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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?"

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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