If the volume in a syringe is doubled, what happens to the pressure?
It will decrease by a factor of 2.
This is basically an application of the ideal gas law:
If you solve for pressure, you get:
Assuming that the amount of gas and the temperature remain constant, this can simply be rewritten as:
In other words, pressure is inversely proportional to volume. So, if you double the volume, it will be like halving the pressure. This also makes sense if you think about it logically. If the gas has more space to move around in, it's going to spend less time pressing against the walls of the container.
Hope that helped :)