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Question:
Today a student asked me, "If you use a CO2 cartridge to propel a toy the cartridge will feel cold. But if you use a rocket engine with solid propellant, it will feel hot. If both are expelling gases, then why is one hot, and the other cold?"



Replies:
Because one is simply the expansion of compressed gases -- perhaps with a phase change (which absorbs heat) while the other is a chemical reaction to produce the gases -- one of the products of the specific chemical reaction used is heat.

gregory r bradburn



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