Which cools faster, a litre of hot water poured into a litre of cold water, or vice versa?

Answer - Pouring cold water into hot water.





Reason - For best mixing, we have to bring the hotter molecules in contact with colder molecules for a longer duration of time. This is because heat transfer phenomena is not an instantaneous process, instead it is a gradual process and needs time and contact.

One example - You can touch the flame of candle for 0.5 second and feel nothing. Touch it for 2 second, you can feel the sensation. If you are brave enough you may go further!

So why pouring cold water brings better mixing and better contact time?

The exact reason is this: the cold water is denser than hot water. If you pour it in hot water, the hot water, being lighter, will start moving up and cold water, being heavier will move down. This brings greater mixing.

While on the other hand, if you pour the hot water over cold water, and if you have poured it slowly, then heat transfer will take place gradually from the interface where both meet and eventually equilibrium will be attained. There is relatively much lesser molecular motion and mixing.

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