# eli5: Why can hydro flasks (and all other thermoses) keep liquids cold for longer than they can keep them hot?

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eli5: Why can hydro flasks (and all other thermoses) keep liquids cold for longer than they can keep them hot?

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It comes down to what hot and cold actually are, hot is atoms moving about more and this movement means that they are bumping into other things and transmitting (losing) that energy to other atoms.

Smaller temperature differential.

Cold drinks are, at lowest, 0C compared to a room temperature of ~20C.

The equivalent differential on the other side would be a 40C “hot” drink, which is just luke warm. A good hot drink is more like 80-90C, a ~60C+ differential.

The amount of heat that moves through a thermos basically scales directly with the temperature difference, so it’s losing heat from the hot drink a lot faster than it’s letting heat into the cold drink.

Heat loss is directly proportional to the temperature difference between the two substances/objects. If you’re trying to keep a mug of iced coffee cold, it’s probably somewhere around 2-8C (refrigerator temperature). Assuming room temperature around 25C, that makes for a temperature difference of about 20C.

If you were trying to keep a mug of coffee hot, it would be somewhere in the range of 70-80C. Again comparing to the room temperature of 25C, the temperature difference is about 50C, over twice that of the cold beverage. So the hot coffee will cool down much faster than the cold beverage would warm up.

An ice-cold drink is really like ~40 degrees F (4 degrees C). Ambient room temperature is around 70F (21C). That’s the temperature gradient between the inside of the vacuum insulated container and the room around it.

Whereas, a hot drink or soup, is best served at around 200F (93C). That means that there is a much larger difference between the inside of the container and the room around it.

Whenever one gradient (difference) is larger than the other, that gap will close faster regardless of the container’s insulating capabilities.

If you put a cold drink in a 200F oven, then the cold would warm up faster than the warm would cool off.

It’s all about calculating the difference between up the inside and outside temperatures. The closer they are, the more stable the inside temperature will be for a longer time.

There are a couple of bits of thermodynamics to consider.

[1] ICE is solid water [look here at the wiki on melting](https://en.wikipedia.org/wiki/Melting)

so if my thermos is full of a half frozen drink (or water with ice cubes) there is a huge amount of energy that needs to transfer into my drink just to make it all liquid at 0 C .

[2] Temperature differential.

a drink at 0 degrees is only 20 degrees from room temperature

my tea (made with boiling water) is ideally 100 degrees, and 80 degrees above room temperature.

The temperature differential between my drink and room temperature is the primary determinant in the rate of change in temperature (the drink approaching room temperature).