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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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 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.
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
“Hot” is a lot farther from room temperature than “Cold” We also perceive hot as warm a lot closer to its original temperature than we do cold
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.
I hope I’m able to phrase this correctly, but it’s based on the 2nd law of thermodynamics: (paraphrased) heat always moves from hotter objects to colder objects. The thermos is acting as both insulator and conductor in this case. It’s conducting its heat to the outside world, albeit slowly, but it’s losing therms more quickly than it can absorb them from the “outside” ambient temps.
Cold does not conduct, or transfer, the way heat does – think of it this way: I can’t cool my house by opening the fridge, that just lets the heat in and warms my food. I can however warm my house by opening the oven, because it’s the heat that’s transferring.
In short, the thermos loses heat faster than it absorbs heat.
I can’t believe it, all the answers except one are wrong. And you’re all waay overthinking it. It’s simply because cold water has ice in it. Ice absorbs as much heat as water at -83C. You’re comparing 100C water with -83C water.
ELI5: ice is colder than hot water is hot.
Nope, the greater the heat differential between outside of the bottle and inside the bottle, the faster the transfer.
Fun note, for hotter coffee, add the cold milk when filling the thermous.
Let’s use food safety numbers. 40F is cold safe and 140F is hot safe. We’ll let room temperature be 70F. Temperature changes faster the farther it is from room temperature. Something cold is only 30F from room temperature, something hot is 70F from room temperature. The hot thing will get cooler much faster than the cold thing will get warmer.
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