The pressure inside the closed jar will increase because as the solid melts, it will occupy a larger volume as a liquid compared to its solid state. Additionally, if the jar is sealed and the volume of the jar remains constant, the increase in the number of moles will also contribute to an increase in pressure according to the ideal gas law.
The pressure inside the closed jar will increase if the solid expands upon melting, and decrease if the solid contracts upon melting. To determine the exact change in pressure, it would be necessary to know the behavior of the solid when it melts, such as whether it undergoes an increase or decrease in volume.
When an unknown solid inside a closed jar melts, the pressure within the jar will remain constant. This is because the volume of the solid will not change during the process of melting, so both the number of gas molecules and the temperature will stay the same, resulting in a constant pressure.
