What you need:
A paper tea bag (the stapled kind with a string and tag on it), scissors, a plate, a matchbox.
What to do:
Remove the staple pin, the thread and the tag from your tea bag. Empty its contents (use them to brew some tea later).
Place the empty bag upright on the plate. Make sure the plate is made of a non-flammable material.
Light a match and set the entire mouth of the tea bag on fire.
What happens?
Once the tea bag has finished burning, what’s left of it suddenly rises up in the air like a charred rocket!
Why?
When you set fire to the tea bag, the air inside it becomes hot. The air molecules, on heating up, become energized and spread out to occupy more space. Since the molecules spread on heating, the hot air becomes less dense.
In nature, a less dense substance always floats above the substance that’s denser. So the hot air molecules start rising up to float above the cooler, denser air below.
As the flame moves down the body of the tea bag, more and more air gets heated and starts rising up. When most of the tea bag is burnt, the remaining ash is light enough to be whooshed up with this rising hot air.
This process of transfer of heat in a molecular fashion is called ‘convection’. Most gases and liquids exhibit convection when they are heated.
In fact, hot air balloons also operate on the principle of convection. When the burner in the balloon heats the air inside it, the air becomes less dense than the surrounding cooler air. The hot air starts to rise and takes the balloon with it!
Picture Credit : Google
