6 ms·

I searched for "mass" and "dens" and didn't find anyone mentioning this, so: Note that hot water is less dense than cold water. Therefore, an "equal volume" of

I searched for "mass" and "dens" and didn't find anyone mentioning this, so: Note that hot water is less dense than cold water. Therefore, an "equal volume" of hot water contains less mass than the same volume of cold water. The article mentions 35°C vs 100°C; going by https://www.usgs.gov/special-topics/water-science-school/science/water-density https://www.usgs.gov/special-topics/water-science-school/sci... and interpolating, it looks like the density difference would be about 3.6%. That seems worth mentioning, even if it's not a large effect. Also, if some of the water starts at 100°C, then presumably some of it will evaporate, and one should measure how much remains... The article does mention this possibility, though it doesn't evaluate or quantify it.

joseda-hg 4y ago
Would using an sealed hidrophobic container isolate the water enough that it would condense into itself as temperature drops?
themitigating 4y ago
I always thought they use volume as a measurement for experiments. Like "we took 100ml of water..."
- eru 4y ago
  Depends on what experiment you are doing. Either mass (approximated via weight) or volume can be the right thing to do. In theory you could also go by number of particles (measured in mol).
  - OJFord 4y ago
    Agreed, if it's about 'is this the case' especially in home freezer context then it's volume that's interesting: ice cube tray sized volumes. (But also are the resultant cubes the same volume? Presumably not.) If it's 'yes this does happen, why?' then it seems right to try same mass, to test that theory.
Supermancho 4y ago
> if some of the water starts at 100°C, then presumably some of it will evaporate, Under controlled conditions, it would not. Ideally it would be in a pressure regulated space.
- autoexec 4y ago
  > Ideally it would be in a pressure regulated space. That probably wouldn't do much to help us answer the question of what will happen in our own freezers which aren't pressure regulated. While I'm glad there are and will continue to be all kinds of investigation and exploration around the Mpemba effect what I (and many others) really want to know is if heating water first would help with real world situations like having ice cubes ready in time for an event. I can accept that it might not always be the case, but considering that at this point we can't say for sure if the effect is real at all it'd be nice if we could get it settled that it worked often enough and potentially decreases freezing time enough to be worth the time and trouble of microwaving our ice cube trays even if all the details and underlying mechanisms aren't well understood. I should probably just start trying it myself and see what I come up with on my own, but even imperfect lab experiments would be much more accurate and trustworthy. I regularly do things like heat a mug full of water in the microwave, get distracted, and then forget all about it for hours.
  - jbay808 4y ago
    > if heating water first would help with real world situations like having ice cubes ready in time for an event. If the mechanism is that 10% of the water evaporates away, allowing the remaining 90% to freeze smaller by virtue of its reduced mass, the take-away for making ice in a hurry is not to use hot water. It is to use cold water, but a little less of it.
    - autoexec 4y ago
      A difference that large should show up in the resulting cubes though right? You wouldn't even need to figure out exactly how long they took to freeze or which tray froze first, just put a heated water tray and a cold water tray in together and compare them after waiting long enough for both to freeze. That'd give you an idea of how much more/less water you'd need to compensate. Honestly, smaller ice cubes is probably your best bet if you need ice in hurry either way.
legitster 4y ago
When we did this as an experiment in high school, we started with the same volume at room temperature. 500mL was brought near boiling, and another 500mL while the control was left out.
- bryanrasmussen 4y ago
  Was the 500ml still really 500ml when near boiling? https://www.cooksillustrated.com/how_tos/5684-when-to-measure-boiling-water https://www.cooksillustrated.com/how_tos/5684-when-to-measur...
achow 4y ago
It says in the article.. A more mundane explanation is that hot water evaporates faster than cold, decreasing its volume and thus the time it takes to freeze. [Edit - Cannot delete my comment now. As 'ketzo' points out below my point is not same as OP's. OP points out that hot water is less dense. Maybe less dense liquid cools faster]
- ketzo 4y ago
  That’s not the same point, actually. That quoted sentence says that hot water loses volume compared to cold water. The person you’re replying to is saying that hot water simply takes up more space as well.
  - atoav 4y ago
    So that means two things: - 1 m³ of hot water is less water (in terms of mass) than 1 m³ of cold water - 1 kg of hot water has a bigger surface area than 1 kg of cold water I am still not convinced that this alone would explain such a phenomena. I'd rather believe there is some weird fluid dynamics and layering involved.
    - wildmanx 4y ago
      > such a phenomena The singular is "phenomenon".
      - atoav 4y ago
        Thanks, English is not my first language.
        hprotagonist 4y ago
        it’s all greek to me!
        beckingz 4y ago
Zenst 4y ago
ANother aspect would be that the hotter an atom is the faster it will vibrate. So would it be that a vibrating atom is able to transfer thermal energy quicker akin to comparing a warm start to a cold start in a way that see's the thermal transfer inertia at the aromic level has better initial acceleration over the colder atom. So even when the atom reaches the vibrational speed of the colder atoms, the momentum of thermal transfer is already accelerated. Sorry if that sounds fluffy but a bit of a theory. One which trying other liquids and testing would give more insight. Or is this effect unique to just water?
- cubancigar11 4y ago
  What you are describing is Newton's law of cooling and won't have an effect here.
  - Zenst 4y ago
    aha - thank you.
- waterhouse 4y ago
  I don't think there's such a thing as thermal transfer inertia, at least at the level of individual atoms (or molecules) within a fluid. In the inertial reference frame of the fluid, each molecule just has its own kinetic energy (and maybe excitation of electrons or whatever) representing its current temperature; I'm not sure what physical state at the molecular level could capture "how rapidly the temperature had recently dropped". At the macro level, one can come up with ideas. Convection currents within the fluid, and in the air touching the fluid, maybe; though, say, once the hot water has cooled from 100°C to 35°C over a period of—what, ten minutes?—I'm pretty skeptical that there would be so much inertia in those currents that they'd persist, and persist strongly enough to accelerate the 35° to -1° cooling, enough to beat the head start of the water that started at 35°. Maybe the initially cool water forms some uniform layer of very-cool water at the top (which, being very cool, doesn't exchange much heat with the very-cool air above it; an ice layer would be an example of this), which is held together by surface tension or something; whereas with initially hot water, that layer is not uniform and there's more mixing (and constantly-created convection as a result)? I have no idea if any of that is realistic. If so, it would suggest that shaking a cup of cold water (perhaps after a few minutes in the freezer) would work as well as having it start hot. ... After writing the above, I saw that (a) infogulch below has the same idea, (b) Wikipedia has an anecdote appearing to confirm it[1], and (c) the article doesn't seem to mention shaking, stirring, or otherwise agitating the cup. [1] The Scottish scientist Joseph Black investigated a special case of this phenomenon comparing previously-boiled with unboiled water;[9] the previously-boiled water froze more quickly. Evaporation was controlled for. He discussed the influence of stirring on the results of the experiment, noting that stirring the unboiled water led to it freezing at the same time as the previously-boiled water, and also noted that stirring the very-cold unboiled water led to immediate freezing. https://en.wikipedia.org/wiki/Mpemba_effect https://en.wikipedia.org/wiki/Mpemba_effect
dhzhzjsbevs 4y ago
Density and freezing outside in could have a pressurising effect too. I'd be curious to know if the defect remains while stirring until frozen.
- waterhouse 4y ago
  Good instinct to try stirring. Search the comment thread (or the Wiki page for "Mpemba effect") for "Joseph Black", and you'll find something intriguing.
comboy 4y ago
I got into this rabbit hole once and remember having the same thought, I checked some paper and they used the same mass as measured when putting water into the freezer. AFAIR it was also tight, closed container so evaporation shouldn't have a big impact (checking mass after sure seems like a good idea). Intuitively hot water should be much slower to freeze. It takes quite a bit of time for it to get to the same state that the cold water is at the start. After that time, the way we look at it, it's at the same point in experiment as if we have just put the colder water in. So we seem to still be missing something.
- Tuna-Fish 4y ago
  > After that time, the way we look at it, it's at the same point in experiment as if we have just put the colder water in. So we seem to still be missing something. Unless there is very substantial mixing, the container of hot water will never be at the same state as the container of cooler water. We did this in high school, with 3 open-topped 10L buckets, one with 25C water, one with 95C water and no mixing, and one with 95C water sitting on a magnetic mixer. They were not in a freezer, but outside at ~-25C. The freezing order was 95C unmixed, 25C, 95C mixed. We also had a few temperature probes in the 95C unmixed bucket, and found that about halfway through, the temperature of the water in the middle about 1/3rd of the way from bottom was sitting at 0C, while the temperature of the water at middle at the top was still >70C. Our final findings were that: 1. The buckets cool mainly by evaporation, which is only happening at the top, and which is much faster for hotter water 2. Water of different temperature stratas can be surprisingly stable even in a small container 3. In the unmixed bucket a stable, slow flow of water down the sides and up the middle forms. New hot water emerges from the middle, gets cooled and pulled to the sides, and then pulled down to the bottom. This circular flow is probably caused by the fact that water at the sides get initially cooled a bit and so it sort of biases the flow. But now there's people saying it still works with closed-top containers, so I dunno. My physics teacher clearly did this every year and knew what was going to happen. The class came up with those findings after some very specific questions by him.
  - comboy 4y ago
    Very cool finding with the flow. It explains why when you take some round container and freeze water in it, the ice will be crystal clear at the edges, but it will not be crystalized so cleanly in the middle (turbulence once edges are frozen) Who knows, perhaps there's even some laminar flow happening at the edges :)
pdpi 4y ago
Density is going to factor in somewhere, sure, but when you have 100 -> 0 vs 35 -> 0, that's almost 3x more energy (given constant specific heat) you need to sink somewhere. a single-digit percentage is a drop in the ocean here.

joseda-hg 4y ago

Would using an sealed hidrophobic container isolate the water enough that it would condense into itself as temperature drops?

themitigating 4y ago

I always thought they use volume as a measurement for experiments. Like "we took 100ml of water..."

eru 4y ago
Depends on what experiment you are doing. Either mass (approximated via weight) or volume can be the right thing to do. In theory you could also go by number of particles (measured in mol).
- OJFord 4y ago
  Agreed, if it's about 'is this the case' especially in home freezer context then it's volume that's interesting: ice cube tray sized volumes. (But also are the resultant cubes the same volume? Presumably not.) If it's 'yes this does happen, why?' then it seems right to try same mass, to test that theory.

Supermancho 4y ago

> if some of the water starts at 100°C, then presumably some of it will evaporate, Under controlled conditions, it would not. Ideally it would be in a pressure regulated space.

autoexec 4y ago
> Ideally it would be in a pressure regulated space. That probably wouldn't do much to help us answer the question of what will happen in our own freezers which aren't pressure regulated. While I'm glad there are and will continue to be all kinds of investigation and exploration around the Mpemba effect what I (and many others) really want to know is if heating water first would help with real world situations like having ice cubes ready in time for an event. I can accept that it might not always be the case, but considering that at this point we can't say for sure if the effect is real at all it'd be nice if we could get it settled that it worked often enough and potentially decreases freezing time enough to be worth the time and trouble of microwaving our ice cube trays even if all the details and underlying mechanisms aren't well understood. I should probably just start trying it myself and see what I come up with on my own, but even imperfect lab experiments would be much more accurate and trustworthy. I regularly do things like heat a mug full of water in the microwave, get distracted, and then forget all about it for hours.
- jbay808 4y ago
  > if heating water first would help with real world situations like having ice cubes ready in time for an event. If the mechanism is that 10% of the water evaporates away, allowing the remaining 90% to freeze smaller by virtue of its reduced mass, the take-away for making ice in a hurry is not to use hot water. It is to use cold water, but a little less of it.
  - autoexec 4y ago
    A difference that large should show up in the resulting cubes though right? You wouldn't even need to figure out exactly how long they took to freeze or which tray froze first, just put a heated water tray and a cold water tray in together and compare them after waiting long enough for both to freeze. That'd give you an idea of how much more/less water you'd need to compensate. Honestly, smaller ice cubes is probably your best bet if you need ice in hurry either way.

legitster 4y ago

When we did this as an experiment in high school, we started with the same volume at room temperature. 500mL was brought near boiling, and another 500mL while the control was left out.

bryanrasmussen 4y ago
Was the 500ml still really 500ml when near boiling? https://www.cooksillustrated.com/how_tos/5684-when-to-measure-boiling-water https://www.cooksillustrated.com/how_tos/5684-when-to-measur...

achow 4y ago

It says in the article.. A more mundane explanation is that hot water evaporates faster than cold, decreasing its volume and thus the time it takes to freeze. [Edit - Cannot delete my comment now. As 'ketzo' points out below my point is not same as OP's. OP points out that hot water is less dense. Maybe less dense liquid cools faster]

ketzo 4y ago
That’s not the same point, actually. That quoted sentence says that hot water loses volume compared to cold water. The person you’re replying to is saying that hot water simply takes up more space as well.
- atoav 4y ago
  So that means two things: - 1 m³ of hot water is less water (in terms of mass) than 1 m³ of cold water - 1 kg of hot water has a bigger surface area than 1 kg of cold water I am still not convinced that this alone would explain such a phenomena. I'd rather believe there is some weird fluid dynamics and layering involved.
  - wildmanx 4y ago
    > such a phenomena The singular is "phenomenon".
    - atoav 4y ago
      Thanks, English is not my first language.
      - hprotagonist 4y ago
        it’s all greek to me!
        beckingz 4y ago

Zenst 4y ago

ANother aspect would be that the hotter an atom is the faster it will vibrate. So would it be that a vibrating atom is able to transfer thermal energy quicker akin to comparing a warm start to a cold start in a way that see's the thermal transfer inertia at the aromic level has better initial acceleration over the colder atom. So even when the atom reaches the vibrational speed of the colder atoms, the momentum of thermal transfer is already accelerated. Sorry if that sounds fluffy but a bit of a theory. One which trying other liquids and testing would give more insight. Or is this effect unique to just water?

cubancigar11 4y ago
What you are describing is Newton's law of cooling and won't have an effect here.
- Zenst 4y ago
  aha - thank you.
waterhouse 4y ago
I don't think there's such a thing as thermal transfer inertia, at least at the level of individual atoms (or molecules) within a fluid. In the inertial reference frame of the fluid, each molecule just has its own kinetic energy (and maybe excitation of electrons or whatever) representing its current temperature; I'm not sure what physical state at the molecular level could capture "how rapidly the temperature had recently dropped". At the macro level, one can come up with ideas. Convection currents within the fluid, and in the air touching the fluid, maybe; though, say, once the hot water has cooled from 100°C to 35°C over a period of—what, ten minutes?—I'm pretty skeptical that there would be so much inertia in those currents that they'd persist, and persist strongly enough to accelerate the 35° to -1° cooling, enough to beat the head start of the water that started at 35°. Maybe the initially cool water forms some uniform layer of very-cool water at the top (which, being very cool, doesn't exchange much heat with the very-cool air above it; an ice layer would be an example of this), which is held together by surface tension or something; whereas with initially hot water, that layer is not uniform and there's more mixing (and constantly-created convection as a result)? I have no idea if any of that is realistic. If so, it would suggest that shaking a cup of cold water (perhaps after a few minutes in the freezer) would work as well as having it start hot. ... After writing the above, I saw that (a) infogulch below has the same idea, (b) Wikipedia has an anecdote appearing to confirm it[1], and (c) the article doesn't seem to mention shaking, stirring, or otherwise agitating the cup. [1] The Scottish scientist Joseph Black investigated a special case of this phenomenon comparing previously-boiled with unboiled water;[9] the previously-boiled water froze more quickly. Evaporation was controlled for. He discussed the influence of stirring on the results of the experiment, noting that stirring the unboiled water led to it freezing at the same time as the previously-boiled water, and also noted that stirring the very-cold unboiled water led to immediate freezing. https://en.wikipedia.org/wiki/Mpemba_effect https://en.wikipedia.org/wiki/Mpemba_effect

dhzhzjsbevs 4y ago

Density and freezing outside in could have a pressurising effect too. I'd be curious to know if the defect remains while stirring until frozen.

waterhouse 4y ago
Good instinct to try stirring. Search the comment thread (or the Wiki page for "Mpemba effect") for "Joseph Black", and you'll find something intriguing.

comboy 4y ago

I got into this rabbit hole once and remember having the same thought, I checked some paper and they used the same mass as measured when putting water into the freezer. AFAIR it was also tight, closed container so evaporation shouldn't have a big impact (checking mass after sure seems like a good idea). Intuitively hot water should be much slower to freeze. It takes quite a bit of time for it to get to the same state that the cold water is at the start. After that time, the way we look at it, it's at the same point in experiment as if we have just put the colder water in. So we seem to still be missing something.

Tuna-Fish 4y ago
> After that time, the way we look at it, it's at the same point in experiment as if we have just put the colder water in. So we seem to still be missing something. Unless there is very substantial mixing, the container of hot water will never be at the same state as the container of cooler water. We did this in high school, with 3 open-topped 10L buckets, one with 25C water, one with 95C water and no mixing, and one with 95C water sitting on a magnetic mixer. They were not in a freezer, but outside at ~-25C. The freezing order was 95C unmixed, 25C, 95C mixed. We also had a few temperature probes in the 95C unmixed bucket, and found that about halfway through, the temperature of the water in the middle about 1/3rd of the way from bottom was sitting at 0C, while the temperature of the water at middle at the top was still >70C. Our final findings were that: 1. The buckets cool mainly by evaporation, which is only happening at the top, and which is much faster for hotter water 2. Water of different temperature stratas can be surprisingly stable even in a small container 3. In the unmixed bucket a stable, slow flow of water down the sides and up the middle forms. New hot water emerges from the middle, gets cooled and pulled to the sides, and then pulled down to the bottom. This circular flow is probably caused by the fact that water at the sides get initially cooled a bit and so it sort of biases the flow. But now there's people saying it still works with closed-top containers, so I dunno. My physics teacher clearly did this every year and knew what was going to happen. The class came up with those findings after some very specific questions by him.
- comboy 4y ago
  Very cool finding with the flow. It explains why when you take some round container and freeze water in it, the ice will be crystal clear at the edges, but it will not be crystalized so cleanly in the middle (turbulence once edges are frozen) Who knows, perhaps there's even some laminar flow happening at the edges :)

pdpi 4y ago

Density is going to factor in somewhere, sure, but when you have 100 -> 0 vs 35 -> 0, that's almost 3x more energy (given constant specific heat) you need to sink somewhere. a single-digit percentage is a drop in the ocean here.