If a sealed syringe that contains some gas is heated, in which direction will the syringe plunger move?
The plunger will move outward increasing the volume in the syringe.
The increase in heat will cause an increase in temperature of the gas particles..
The increase in temperature will cause an increase in motion.
The increase in kinetic energy (motion) will cause an increase in pressure inside of the syringe.
The increase in pressure will cause an increase in the volume of the syringe.
Charles' law says that volume and temperature are directly proportional. As the temperature increases the volume will increase.
So an increase in heat and temperature will cause an increase in volume causing the plunger to move outward.