Question

Take two similar transparent glass bottles having wide mouths. Put a few crystals of potassium permanganate or pour a few drops of ink in one bottle. Fill this bottle with hot water. Fill the other bottle with cold water. Cover the cold water bottle with a thick piece of paper such as a postcard. Press the postcard firmly with one hand and hold the bottle with the other hand. Invert the bottle and place it on top of the hot water bottle. Hold both the bottles firmly. Ask some other person to pull the postcard. Observe what happens. Explain.

Answer 1

we know , all matters found in nature form by very small particle e.g. electron , protons etc .
here first bottle in which hot water and some ink with KMnO4 and second bottle in which cold water and some ink with KMno4 given.
if we invert the cold bottle in the top of hot bottle we see ink is diffusing from first in which hot water found .
this is due to small particle found in matters ( in mixture found In bottle ) move freely with high kinetic energy so their RMS velocity is higher then cold bottle . so, particle move hot to cold .
which we observed by experiment .

in other way we can say that
any matters which is hot have higher kinetic energy then cold matters .so higher velocity of hot matters then cold matters . hence maximum chance to flow of particle hot to cold when we connect by one direction .
this is based on Graham's law of diffusion . we study this theory details in 11th or 12 th class

Answer 2

Given:
Two transparent bottles and fill first bottle with Hot water and drops of KMnO4 crystals or ink drop and fill the other bottle with cold water.
observation:
case 1: In Hot Water as KMnO4 crystals are added due to diffusion the puprple colour spreads through hot water.
case 2:
If we invert cold water bottle over hot water bottle containing KMnO4 crystals.what happens.
We observe that , Hot Water will rise and cold water sank. As the motion continues the colour gets mixed.
Reason:
1.Due to difference in density of hot and cold water.
2.Hot Water is less dense than cold water.
3.Gravity can separate fluids by their density.Because cold water occupies more mass per unit volume than hot water.
4. The force of gravity is more cold water than on hot water.
5. This causes cold water to rush downwards and hot water to lifted upwards.
6. This motion of fluids is called convention.
Note: if we invert hot water bottle over cold water bottle , we observe hot water stays at top and cold water at bottom. Because due to difference in density and gravity already cold water is at bottom.
Hope it's clear.