Why doesn’t coffee immediately mix with cold milk, but mixes with hot milk pretty easily?

Why doesn’t coffee immediately mix with cold milk, but mixes with hot milk pretty easily?

You can check the answer of the people under the question at Quora “adding cold milk to hot coffee

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  1. The answer to this question applies not only to coffee and milk but also to other solutions like water and salt/sugar.
    If you look at the milk with a microscope then you will find milk molecules moving randomly in a Zigzag manner. This type of motion is called Brownian motion.

    Why doesn't coffee immediately mix with cold milk, but mixes with hot milk pretty easily?


    Now imagine that you have a cup of milk and you are looking it through a microscope. You will find billions and trillions of milk molecules having Brownian motion like this

    Why doesn't coffee immediately mix with cold milk, but mixes with hot milk pretty easily?


    File:Brownian motion large.gif
    This Brownian motion is dependent on kinetic energy of the molecules (Kinetic energy is the energy possessed by the particle in motion). Kinetic energy is dependent on the temperature of the liquid (in our case its milk) .
    If temperature of liquid is more, then the kinetic energy of the molecules in liquid will be higher. Hence their motion will be much faster as compared to the liquid whose temperature is less and is relatively cold
    If the liquid is cold, liquid molecules will have lesser kinetic energy and their motion will be slower
    (You can imagine this situation by taking an example of an ant’s nest. Imagine that you are looking at a group of ants near their nest doing some activities. This will represent state of molecules of cold milk. Now suppose if you disturb them then what happens ? Ants start to move more quickly. This will represent state of molecules of Hot milk)
    For coffee to get dissolved in the milk, coffee molecules have to fit into the spaces between these milk molecules.
    Now when you add coffee to the milk, coffee molecules starts diffusing into the milk. This diffusion happens due to the Brownian motion of molecules in the milk. In case of cold milk you have to stir it to dissolve the coffee. Can you guess what are you actually doing while stirring the cold milk to get your coffee dissolved ? By stirring you are creating a disturbance in molecules of the cold milk or in other words you are making the molecules of milk move faster and allowing coffee to get fit into the spaces between milk molecules and get dissolved. In case of hot milk, the milk molecules are already moving faster and hence only little amount of stirring is required to get coffee dissolved.

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