science questions

[Journer]

so i was thinking about this while i was in the bathroom today (someone left a shit load of ice in the sink)...

does hot water make water hotter quicker than cold water makes it cold?

such as...lets say the target temp is 64oF.

lets say the first test, you start at a water temp of 32oF you add in hot water (which is 96oF) until the temp reaches 64oF

second test, you start the water temp at 96oF and add cold water (32oF, well maybe 33) until it reaches 64oF.

would the cold water bring the temp down quicker than the hot water would bring it up? ...

i thought about trying this this weekend...i'm curious if the time and volume will be similar

anyone got some input?


another though:

does water heat or cool quicker?
ex: you put water in a pot, set it on top of one side of a TEC or something that can output exact temp. and see how long it takes to get to desired temp. then you use other side of pelt that outputs exact opposite temp, using the same pot (waiting until it is back to normal temp) and add water and see how long it takes to get to reverse temp. which is quicker?

 

[YOyoYOhowsDAjello]

Originally posted by: Journer
target temp is 50oF.

lets say the first test, you start at a water temp of 32oF you add in hot water (which is 86oF) until the temp reaches 64oF

second test, you start the water temp at 86oF and add cold water (32oF, well maybe 33) until it reaches 86oF.

 

[spidey07]

No. Look into specific heat.

Also, ambient temperature and surface area would come into play.

 

[Toastedlightly]

In a perfect world, they will be the same. If the change in temp is the same, they both have about the same specific heat (maybe varies by .0001 or something absurd).

 

[Howard]

You can't add any cold water to reach 86, you're already at 86.

 

[BrownTown]

The AMOUNT of water needed to raise or lower the temperature would be the same in mass (not volume). The rate at which it happened though WOULD be different, the hot water will diffuse into the medium water faster than the cold will.

 

[potoba]

This is not a chemistry question. It's physics. Well, i think if the rate of diffusions between hot and cold water is the same (Well, they're not, but close enough), it'll be the same if the amount of heat transfered is the same. If you want exact answer, research about transport properties of liquid phase. It's covered in thermodynamics or kinetics and dynamics courses.

 

[Born2bwire]

Originally posted by: BrownTown
The AMOUNT of water needed to raise or lower the temperature would be the same in mass (not volume). The rate at which it happened though WOULD be different, the hot water will diffuse into the medium water faster than the cold will.

But a pint's a pound the world around. Unless your French... or British... or German... You know what, screw you all!

 

[Tiamat]

edited because I just dont have the patience to make it completely correct.

 

[Chaotic42]

Originally posted by: Howard
You can't add any cold water to reach 86, you're already at 86.

What about significant digits?

 

[Journer]

Originally posted by: Chaotic42

Originally posted by: Howard
You can't add any cold water to reach 86, you're already at 86.

What about significant digits?

typo...meant 64oF

 

[Journer]

Originally posted by: BrownTown
The AMOUNT of water needed to raise or lower the temperature would be the same in mass (not volume). The rate at which it happened though WOULD be different, the hot water will diffuse into the medium water faster than the cold will.

well what if you were to pour 100oF water onto 0oF ice
and then 0oF ice into 100oF degree water...

would the ice melt faster or would the water cool quicker?

 

[eits]

i think it'd be easier to get to a target water temp by having hot water and adding cold.

 

[Journer]

another question:

how much 32oF water would it take to cool something that is 64oF to 32oF ? double, triple, a shit load?

 

[Howard]

Originally posted by: Journer
another question:

how much 32oF water would it take to cool something that is 64oF to 32oF ? double, triple, a shit load?

Depends on the environment and time period

 

[YOyoYOhowsDAjello]

Originally posted by: Journer

Originally posted by: Chaotic42

Originally posted by: Howard
You can't add any cold water to reach 86, you're already at 86.

What about significant digits?

typo...meant 64oF

Is 64 supposed to be the midpoint between 32 and 86?

 

[dullard]

This thread is so messed up, but it could be a fun thread. How exact do you want the answers? I guess to answer that, I think you need to ask a very specific question. No where in that rambling is a coherent question.

So, to get the ball rolling, first do this.

Alt 2 4 8

Try it with me

1) Alt (hold it down).
2) On the keypad press 2.
3) On the keypad press 4.
4) On the keypad press 8.
5) Let go of the Alt key.
6) Notice that the 2, 4, 8 keys are drawing a circle on the keypad (it just doesn't complete the circle with the 6 key).

Try again: Alt 2 4 6.

Once you are done, post a question and we'll tackle it. And give us details. How much of each bucket of water are we starting with, the same amount? Are you pouring both the hot and cold water in at the same rate? Do you want to get the AVERAGE temperature to 64°F or all of the water to 64°F? Can we use 32°F water or must we use 33°F (both are possible). What temperature is the environment? Or is the model to assume no heat transfer with the environment? Etc.

 

[dullard]

Originally posted by: Journer
how much 32°F water would it take to cool something that is 64°F to 32°F ? double, triple, a shit load?

Mathematically, you could NEVER add 32°F water to 64°F water and have the final result be exactly 32°F unless the environment is below 32°F. But as an engineer you could give a cutoff range, could we say that 32.01°F is close enough to call it 32°F? Or what type of precision do you need?

 

[KayGee]

Originally posted by: Tiamat
This is a question related to the heat transport via conduction. Depending on how rigorous you want to do this, the equation is generally the following:

Q = kdT

where Q is heat flow, k is the thermal conductivity of the item and dT is the temperature delta.

Since the temperature difference is the same in both cases, and k is the average thermal conductivity at the average temperature, the heat flow rate will be identical for both cases.

This assumes perfect mixing, which is a decent assumption if you are literally dumping both containers holding identical masses of water into a third container.

Of course, the ambient conditions should be at the average temperature as well.

heat transfer in liquids/gases occurs through the process of convection. there will only be conduction within the material that the vessel containing the liquids is made of.

if the OP contained additional information, one might be able to predict an answer with a certain degree of accuracy. what is the container made of, what is the ambient temperature, flow rates, etc.

theoretically, if conditions are identical, it would take the same amount of time for the same temperature difference.

 

[piasabird]

Hot water and cold water should have different properties. As water heats up it expands, and the molecules move faster. This is something you may have to take into account. Another factor may be the Humidity level and/or the air pressure of the air and the temperature. I dont know how to put this into technical scientific terms. However, I think you will find the problem is quite a bit more complex than you are considering.

 

[dullard]

Originally posted by: KayGee
heat transfer in liquids/gases occurs through the process of convection. there will only be conduction within the material that the vessel containing the liquids is made of.

Liquids and gasses both convect AND conduct heat.

 

[dullard]

Originally posted by: piasabird
However, I think you will find the problem is quite a bit more complex than you are considering.

It all depends on how exacting he wants to be. I'm all prepared to discuss the heating created during the pouring of one liquid into another. In the very least, some kinetic energy is converted into thermal energy.

 

[KayGee]

Originally posted by: dullard

Originally posted by: KayGee
heat transfer in liquids/gases occurs through the process of convection. there will only be conduction within the material that the vessel containing the liquids is made of.

Liquids and gasses both convect AND conduct heat.

not trying to be a smartass, but that's the first time i've heard that. do you have a source? you aren't referring to conjugate heat transfer? i'm just curious.

 

[Tiamat]

Originally posted by: Journer
another question:

how much 32oF water would it take to cool something that is 64oF to 32oF ? double, triple, a shit load?

Depends on the precision of your thermometer or thermocouple.

If you want to simplify your problem to within whole degree precision, you can make a quick estimate for this problem.

Equal masses of liquid water well-mixed will reach the average temperature. Since temperature is the driving force for heat transfer, the greater the temperature delta, the faster the rate of heat flow. As you approach the average, the delta temperature is small, causing the heat flow rate to be small.

If you have 1kg of 33°F liquid water and add it to 1kg of 32°F liquid water, the time it takes to reach the average temperature (32.5°F) is high enough that the ambient air (68°F) will warm the waters and prevent the approach of the average temperature (even though thermal conductivity through air is very low). So, you typically have to relax your goals and have an approach temperature that is satisfactory to you or you buy extra ice. A surplus of ice will keep the system in thermal equilibrium at 32°F assuming pure water, 1 atm.


So, to answer your question for a real world application that I actually use in the lab, to cool down a bath of liquid water from 18°C to 0°C requires 10 times the mass in the form of liquid water at 0° held there by its own ice/water bath at 0°C. My thermocouple and associated electronics has precision to 0.1°C.

 

[Tiamat]

Originally posted by: KayGee

Originally posted by: dullard

Originally posted by: KayGee
heat transfer in liquids/gases occurs through the process of convection. there will only be conduction within the material that the vessel containing the liquids is made of.

Liquids and gasses both convect AND conduct heat.

not trying to be a smartass, but that's the first time i've heard that. do you have a source? you aren't referring to conjugate heat transfer? i'm just curious.

It occurs but to a lesser degree. This can be seen by just observing the thermal conductivities of water in its typical 3 phases. Solid has a decently high thermal conductivity, where as liquid and gas have much lower. This just means that the heat flow is governed more by convection for the latter two and less by conduction, but since atoms of each will contact one another -- there is no deleting conduction from the equation.