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
On a molecular level, ice is ice (there are exotic forms of ice, but none of them will be forming in your freezer). However, the hardness of a substance is based on a lot of structural factors that take place above the molecular, crystal level. Think about snow – its solid parts are pure ice, but is much easier to scoop than hard ice because all of its ice crystals are contained in small, separate flakes which are not strongly attached to each other.
Which brings us to ice cream. Ice cream is not just water, nor is it a uniform structure. There are a lot of proteins, fats, and (if it hasn’t been melted) air pockets in between the water crystals. Even the water itself is not uniform – there is sugar dissolved in it, and there will be tiny pockets with higher concentration of sugar it and pockets with less. The freezing temperature of water changes the more sugar you dissolve in it, which means that the water in the ice cream will not all freeze at the same time or at the same temperature.
What this means is that depending on the temperature of your ice cream, different amounts of it will have actually turned to ice. At extremely low temperatures it’s like solid ice with protein and fat molecules stuck inside it. At higher temperatures close to freezing it’s mostly liquid sugar water held together by a few “threads” of purer ice, which can be easily broken, making it easier to scoop.
No. There is no such thing as frozen and really frozen. Before looking at ice cream, we have to understand molecules. As you already know, water is an oxygen atom connected to two hydrogen atoms. When molecules bind to each other, they become bound which is the frozen state. Now when frozen, the molecules are bound like bricks on a house. They can’t “move” but they can vibrate. So, at the molecular level, they cannot freeze harder when they are already bound to other water molecules like bricks.
Ice cream is obviously different, not exactly pure water. For this instance, Impurities are classified as things different from a water molecules like elements ranging from sodium, iron, magnesium to complex molecular compounds like citrus, fats, proteins, carbohydrates, and salts. We have created ice cream combining these elements and compounds with water. The impurities is what makes it easier to break or weaken a substance, in this case ice cream. That few degrees warmer is the “breaking/scooping” point of those impurities because they are not frozen even though water molecules are frozen within the ice cream.
Answer your other question, yes. If you add more energy you reach your goal faster.
I don’t know the science behind it, but I have always found it interesting (and sometimes frustrating) that ice rinks vary. The surface is supposed to be at 22-24f for ice hockey and 26f for figure skating. It makes a huge difference. (I may have these temps off but point still stands, different frozen temps for different sports.)
Something made purely of a single type of molecule (pure water) does change state sharply at a fix temperature (for a given pressure but as long as you are in open air, let’s ignore the pressure).
Anything that is not pure (ice cream, any salt/water mix, sugar/water mix, and even metal alloys like steel) start behave silly. Because inside there are molecules that are apply to freeze and others that don’t yet have any will to freeze. So there’s a transition range in your ice cream, where some ice crystals do form inside making it not liquid anymore, and it start to behave like a gel, then you cool it more and more crystals are formed and the ice cream can stand up but is not solid, and then if you freeze it more all the molecules inside become crystals and ice cream becomes rock solid.
There’s an internal fight in the ice cream between some more watery parts that want to get solid ASAP and some more greasy parts that want to stay liquid for longer.
In this transition the inside of your soft ice cream would look like wet sand if looked with a very powerful microscope.
Note: one of the most strange things I ever seen is when I grind titanium parts. If you make it hot enough, the alloy melts on the grinder while at the same time it stays together like solid steel. It’s like trying to make a clear cut into butter in a summer day, and instead of cutting it goes all over the place. And it’s at 1000 degrees and with the strength of steel. Awful material to work with.
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.
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.
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
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.
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.*
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.
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