This is really a question about convection.
If you hot pot contains air the convection currents in the air will mean that it is hotter at the
top than at the bottom. The cooler air will be at the base of the pot.
Putting a block of ice
at the bottom will cool the air at the base even more but will do little for the air at the top
because air is a poor conductor of heat.
However if you put the block of ice at the
top of the pot then the hot air rising here will be cooled and so fall back inside or outside the
pot. Hot air rising up from below will replace the cooled air and so the result will be a more
rapid cooling of the pot.
Think about an ordinary domestic refrigerator. The icebox is
always at the top because any hot air rising in the fridge will be cooled by the icebox and this
will keep the general temperature of the fridge low.
As to any other experiments:
(a) refer to the fridge
(b) try a block of ice
under a pot and a block of ice above and see what happens (I wonder what will happen of
the block of ice actually rests on top of the pot and prevents air escaping) Actually this will
help because the warm air rising up will be cooled, fall back into the pot and so aid the
general rate of cooling
(c) put a block of ice in the air with a thermocouple or sensitive
digital thermometer just above the block and another just below (neither touching the block).
Record the temperature against time
(d) do the reverse experiement with a bunsen
burner. Hold you hand (not for too long) just above the flame and then below the flame down
one side of the burner and see which gets hotter quickest – it won't take long!
(e) try and
produce some shadows of the air currents around an ice block to plot the movement of the
warmer air