Final answer:
Doubling the pressure of a gas, with all other factors held constant, would not double its volume. It would halve the volume according to Boyle's Law, in contrast to the effects of doubling the temperature or molecule count, or halving the pressure.
Step-by-step explanation:
If all else is held constant, changing which factor would not double the volume of a gas? Among the options given, the one that would not double the volume of a gas is doubling the pressure. According to Boyle's Law, if temperature and amount of gas are kept constant, the volume of a gas is inversely proportional to its pressure. Therefore, doubling the pressure would actually halve the volume instead of doubling it.
In contrast, according to Charles's Law, if pressure is constant, the volume of a gas is directly proportional to the absolute temperature. Therefore, doubling the absolute temperature of a gas at constant pressure would double the volume. Similarly, halving the pressure would also double the volume because of Boyle's Law, as long as temperature remains constant. Finally, doubling the number of gas molecules would double the pressure if the volume and temperature stay constant, which according to Gay-Lussac’s law, would also lead to a doubling of the volume if pressure is then adjusted to the original value.