higher altitudes. This is usually because the boiling point of water is lower there (because at higher altitude the atmospheric pressure is a bit lower than close to sea level). "Boiling" your food there gets it a bit less hot, which in some cases makes an noticeable difference. A pressure cooker, on the other hand, works the other way. It traps the gas in the cooker, increasing pressure as temperature goes up - This allows the remaining water in the pressure cooker to get hotter than 100 C (without turning to steam) and thereby getting the "boiled" food hotter than would have been possible in a normal pot.

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