Today while in school, I observed the thermal decomposition of lead nitrate. The peculiar thing I noticed was that the lead nitrate powder in the test tube starts turning yellow (due to the formation of lead oxide) from the top and then towards the bottom. This indicates that the certain decomposition temperature is first reached at the top and later at the bottom.
I tried to apply a logic that as soon as some heat was provided to the bottom of the powder in the test tube, it was quicky transferred to the powder present above it and so on till the top. At this point, since the heat had nowhere to go (since air is a bad conductor is heat) it got accumulated at the top and hence it reached a higher temperature.
However I'm unsure of my logic because for the top powder to reach the specified temperature, the lower powder must first reach either the same or a higher temperature (otherwise the powder present above it would not get heated).