# Can an isobaric and isothermal process happen at the same time without being in a process of phase change?

On a piston cylinder system, when I transfer heat to it, can it be an isobaric and isothermal process? Consider it is not a phase change process, because it is possible there.

Don't rely your answer on ideal gas equation, since it is very fast and easy to know the answer to my question using this equation.

What happens in particles that they can't have constant pressure and constant temperature troughout the process? How do they react?

Some people (myself included) regard an isobaric process as one in which the external pressure $P_{ext}$ is held constant throughout the change until the final volume is attained, even if, during the gas deformation, $P_{ext}$ does not match the initial- or final thermodynamic equilibrium pressure of the gas. This is an irreversible change. This is just a definition that is sometimes used for a constant pressure process.
Some people (myself included) regard an isothermal process as one in which the cylinder is in contact with a constant temperature reservoir throughout the process, so that the gas temperature at the boundary is constant at $T_{res}$ during the change until the final volume is attained, even if, during the gas deformation, $T_{res}$ does not match the initial- or final thermodynamic equilibrium temperature of the gas. This is an irreversible change. This is just a definition that is sometimes used for a constant temperature process.
So you want constant $P,T$ while $V$ presumably changes due to the moving piston. But if $V$ is increasing (and if external $P\neq0$, i.e., not a so-called "throttling process"), then the gas is doing work on its surroundings. So if the process is adiabatic, then that work energy has to come from within the gas itself, whereby $P,T$ has to change (a la first law) correspondingly. Otherwise, if you want the gas's $P,T$ constant while it's expanding, you'd have to supply outside energy to make up for the energy lost to the piston head, and the process won't be adiabatic.
edit Re your "what happens in particles?" part of the question, just consider a gas molecule colliding with the piston. If the piston's not moving, then, ideally, the collision's elastic, and macroscopic $P,V,T$ aren't affected. But in the non-equilibrium situation where the piston's expanding, some momentum's transferred from the molecule to the piston, resulting in corresponding changes to $P,V,T$.