# Why do thermos flask bottles advertise 24hrs cold and 12hrs hot. Shouldn’t it be the same amount of time for temps in both directions?

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Why do thermos flask bottles advertise 24hrs cold and 12hrs hot. Shouldn’t it be the same amount of time for temps in both directions?

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The difference in temperature determines how fast heat transfers.

Ice water at 0 C is 25 degrees C from 25 C room temperature.

Hot water at 80-100 C is 55-75 degrees C from 25 C room temperature.

Hot water is more than double the temperature difference of cold water, so it changes temperature faster. Thus, the water inside will remain hot for a shorter time than cold water remains cold.

I really shouldn’t be answering here, but there’s also an issue of entropy I think. the molecules of hot water are moving around quite a bit. Cold water, or even ice, the molecules aren’t moving much at all, it’s going to take more energy to get them moving around, ie heat up. And while I’m here; 32F (freezing) to 69F (room temperature) is thirty-seven degrees. 176F to 69F to is one-hundred and seven degrees.

The rate at which heat flows is portional to temperature difference. This matters because if you measure the ambient temperature you keep the thermos in you will likely discover it is much much closer to the cold contents temperature than the hot contents temperature. The larger temperature gap means heat transfer is faster.

I think they assume that if you’re keeping something cold you’ll have ice in it. Water undergoing a phase transition like from solid to liquid requires a good bit of energy and can essentially act as a “cold reservoir”

I see people pointing out that hot stuff is further away from room temp than cold stuff and that the difference in temp inside vs outside the cup causes the hot to become room temp faster. While this is true to some extent in that it will lose energy faster than the cold will gain, it is an exponential process and the heat transfer slows as a proportion of the temp difference. So the hot stuff will reach room temp later than cold because it started off with a larger temp difference

Cold liquids range down to zero Celsius if you have ice cubes in them. Room temp is about 21 Celsius. Just boiled liquid would be what 90-100 C. So going from 0 to 21 is 21 degrees of temp change of 21 degrees. 90 to 21 is 69 degrees. The bigger temp change means a faster change. So what do we consider cold versus hot versus lukewarm? A hot liquid turns lukewarm faster.