Boiling and Freezing Water

Generally yes in both cases. You can rig up very special cases where hot water freezes faster than cold water, but you really have to work at it and the effect is small. Old urban myth.

Does water boil faster if you start out with hot water as opposed to cold water?

Yes especially if you start with it at 99^oC

Del

Un earth or in space, because it also largely depends on the airpressure at what temperature water boils (or freezes)

Himmm...it depends on the location as well. Sould boil faster on the moon instead of Jupiter.

Unless you happen to want to boil and freeze it all at the same time triple point

Ronseto

I was taught that warm water freezes faster, in time, than cold water. I know that is subjective, but the idea is that there is a cooling plateau colder water has to overcome on its way to the freezing point. Why? It seems like there is some value of velocity to warm water reaching its freezing point before cooler water. We did an experiment and it held true(about 30 yrs. ago).

Heating water is a different story, I believe.

http://www.usd.edu/~kmccolli/Hot%20Water%20Vs%20Cold%20Water%20Freezing.htm

Assuming the same amount of power applied to cooling the water, is the "warm" water any different at the moment it reaches the temperature of the "cold" water?

I would guess any observed difference was due to an error in procedure.

Energy is mass.......mass has momentum.......

I think you are attempting to make an analogy with motion that does not apply to thermodynamics. When you quit applying energy to the system, it doesn't magically keep getting hotter or colder just because it was previously going in that direction. Where would this magical energy come from? Sorry, when you quit heating or cooling something its temperature quits changing. It's that simple.

Actually, it occurs to me that your analogy, beyond not applying to thermodynamics, doesn't even work in the context of the laws of motion. What you propose is to say that because of an initial high rate of acceleration (dv/dt) that a mass should continue to speed up after you quit adding energy to it. If that were true, a bullet would continue to speed up after it left the barrel. It doesn't. Once you quit adding or removing energy from a system, its velocity or temperature becomes subject to the effects of the background environment and it will instantaneously begin to move in whatever direction that those background forces causes it to go. For the bullet, friction and gravity will effect it. For the heated/cooled mass, it will move toward the ambient temperature, unless of course it is already there and it will do so instantaneously, at the interface boundary. Therein lies the point of confusion with liquids. The boundary of interface is not clear nor is it singular. There are inevitably swirling gradients all through the liquid due to unseen convection currents and the inability to accurately characterize these leads to erroneous conclusions about what has actually happened.

Not to be too pedantic (well, OK, to be very pedantic ), energy is not mass; mass is a form of energy. And, mass does not have momentum; I have a massive cat sitting on my desk half in front of my monitor that hasn't moved for a long time.

You might like this thread;

http://cr4.globalspec.com/thread/6446/A-story-of-a-cup-of-Milk-water-and-ice-cream-in-refrigerator

Hello,

The problem is the same independent of matter you boil/freeze, water, steel, lead, tin, copper....

If heating/cooling power is constant, usually the rate of temperature change is dependent on the temperature gradient between your subject and its surrounding environment: If you let a ladle with a melt metal above its melting point temperature (overheated) and plot the temperature versus time you'll see that the curve is steeper at the beginning, then goes down slowly till freezing point is reached. At this point, temperature remain constant (as far as you have a pure element or eutectic alloy) because the energy loss is through latent heat instead of temperature change. Once completely solidified, the temperature gets down with variable speed, going slower as gradient is lower.

Reverse way is the same.

Kind regards

Up here in Canada when it is below -35C if you throw a cup of boiling water into the air it freezes before it hits the ground.You can hear the sound of it vaporising into ice crystals.

Oh that must be cool...er, cold!!

I tried to supercool water once...of course it failed coz it's warm here, in India!!

Yep, it does. Some snow machines use that principle.

Of course, droplet size has a major impact on the freeze rate as would levels of contaminants in the water.

The issue of rate of freezing (a heat transfer rate problem) of water is generally related to the heat transfer conditions and not to the heat capacity and latent heat issues.

Your proof makes sense, my dad made us a slide slope that way and told us about hot water freezing faster than cold.

If I remember my high school physics correctly, water/steam skips the liquid calorie/btu transition stage and goes directly to crystaline/ice stage requiring less time and energy to occur. Note it's the steam that this happens to and is probably where the old adage came from that hot water freezes faster than cold water, the error is in what is the definition of hot in this case.

I was in a discussion once where the comment warm or hot water "freezes better" meant that it produced clear ice (less dissolved gas) when compared to cold water. That was interperted that better was was faster and caused much confusion. ROSETO, I am in Mobile, is it snowing where you are in MS?

Paddler

As my physics professor explained it the the energy difference at super heat and latent heat is high enough that the energy to just change the liquid between the two is fractional. Check the numbers to figure out the ratios. (1979 so I don't remember what the final outcome was).

If that does not cover the difference completely it may be possible that the gas absorption in colder water changes the heat conduction transfer from the container to the water and from water to water. An instrument calibration shop could test this or may even know if the absorbed gasses in water change the heat conduction properties of water.

Just a thought

Brad

Just thinking(off on a tangent as usual)

The difference between snow and how it forms---Hail and what conditions cause it---And freezing rain.

Water freezes at 32°F @ sea level

Independent of any anticyclone or low pressure area surrounding the place?

What sea level? Dead sea level is the same as Atlantic or Pacific?

According to Gibbs a pure matter phase change temperature is constant for a given pressure. Changing pressure (even slightly) results in temperature change (even slightly)

Kind regards