Volume of gas, temperature and

Consider the system shown in Figure 13.5. To demonstrate the relationship between temperature and volume of gas, we keep the number of gas particles and gas pressure constant. If our valve is closed and if our system has no leaks, the number of particles is constant. We keep the gas pressure constant by allowing the piston to move freely throughout our experiment, because then it will adjust to keep the pressure pushing on it from the inside equal to the constant external pressure pushing on it due to the weight of the piston and the atmospheric pressure. The atmospheric pressure is the pressure in the air outside the container, which acts on the top of the piston due to the force of collisions between particles in the air and the top of the piston. 

Ideal-gas law An equation describing the theoretical relationship of pressure, temperature, and volume of gas. 

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