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I have a character that can control water (rain, specifically), and am curious if it’s possible for them to turn these droplets solid via pressure rather than built in ice magic. If they exert enough pressure onto the raindrops, could they make them solid? And if so, what happens when they fall and hit the ground?

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    $\begingroup$ It will be nearly identical to hail, it really depends on how much power your mage has. $\endgroup$
    – Zautech
    Mar 6 at 6:04
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    $\begingroup$ Maybe you can stir the pressurised water like how ice cream is made, the air goes in to prevent the pressurized water from turning into a giant block of ice, but now I need to finish my ice cream. $\endgroup$
    – user6760
    Mar 6 at 6:36
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    $\begingroup$ While your character cannot make raindrops solid, I suspect that they could make them act solid by constantly exerting an inward telekinetic pressure on each raindrop, which would make it much harder to deform the raindrop, which in essence would make them feel solid as they hit you $\endgroup$
    – M S
    Mar 6 at 15:24
  • $\begingroup$ does this help?. $\endgroup$
    – Pelinore
    Mar 6 at 18:22
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    $\begingroup$ @user6760 Not sure where you got that idea about how ice cream is made. When I've made it, it was in an open container. $\endgroup$
    – JimmyJames
    Mar 6 at 19:22

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No, kinda, just be careful what you ask for

Here's the simple chart. Click it to enlarge it. As you can see from the simple graph, there is no temperature where you start with liquid water where you get ice as pressure increases.

enter image description here

Here's the more complicated graph. Once again, click to enlarge. Now you can see that if you increase pressure far enough, you'll get ice. But...

  • You have to get into the ~70GPa (you know, ~70,000,000,000 Pa, sea level is 101,325 Pa).

  • And what you get is phase X and phase XI ice. It's not just "ice." The properties are a bit different and heaven help whomever is nearby when that pressure is released.

enter image description here

So, science-based answer, for all practical purposes, no, you can't reasonably increase the pressure to get ice... at least not without dealing with all the consequences that goes with it because so much pressure is needed. Science is indeed a two-edged sword.

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    $\begingroup$ If a mage could compress water this much, imagine what he could do to the water inside of an enemy. $\endgroup$
    – Nosajimiki
    Mar 6 at 14:49
  • $\begingroup$ On releasing the pressure, the ice would just turn liquid again, without fireworks or anything. Or rather it would turn into regular ice, because it would cool under the expansion. Of course to pressurise a rain drop, you have to pressurise the surrounding air, too ... $\endgroup$
    – Karl
    Mar 6 at 20:12
  • $\begingroup$ Supposing they can internally generate this level of pressure by magical means (not using the surrounding air but the water itself), if this pressure was released suddenly, would it cause an explosion? What sort of consequences arise? $\endgroup$
    – Suavé
    Mar 6 at 21:10
  • $\begingroup$ @Suavé There are aspects of this I can't explain. Normally ice is less dense than water, but with a constrained force like you're suggesting, it must have a density higher than water. That might be an aspect of type X or type XI ice (that's the part I don't know enough about). But, let's ignore all that - we have a path to success. But, how much damage due to the release? Using a handy tool we learn that the massive force is equal to the explosive force of a bit less than one ounce of TNT. (*Continued*) $\endgroup$
    – JBH
    Mar 7 at 3:09
  • $\begingroup$ Now, so far I've looked at the massive pressure needed... but not at the itty-bitty amount of water used. I don't know enough chemistry to explain to any detail, but that ounce of TNT will likely be reduced to a firecracker due to the small amount of water involved. However, what's the point of converting just one drop of water? Given its weight, solid or liquid the wind could move it out of the way of, say, a target. Now, convert thousands of raindrops or millions and what to you get? An aggregate explosive force (ignoring some important complexities) of about 6 tons of TNT. (*Cont*) $\endgroup$
    – JBH
    Mar 7 at 3:12
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Ice floats on water. That is because ice is less dense than water. It will take very high pressure to make water solid at room temperature. If the mage magically squeezes water into solid form, and then lets go, the water should immediately revert to the previous state.

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  • $\begingroup$ Violently? Can we weaponize this? $\endgroup$
    – Zautech
    Mar 6 at 7:37
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    $\begingroup$ Actually, the ice would more likely violently sublime into steam and explode if pressure drops that hard. $\endgroup$
    – Trish
    Mar 6 at 11:26
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    $\begingroup$ "If the mage magically squeezes water into solid form, and then lets go, the water should immediately revert to the previous state" probably not, or perhaps not, my gut is telling me that if you compress it you will generate a lot of heat > meaning that you'll be driving the heat out of the water > giving you super cooled water > release the pressure at the right point so it can expand and the super coold water will instantly form into ice // but I'd have to go look at stuff to see if my gut was right about that. $\endgroup$
    – Pelinore
    Mar 6 at 18:30
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    $\begingroup$ @o.m. the point is that pressurizing anything drives energy out of the thing in the form of heat, you're still just applying pressure, radiated heat is a byproduct. $\endgroup$
    – Pelinore
    Mar 6 at 18:46
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    $\begingroup$ @o.m. when you have both [science based] and [magic] tags it's only sensible to assume that they're asking for among other things the science based byproducts of doing this or that with magic, "if you could magically do this (whatever 'this' happens to be in the question) what are the real world science based side effects of doing this thing" is often writ large even if unspoken in any question that includes both of those two tags, the effects are science based, the thing (compressing stuff with magic) is magic, what I've said does not question the magic one jot, you are wrong to suggest so $\endgroup$
    – Pelinore
    Mar 6 at 20:04
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If you look at the water phase diagram, you see that it is possible to make liquid water turn solid by applying pressure in the order of some tens of kbar.

enter image description here

As long as the pressure is applied, the water would stay solid, and act as ice. In this case, it would not be distinguishable from hail or snow.

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Okay. Nice answers so far. The temperature / pressure diagrams are surprising to me in this context... though I have seen these bfore.

I think the conclusion drawn ("No, not possible") in one of the other question is mistaken.

However, the "mistake" is simply to take your question at face value. Let's suppose you had put your questions slightly more generally and you had asked the almost identical question,

I have a character that can control water (rain, specifically), and am curious if it’s possible for them to turn these droplets solid via pressure rather than built in ice magic. If they cause a sufficient increase or decrease in pressure , could they make them solid? And if so, what happens when they fall and hit the ground?

If that is your question (apologies for the minor change) then the answer is, 'Yes!'

That is, they could cause a rain drop to freeze by doing this,

  1. temporary increase in pressure.
  2. while the temperature is increased, although the liquid will not solidify, it will increase in temperature and in doing so, there will be a temperature gradient that causes heat to diffuse from the (pressurized) water droplet into the surrounding air. If this gradient is maintained long enough (perhaps just a few seconds depending on the gradient -- ie depending on the pressure increase) then,
  3. When the pressure increase is relieved, the pressure will of course drop and the temperature will decrease as a result (since some heat energy diffused out of the pressurized droplet in step 2, while the presssure was high). You can see from the chart, and common sense, when the temperature falls, the water will freeze.

This assumes that the air can absorb enough heat from the (temporarily) pressurize droplets without heating up so muhc that the heat is instantly transfered back into the water droplets when the pressure is restored to normal... but that seems to be possible since the droplets are (I Presume) falling through the air from to a region where the air is heating (due to teh pressurized water droplets) to a region where -- a moment prior-- there were no pressurized water droplets. ... this suggests that there are some limites on this technique. There is no free lunch. But it seems reasonabel to suppose that there is at least a leading "front" of falling / freezing droplets that survive any reheating process to remain frozen long enough to reach the ground in solid form.

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    $\begingroup$ Please check point 2, I think you have miswritten something. That apart, you are supposed the question as asked, not to make your own question and answer it. $\endgroup$
    – L.Dutch
    Mar 7 at 6:05
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Let's suppose you can somehow pressurise raindrops without pressurising the surrounding air, right? You need about 10 kbar to get to a solid phase water above 0°C, and air at 10 kbar has the density of a solid object, that doesn't sound handy.

Compressing the water will release quite a bit of heat, so does the phase transition to ice. The solid particles fall, and transfer the heat to the surrounding air. At that point, you can release the pressure, which will turn the high pressure ice modification into regular ice that's significantly colder.

So whomever you're aiming for with that get's caught in a hailstorm. Not very nice!

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