[alexander]

well, it depends on how you setup the experiment, if you boil both waters first and then reheat them, and use distilled water, then the chance of those being the reasons this happens, are zero to null

[Tolouse]

Originally Posted by modest

The perfect science fair project

i remember someone in middle school (back in the early 90's) someone did do this as a science experiment

i never quite got the concept on how it worked though, and still find myself wondering how the molecules bond when they are just moving at such a fast speed whereas the cooler molecules are slower thus should be able to bond a lot quicker

maybe i'm just thinking too much about it, but since middle school, i have used hot water in my ice trays more than cold water

and everyone just thinks i am weird even after i tell them that hot water freezes faster than cold water

[modest]

Originally Posted by Tolouse

i remember someone in middle school (back in the early 90's) someone did do this as a science experiment

i never quite got the concept on how it worked though, and still find myself wondering how the molecules bond when they are just moving at such a fast speed whereas the cooler molecules are slower thus should be able to bond a lot quicker

maybe i'm just thinking too much about it, but since middle school, i have used hot water in my ice trays more than cold water

and everyone just thinks i am weird even after i tell them that hot water freezes faster than cold water

Yeah, if you just think about it in terms of temperature then it doesn't make any sense. I think that's why people have trouble believing it. But, there is a lot more than just temperature as a single property of the water. There is how much gas is dissolved in the water. There are convection currents distributing the water differently between the two. The colder one may form a thin layer of ice on its surface preventing further evaporation while the hotter one continues to evaporate until it gets far colder. There are so many variables. That's why I think it's a good science fair project. All those graphs and controls - not to mention; it's unbelievable and therefore interesting.

-modest

[HydrogenBond]

The mechanism hasn't been settled. Evaporation, dissolved gases and thermal momentum have all been proposed. One of the latest ones has to do with the structure within water. At colder temperature, the water is more hydrogen bonded but in shapes that are somewhat different than in ice. To form ice there needs to some rearrangement. In the warmer water, the structure is more open so the molecules can rearrange faster into ice.

A loose analogy is two people building a card bridge. One is starting with a deck of card and the other with a card house. The house looks closer but we got to carefully remove some of the cards to make this into the bridge. By then the person with the deck is catching up and finishes first.

Plato proposed water was an icosahedral. It turns out he was right in the sense of hydrogen bonded structure, which turns out to be more common in the cold water. The figure show a transition between two states.

[alexander]

Modest, let me, and this is not to be taken as a hostile action or anything of that matter, reflect back on your post.

You say that it depends on the amount of gases dissolved in the water, however, as i have said before, the cool water can have the same level of gas absorbed in it as the hot water, the fact that the hot water is not boiling, keeps the levels about equal throughout the experiment. You could also used freshly-made distilled water, one that will not have any gas dissolved in it, and the experiment would still turn in favor of the warm water

If you think of it temperature-wise, it really does not make sense, but, if you think of it in terms of energy, and steam, the picture starts to become clearer. Then take into acount the convection currents, that distribute the chilled water on top, to the bottom, perhaps if the top of the water is infact brought down to a lower temperature, but the rest of the mass of water is still fairly warm, you will have convection currents going, even though, the water leaving the top is colder then the water on top of the cold-water comparison. Mind you the water is only said to freeze at 0 degrees C, in reality, much of the body of water gets super-cooled prior to freezing, storing enough energy to turn to ice (and that takes a lot of energy, less, though, then turning the water to steam).

Then you think about the colder water, as a body that will cool fairly uniformly, and when it reaches 4 degrees C, the convection flows will stop, and the colder water will stay at the top, such are the density properties of this weird liquid.

The other thing is heat, may change the surrounding "microclimate" of the area around the glass, releasing more heat, will in term cool the water more. Also there may be consideration of water vapor forming on the outside of the glass, that may aid the thermal bond of the glass to the shelf it stands on, thus once again, drawing out more energy, leading to shorter freeze times.

But this is in line of questions like "can you boil water, using a burner and a single piece of paper, or a plastic bottle" and so forth, i'll try to come up with more

[modest]

Originally Posted by alexander

Modest, let me, and this is not to be taken as a hostile action or anything of that matter, reflect back on your post.

Least hostile post ever

Originally Posted by alexander

You say that it depends on the amount of gases dissolved in the water, however, as i have said before...

The point of my post was that there are many variables besides just temperature. Dissolved gas is necessarily one of those. You correctly point out a good control to eliminate that variable. We could also agitate the water and eliminate convection or distribution as a control. We could seal the container and eliminate evaporation in a control. I was pointing out that there are many variables to test in this manner. It looks like you agree, and in fact, I agree with the rest of your post.

-modest

[alexander]

sweet, well i've been looking at some more experiments you can do with water for science fair projects

1:
floating paper clip (using the surface tension to make people wonder). take one paperclip and unfold it to use as the lowering tool, put another paper clip on top, lower the paperclip into the water, slowly, once the paperclip on top floats off, you can remove the tool clip
1.5:
now drop a drop of dish soap into the water, see what happens

2:
using water as a prism, in a clear round glass, filled with water, you put a mirror angled upwards. turn the lights off, and use the flashlight to acheive a rainbow

3:
Unmixable water, using 2 jars (baby food jars work well), some water, and a playing card, and 2 color dyes, you can do the following. First, fill the bottom jar with cold water, add some food coloring, like blue, mix it, finish filling the jar, all the way, to the rim, until water looks like it's almost about to run out.
take the second jar, put a little hot water into it, add some food coloring, different color from the first jar, and also put in enough warm water to where it looks like it's just about to float over.
take the playing card, and put that on top, tap gently to remove air, and make a "seal" in a sense, where the card is in flushwith the top, and there is no air in the jar. now, put your hand over the card, and flip the card and the jar over. now position the contraption over the first jar (make sure the lines meet, to prevent leaking). now, carefully remove the playing card from the middle. (what do you think is going to happen?)

off to find more fun things to do with water

[alexander]

oh, i forget, all the cool effects you can get out of water and corn starch:

YouTube - Amazing Liquid

i'll describe some other cool ones when i'm back, after lunch

[HydrogenBond]

Although water is very common, it is argumentatively the most complicated substance. Water has 63 anomalies that have been investigated, including hot water freezing faster than cold.

Water anomalies

[TheBigDog]

I am a long way from HS chemistry, but I remember hypothesizing about the behavior of water versus other molecular structures, and always thought that the "bent" shape of a water molecule had to be significant, versus the linear shape of something like CO2.

Bill