My freezer is set at -21°C (-6°F) and tubs of ice cream come out hard as a rock and are near impossible to scoop. But if I set it a few degrees warmer, yet still way below the freezing point of water, I can scoop it easily. So, is there such a thing as both frozen and *really* frozen? Conversely, a boiling point is a boiling point, I believe. Heating water to a temp above 100°C gets you the same steam that you got at 100, just faster. Right?
In: 102
My ELI5 take on it (excluding ice cream, of which I assume you read the great answers prior to this one): Ice (and we’ll assume pure water, only H2O) is made up of molecules. Which are kept apart by their energy levels and repulsion of electrons. As the liquid water cools the electrons contract, less kinetic bouncing around, and the atoms come closer together. Eventually, if enough energy is given off (taken away) they start to cling together in lattice arrays. Some solids form different shapes…ice being an aligned pattern keeps it clear. Getting colder will allow the atoms to scrunch together a bit closer…but not at the temps/pressures your freezer is able to perform and for you to determine a noticeable impact. You can also form solids at greater pressures…where the pressure constrains the atoms…it’s why water can boil at room temperature in a vacuum. The molecules aren’t constrained by pressure and have ‘room to spread out’.
Also, remember your freezer isn’t always the same temperature. It likely has defrost cycles to keep ice from building on walls and surfaces. Much like freezers that have to be defrosted manually, if air vapor gets in it crystalizes on the surfaces of your food. Your freezer operates when a thermostat tells it to…which means the thermostat temperature sensing device is the only ‘reasonably same temp’ spot in the freezer. Items close to the walls will gain more heat than ones in the center, if the heat gets high enough to warm the temp sensor/bulb then the compressor runs. So food goes through temperature changes inside the freezer…minor and small changes, but changes nonetheless. Which can impact how food lasts in a freezer. And as others have pointed out, ice cream is a finicky blend of stuff.
Heating water ‘faster’ is a bit of a long thermodynamics type answer. Energy is produced, or added into the material, slowly heating each atom/molecule…which vibrate more and more. More kinetic energy more ‘heat/steam’ – it’s based on factors such as surface area, and contact time. But simply…yes, more heat usually means more/faster steam. But steam is an expansion of molecules into the air…and boiling in a bottle would increase the pressure and change the overall system. It is a balancing act between pressure, temperature, gravity (at least for our tests), and quantity of material. A good example of this, is; you can heat water and boil it in a plastic container. Because the water pulls away the supplied heat faster, and at a temp less than that of melting plastic, than that which water boils at. A pressure cooker prevents the water from boiling until higher temperatures are reached…as 212F (100c) is the hottest (at sea level on earth) you can get water before it wants to start transitioning. Up on a mountain the water may only get to 180f or 170f…or possibly low enough to not kill bacteria (arguably 165f depending on who you ask…but more reasonably at 140f – no one ‘cooks’ on everest…but it’s 154f for boiling up there).
Yes, things can be frozen more or less solidly at different temperatures. Neil deGrasse Tyson talked about this on a recent episode of StarTalk if you’re interested in hearing more about it!
Water will not freeze at 0, it needs to go a bit lower than zero to freeze. Likewise ice won’t melt at zero, it needs to go a bit above 0 to melt
Ice cream is a bit of a plastic-behaving substance, so it will be harder at lower temperatures. Depends on the mixture, what is in it, but it is a mixture and not a solution, which is why it has that plastic behavior. It can deform without fracturing. As with most plastic substances, there is a transition zone, a range of temperatures where the material is a solid but still readily deformable.
As to the phase change question, a pure substance will give a pure gaseous version at the boiling point and will not rise above the boiling point until there is no liquid to accept the heat and create vapor.
The big issue with ice cream and freezing is that it is a very impure mixture, so the water ice part of the issue is only a part of the reason for its behavior. The water will (mostly) freeze at a temp a little below 0 C (usually salt in the solution so freezing temp is lower, if I understand ice cream making correctly, which I have never studied or tried so could be wrong). The fatty stuff though solid is not crystalline and not easily made into crystalline substance in the presence of all the water and salts. The fatty compounds can slide past each other, so “plastic” behavior. Have to get really cold to make the fats hard to move.
The character of ice changes as it gets chilled well below the freezing point of water. I used to do a lot of ice carvings for fancy buffets. When the 300 pound carving block arrives it is chilled to near zero degrees F. way below the melt/freeze point of water. It is extremely brittle and cannot be carved without shattering until it is allowed to “temper” at close to the melting point.
without going into too much detail, I believe ice will become slightly more *brittle* (which isn’t the same as hardness). As will most things as they cool further. Your ice cream situation however is a mixture of many things not just water. Kind of how alcohol doesn’t freeze at 0 despite being a majority water.
Alternatively, steam isn’t always just steam. it can become something classified as superheated steam, and then eventually it can become something called plasma and obscenely high temps/pressures.
For funsies, its also possible for water to exists as liquid, solid, and gas *simultaneously* at the *triple point.*
Water freezes solid at 0C. If you maintain 0C long enough, it will be “solid”.
Ice cream is not water. It has a lower freezing temperature.
The reason a freezer is set lower than zero, is because you are not just freezing water. You are freezing food with variaying ingredients. Stuff with salt in it and such. 0c probably wont fully freeze everything in there.
As for boiling points. Being much hotter will convert more of the water to steam, which blasts more steam out. Also consider air pressure effects what temperature something boils at. So people who are at high elevation will have there water boil before 100c
I actually had to take a dive in to the subject and I did find some graphs about this. But couldn’t read much further other than abstracts and maybe few graphs, because fucking everything is behind a paywall!
But indeed it would appear that ice’s compressive and tensile strength, along with strain rate, does increase as it gets colder. However, there are way more variables to this than just temperature, including grain size and crystal structure which is a complicated thing of it’s own.
But there are different types ice we know of. As someone who lives in Finland and where seas freeze there is a term for this really strong sea ice, called “Teräsjää” *steel ice*. Which is the “default ice” that all other ice is measured against. It is what is used to calculate whether ice roads (as in road built on frozen sea) can be used to access places.
But about boiling. It isn’t as simple as that. You can check the phase [diagram of water](https://upload.wikimedia.org/wikipedia/commons/0/08/Phase_diagram_of_water.svg), from which we can see that at 1/1000th of bar pressure, water boils at -25Celcius (249K). Going up, we know that at 10 bars water boils at 200 Celsius (474K).
But ice does sublimate at any temperature. Basically that if air is has less humidity in it than it can carry, water molecules will move to it from ice. This is why if you put a tray of ice in to the freezer and leave it there for a long time it disappears. Just like water when liquid will evaporate in to the air if it is dry, no matter the temperature.
In the boiling situation, for example, you physically cannot heat liquid water above 100 C at 1 atm of pressure because the liquid transforms to vapour.
On the opposite surface of where the heat is being applied, i.e., where the bubbles are forming, the vapour bubbles can continue to heat above 100 C and form a kind of ‘insulation’, so that’s why the temperature at that surface might be higher than the bulk liquid.
The temperature difference between the heat source and the thing you are heating controls how fast the energy is transferred. So if the heat source is 110 C, it will take a very long time to start boiling, and if it’s 800 C gas flame then it will go faster.
Related to what I said above, if the heat source is extremely hot, you will form a layer of insulating bubbles and the speed after some point will slow down again. This is called film boiling, and the good efficient kind of boiling is called nucleate boiling.
For the ice cream situation, ice cream is complicated. You will probably find that it scoops easily when it’s new, but then gets harder every time you remove it. The post by /u/honey_102b explains in more detail.
Freezing just means “has transformed from a liquid to a solid”, so a ‘really frozen’ doesn’t exist.
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