Constant-pressure calorimetry | Thermodynamics | AP Chemistry | Khan Academy

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Constant-pressure calorimetry is used to measure the change in enthalpy, ΔH, for a physical or chemical process. In this technique, a process is carried out in solution in a coffee cup calorimeter, an inexpensive device composed of two Styrofoam cups. The amount of heat transferred in the process (q) can be calculated from the mass, specific heat, and temperature change of the solution. Because the calorimeter is at constant (atmospheric) pressure, q is equal to ΔH for the process. View more lessons or practice this subject at www.khanacadem...
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[@saigade1236]

Why is a loose-fitting container under constant atm pressure?

[@zafirnaseem578]

The loose fitting lid allows for constant air flow between the inside and then outside. Hence, the pressure inside matches the outside pressure which is atmospheric pressure, leading to the constant atmospheric pressure.