Why does the piston of syringe move outwards when the air inside the syringe is heated to a constant temperature and after a while the piston of syringe will stop moving?
When the air inside the syringe is heated, the air molecules gain kinetic energy. They move with faster average speed resulting in more frequent collisions with one another and the inner walls of the syringe.
The air molecules also collide onto the inner walls of syringe with greater average force per unit area of the walls. As pressure is force applied per unit area, the pressure on the inner walls of the syringe increases resulting in the piston of syringe moving outwards.
The piston of syringe stops moving after a while as the pressure acting on the inner walls of syringe is equal to the atmospheric pressure of air molecules acting on the outside walls of syringe.