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I have a small Butane blowtorch, and use it to heat an Aluminium plate. The plate is thick so takes 20 seconds or so to really start heating up.

As soon as I apply the heat, droplets of some liquid start "condensing" on the surface of the metal around the general area of the flame.

The liquid is colorless and odorless and is at room temperature, note this happens when the metal is still at room temperature as it hasn't heated up much yet.

What is this liquid exactly and why/how does it form? I suspect it could be a $C_{4}H_{10} + O_{2}$ reactant but not sure. Thanks!

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This is a guess - but I think it might be water formed as a byproduct of butane combustion precipitating on the plate:

$$ \ce{2C4H10(g) + 13O2(g) -> 8CO2(g) + 10H2O(g)} $$

Aluminum has very high thermal conductivity, so it is possible that the plate is rapidly absorbing heat from the water vapor, causing it to condense. If this stops happening right around the point when the aluminum temperature reaches 100 C, that would be evidence to support this hypothesis.

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  • $\begingroup$ That sounds right to me then. I updated my question with a thought that the $C_{4}H_{10} + O_{2}$ reactant could involve water and that makes sense now, I'll try with a smaller piece of Al tomorrow to test the theory. Thanks! $\endgroup$ – baharini Jul 12 '14 at 1:58
  • $\begingroup$ I added the reaction equation for you. If you try this, be sure to hold the thermometer right against the metal at the spot where the water is forming. That spot will heat up much faster than the rest of the plate. Also, make sure the thermometer is rated to around 600 C or you might break it accidentally. $\endgroup$ – thomij Jul 12 '14 at 2:12

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