Great question! To answer quickly: when a
molecule has enough energy to float away, it
will. Think of it almost like an airplane
flying. When the airplane has enough energy (jet
fuel), it can escape the gravity of earth and fly
In more detail, what you're asking is related
to what we call "phases of matter".
There are three main phases of matter:
solid, liquid, and gas. When the molecules in
a solid gets enough energy, they will turn into a
liquid, and when the molecules of a liquid get
enough energy, they will turn into a gas. If
you're looking for a little more information, like
what happens to the temperature of a solid/liquid
when it turns into a gas, check out this wikipedia
on phase changes.
The water molecules that are hot wiggle around
a lot and push away the molecules near them ,
so they're not packed so close together. That
makes the hot water less dense than the cold
water, and they 'float' to the top.
The water molecules that are cold are more
sluggish, so they just crowd together a little
more. That makes them more dense, so sink to
the bottom. I could do an experiment with hot
and cold water and maybe food color to test this,
but I'll just give you this cool link:
How atoms and molecules are affected by Heat:
atoms and heat
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