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I have two transparent bottles of nearly the same base diameter and cap diameter (Implying that both bottles have similar cap and base area respectively). But one is taller than the other by 15 cm.

The bottles are made of the same material (PolyPet) and I poured cold water at the same temperature to both of them and observed.

I found that :

  1. If I opened the cap the temperature of water just around the cap was higher than that of water below it. The water below it had a comparably low temperature.Does that mean that waters acts as an insulator to prevent heat exchange (to some degree)?

  2. The smaller bottle cooled down later while the taller one faster. I thought that the reverse would happen as it would be relatively easy to cool down a smaller quantity of liquid than a larger one. But the opposite happened.

I could not find any reasons for the above. I repeated the above steps thrice getting the same results.

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closed as unclear what you're asking by Floris, ZeroTheHero, Michael Seifert, honeste_vivere, AccidentalFourierTransform May 26 '17 at 10:45

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

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    $\begingroup$ Need a bit more detail. Are these bottles transparent? What is the temperature of the surroundings? Is there any (sun) light or other source of heat? What is causing "cold" water to cool (further)? Are you putting these bottles in a fridge? If so - are they standing on an actively cooled surface? $\endgroup$ – Floris May 23 '17 at 15:57
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    $\begingroup$ Wouldn't the water heat up if you put it in the sun? $\endgroup$ – Floris May 23 '17 at 16:22
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    $\begingroup$ -1. This is a very confusing question. $\endgroup$ – sammy gerbil May 23 '17 at 17:53
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    $\begingroup$ Confusing: you still mix "heating" and "cooling". Among other things. $\endgroup$ – Floris May 23 '17 at 20:55
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    $\begingroup$ My confusion is the same as repeated by Floris : your title asks about "heating up" but your experiment talks about cold water being cooled (with no mention of freezing) by the sun's rays (cooling by evaporation?). ... Also, there is no mention of temperature readings, so we have no idea of the size of the effect. $\endgroup$ – sammy gerbil May 23 '17 at 22:51
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Assuming ideal conditions, here are some thoughts:

1) Warmer water is less dense than cooler water, so it follows that the warmer water will tend to move toward the top of the bottle.

2) The surface area of the taller bottle is greater, so rate of heat transfer out of it is greater than the rate of heat transfer of the smaller bottle. Perhaps the specifications of the bottles are such that this discrepency in rate of heat transfer outweighs the difference in mass that is cooled.

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    $\begingroup$ the surface area to volume ratio is greater for the small bottle though. Heat is roughly proportional to mass which is proportional to volume. If the law of cooling applies. Then the rate of cooling should be directly proportional to this ratio and the temperature gradient as a function of time. I think the experiment should be further questioned before we try to explain the second point $\endgroup$ – lucky-guess May 23 '17 at 16:56
  • $\begingroup$ @rpfphysics If the difference in the height of the bottles is primarily due to an elongation of the bottle's neck, there will be greater surface area per unit volume. I tested this with the simplified scenario of two stacked cylinders of different radii, then seeing that the ratio of surface area to volume increases when the height of the cylinder with the smaller radius is increased. $\endgroup$ – user122423 May 23 '17 at 22:02
  • $\begingroup$ @Ziggurat The question says "... nearly the same base diameter and cap diameter...But one is taller than the other by 15 cm". This implies that we should assume they are the same size if we assume anything. $\endgroup$ – JMac May 24 '17 at 10:18
  • $\begingroup$ @JMac That's what I assumed; the radii remained the same, and the height changed. $\endgroup$ – user122423 May 24 '17 at 11:40
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Changing from heating to cooling does change the situation somewhat.

Depending on how you allowed them to heat, you could definitely say that the heating "insulated" the bottom of the bottle (if being heated from above).

When you heat water, the lower density water moves up (heating lowers the density). This creates natural convection. This process assists the heat transfer by circulating the water.

Heating from the top creates stagnation. The warm water is already on top, so it will not circulate. The water transfers it's heat the same way a solid object would, by conduction (which is much less effective, it's like not stirring vs. stirring).

The tall one heating up faster seems odd. That may have had to do with orientation though. Without more information on your setup and conditions we would only be guessing why that occurs.

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  • $\begingroup$ Could you please explain how hot water stagnates ? $\endgroup$ – DarkSideofPhy May 23 '17 at 16:46
  • $\begingroup$ I did. Lower density waters moves up. Heating lowers the density. Heating from the top means there's nowhere "up" for the less dense water to move. $\endgroup$ – JMac May 23 '17 at 16:47

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