[Morphic flip]

Why, if you take a glass of cool water and shake it up, you can feel the class get colder?

[enahs]

I am assuming you mean a half full/half empty glass? Where normally you have a large surface area of the glass not in contact with water, and then you bring that area in contact with water? And I guess you mean sealed container, or are you shaking some of the water out?

[Morphic flip]

Yeah sorry, I didn`t explain fully.
Your correct in all assumptions of what I stated. But normal drinking glass half full, not sealed, just if you swirled the water round.
Why it makes the glass feel cold.
Cheers.

[Bakegaku]

Like enahs mentioned, it's because the top part of the glass is not in contact with the water, thus it is about the same temperature as the air around it.  When you swirl the water, it causes that part of the glass to come in contact with the water and cool down.

[Mitch]

Like enahs mentioned, it's because the top part of the glass is not in contact with the water, thus it is about the same temperature as the air around it.  When you swirl the water, it causes that part of the glass to come in contact with the water and cool down.

Not true, since this occurs even if the water bottle has been allowed to reach thermal equilibrium in the room. I've thought about this before but I don't want to give away the answer just yet. But this phenomenon is also related to the following observation. If you enter a room where all the objects have been in thermal equilibrium and you decide to touch a piece of metal it will feel colder than if you decided to touch the carpet.

[nearly.alex]

i'm not certain, but when the water isn't being swirled the heat from your hand goes to the glass then to the air, but when the water touches that bit of glass it transfers the heat quicker, as i think water is a better thermal conductor (if thats a term) than air, ie it feels colder like the metal being a better thermal conductor transfers the heat from your hand quicker than when you touch the carpet.

alex

[enahs]

Temperature and heat are related, but different.

Heat is the flow of energy. If you run water over your hand that is the same temperature as your skin, it will still feel like it is cooling you down.

Water, along with its many other fascinating properties, has a very high specific heat. Since your said simplistic for younger person; this means water requires much more energy in the form of heat to raise the temperature of water by 1 degree, then most other substances.


Energy (an thus heat) are always trying to go to as low of a state as possible. When your swirl the water around and contact the other portions of the glass, the water is able to absorb a substantial about of energy in the form of the heat from the glass, while not actually raising its "heat content" much at all, due to its large specific heat value. This allows the glass to go to a lower energy state, while waters essentially remains the same in this particular case; making the universe and the laws of thermodynamics happy.

It is this flow of heat that makes it feel colder. The heat flows from the glass to the water, and to maintain thermal equilibrium (same temperature) the heat flows from your hand to the glass. The glasses temperature does not cool down on a macroscopic level, you simply feel the flow of heat and your brain interprets it as it being colder, when in fact it is not.

[Mitch]

enahs nailed that one. +3 scooby snacks given.

[english]

I don't know if this relates any at all but why is it that when you place a bottle of water in the ice box and let it reach a temperature decently close to freezing point, but not yet, and then take it out and open it the water begins to solidify, but not as a crystal, but more like a cross between the solid and liquid states.

Know what I'm talking about?

The water is obviously colder than its surroundings (the air), so it doesn't follow that the flow of heat is from the water to the surroundings.

Therefore, if the flow of heat is from the surroundings "into" the water shouldn't this negate the chance of the water partially solidifiying, as it were?


I swear I'm not hallucinating.   

[Yggdrasil]

In this case the water is supercooled.  That is the water is in a metastable state where the temperature of the water is below its freezing point.  When you take the water out of the freezer and open it, you disturb the system enough to disrupt the metastable state and make the water go toward its most stable state (ice).  Why does this occur?  Well, freezing (crystalization) has its own activation energy.  Freezing requires an event called nucleation where ice crystals must overcome the surface tension of the water.  In a relatively smooth container (so that there aren't any rough edges where nucleation is easier) at low temperature and with little disturbances, water can easily supercool due to the fact that ice crystals cannot nucleate.  However, when you distrub the system, for example, by opening the bottle, you allow ice crystals to nucleate and catalyze ice formation.

So heat is not flowing into the system in this case.  The water is already below its freezing point so freezing is a spontaneous process.  All it needs is a little jump start to get going.

Another example of nucleation is the infamous diet coke + mentos experiment.  Soda is supersaturated with carbon dioxide.  However, carbon dioxide will normally evolve very slowly from the soda.  But, when you introduce a mentos candy, with a rough surface that serves as a point of nucleation, carbon dioxide rapidly escapes from solution, producing a dramatic geyser of soda.