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Answer: the spaces between the gas molecules shrink. This makes the most sense since the volume is decreased, but the same number of gas molecules remain. Number of collisions would increase since there's less room. None of the gas leaks out as well. The temperature is going to be halved, not doubled. Due to PV=nRT. If you half the volume, you also have to half the temperature to keep the equation equal. This it true since the pressure is kept constant.
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