I wish on a summer day, if I carry lemonade with myself on a journey, I wish its all cold! While in snowy winters, I want my tomato soup to be as hot and warm as I cooked it, before departing.

But how is it possible? Now I can’t take an oven or a refrigerator with me, on my way…right ?

One thing can be done, I can take that thermos, I was gifted on my birthday. Yes! That’d help.


Thermos (technically, a vacuum flask) has its own story! There is science behind it, particularly Physics.A thermos is nothing but ‘a bottle inside a bottle’ seperated by an evacuated space. 

We all know heat, it makes something hot or cold. Basically, Aheat is the energy of flow, the internal energy of the particles (usually molecules) of the system, which flows from from one thing to other, because of temperature difference; for advanced readers, temperature gradient!

Also, by the second law of Thermodynamics, we know that heat transfers from a body at higher temperature to a body at lower temperature (reason lies in entropy, the randomness of particles). And there are three modes of transfer of heat namely, Conduction, Convection and Radiation. 

Conduction is the transfer of heat from particle to particle, because of which my mother suddenly pulls her hand back involuntarily, while touching a tablespoon in hot coffee after some time she comes in the kitchen! Convection keeps the temperature of the overall system same, by simultaneous movement of less dense, hotter molecules moving up and cold, the denser ones moving down, like ACs cool our rooms and the same phenomena associated with see and land breezes. And radiation which requires no medium, like electromagnetic waves (EM waves).

Modes of heat transfer

Back to thermos!, since the space between the two bottles is evacuated, (though at some place, near the cap at the top, they are still in contact, but heat transfer is minimal there) there are no air molecules to transfer the heat from my soup (at a higher temperature) to the environment (at a lower temperature) and vice-versa with my lemonade. Hence, no conduction takes place. Convection, I think, I shouldn’t include since its inside my system (my soup or my lemonade) only, which maintains its overall temperature. 

Thermos flask

Now, being a vacuum, radiation can still occur since without even a medium too, there is a heat transfer through radiation.How can I prevent my soup turning cold, and my lemonade becoming warmer through radiation? 

So, yes! I have something for that also. It’s a bit expensive though… its silver which does not let pass EM waves pass through it. So, thermoses, have coatings of silver linings inside their inside bottle to prevent any heat transfers through radiation.

So, now by the magic of thermodynamics and silver, I can feel the hot soup and the chilled lemonade! Thanks to physics behind it, silver and my friend who gifted me.

And, see from the hour I made it….its …as it was!