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Stumped by a 10 year old

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Gravity_Ray

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This afternoon one of my nephews asked me if water freezes at the same temperature that ice melts?

Now I know I am not that smart, but you’d think this would be general knowledge. My guts told me “of course”, but then I started thinking (which my wife has told me is when I get in trouble) and I figured well I know pure water can go below 0 degrees Celsius and still be liquid because water needs some kind of particles in it so that it can convert to ice around these particles. And I have seen ice cubes above 0 degrees Celsius. So I over thought the problem.

Anyway I started explaining all these complicated things to my nephew and he cut me off and said, “The answer is yes”.

A simple answer for a simple question that totally stumped me… “And a child will lead them”. Isaiah 11:6
 
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SteveCNC

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From my old chemistry class I took in college , hydrogen bonding occurs at 4 degree C if I remember right , as for melting I would think it would start at 5 degree C perhaps .
 
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neilsox

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I think gravityray is analyzing correctly and Steve was misinformed or misunderstood. Usually melting and freezing occur at zero c = 32 f. Water has it's maximum density at 4 degrees c. Impure water typically freezes at lower temperatures. Usually the impurities are left in the portion that has not frozen yet = are not dissolved in the ice, so the ice melts at zero c or not much cooler. Super cooled water can occur if there are no nuclei to start the freezing, so "of course" is not exactly correct. Your wife also is missing that the truth is often in the details. Neil
 
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SteveCNC

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I have an odd memory for numbers and that particular number was on our final exam so that's why I still remembered the bonding temp .

As the temperature of liquid water is reduced, random movements of the
water molecules are likewise reduced. At 4 C, molecular motion has been
diminished to the point where the hydrogen bonds that held them together
in the liquid phase are beginning to settle into an optimum (best)
arrangement that will ultimately bind them together in the ice
latticework at 0 C. From about 4 C to water's freezing point at 0 C, the
molecules are no longer able to so easily slip past each other as they
did in the liquid phase.
water at 4 Degrees Celsius

that is the point where ice crystals begin to form on the surface generally at least in chemestry class ~20 years ago , it won't freeze solid till 0 C at sea level , course my college was on a hill maybe 800 foot above sea level not sure that would make any real difference . I happen to still have every text book I ever bought for college and my chem book was pretty handy but I couldn't post it without a lot of trouble .

We didn't do the melting temp but it is likely any temp above 0 C , after re-reading the text it was only beginning to occur so it wasn't stable ice crystals at 4 C they don't get stable till 0 C .
 
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Shpaget

Guest
Are you saying that you can freeze water even if you cool down to no less than +3 degrees Celsius? (assuming normal atmospheric conditions apply, seeds for ice crystals are available and no freezing point altering chemical is present)

Doesn't sound right.

From my chem classes and memory that still serves me somewhat the answer is following:
The temperature is the same, but for the change of state to occur the heat must be provided (for melting) or taken away (freezing).
Meaning you can have both ice and water in the same container at the exactly the same temperature, provided that you keep it perfectly (or at least adequately) insulated from the environment.

It is called triple point and for water it is 273.16 K (0.01 °C) with partial vapour pressure of 611.73 pascals.
 
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trumptor

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I agree with Shpaget, that's the way I learned it. Also, you may see ice at greater than 0C but the ice is melting. It may take time for it to melt because it needs to absorb a certain amount of heat just to change states from solid to liquid.

Latent heat is absorbed or released during any changes of the state of the water and the temperature should remain at 0C until the phase change is complete from solid to liquid or vice versa.

I've never heard of water requiring a seed particle on which ice would form from the liquid.
 
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adrenalynn

Guest
The short answer to the short question would be that water freezes and thaws at the same temperature.

It's a state-change, which means it doesn't happen spontaneously or all at once. As energy is given or taken the state will begin to flop over. The more subsequent surface area exposed to the molecules "looking" to change state by those that have, the more rapidly the state change will begin to cascade, as well as the more energy that is imparted, the faster the process will occur.

As far as speculating an "exact temperature" - I haven't heard anyone asking the all-important "at what pressure" question yet. :)
 
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dangineer

Guest
Some of you guys are pretty close to the answer I was looking for. Basically, water freezes and melts at the same temperature. Whether its freezing or melting depends on the net direction of heat flow, NOT the temperature. The freezing/melting point can change and depends mostly on pressure and salinity. The fact that water does not freeze all at once is due to a number of factors, such as non-uniform conduction patterns within the water (due to temperature gradients, i.e. not all of the water is at the freezing point), the presence of particles and non-uniform salinity concentrations. So, the answer is no, water does not freeze and melt at different temperatures.

An interesting note, water that would normally freeze at 0 C can freeze while the ambient temperature is above 0 C if enough heat is lost through radiation.
 
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MeteorWayne

Guest
And it can also exist as liquid well below 0C if not disturbed (see supercooled water)
 
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orionrider

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Is a 100cc glass containing 50cc of water half full or half empty?

If you were filling the glass it would be half full.
If you were emptying the glass it would be half empty.

So it depends on the previous state of the situation. Same for the freezing/thawing cycle.
 
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adrenalynn

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orionrider":2ib5lmxz said:
Is a 100cc glass containing 50cc of water half full or half empty?
It's half dirty, and you can't wash just half your glassware. Believe me - I've tried. :roll: :lol:
 
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SteveCNC

Guest
orionrider":121o015u said:
Is a 100cc glass containing 50cc of water half full or half empty?

If you were filling the glass it would be half full.
If you were emptying the glass it would be half empty.

So it depends on the previous state of the situation. Same for the freezing/thawing cycle.
hmm seems to me the glass is twice as big as it needs to be .
 
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CalliArcale

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MeteorWayne":10jr970x said:
And it can also exist as liquid well below 0C if not disturbed (see supercooled water)
And there's the question of purity as well. Adding salt will affect the melting/freezing point (which is why it gets used in old-fashioned ice cream manufacture, and to battle road icing in the winter).
 
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orienteer

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SteveCNC":13kdt8ir said:
orionrider":13kdt8ir said:
Is a 100cc glass containing 50cc of water half full or half empty?

If you were filling the glass it would be half full.
If you were emptying the glass it would be half empty.

So it depends on the previous state of the situation. Same for the freezing/thawing cycle.
hmm seems to me the glass is twice as big as it needs to be .
Not if you are freezing the water, since 50 cc of water makes 55 cc of ice
 
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orionrider

Guest
Water cooling down is dumping energy, but e=mc², so in fact it becomes lighter, bending space-time less than before, contracting in the process... :shock:
This is not fair... :cry:
 
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