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
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.
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!
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).
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