Why does a pressure cooker work

The Clausius-Clapeyron equation quantifies the way a boiling temperature changes as a function of the applied pressure. At the boiling points of 7i and T2, the external pressures pi and p2 are the same as the respective vapour pressures. 

It does not matter which of the values we choose as 1 and 2 provided that 7i relates to pi and T2 relates to p2. It is permissible to swap 7i for T2 and pi for p2 simultaneously, which amounts to multiplying both sides of the equation by -1.  

We need to understand that the Clausius-Clapeyron equation is really just a special case of the Clapeyron equation, and relates to phase changes in which one of the phases is a gas. 

Worked Example 5.2 What is the boiling temperature of pure water inside a pressure cooker Let T be the normal boiling temperature T(boii) of water (i.e. 100 °C, 373 K, at p ) and let p2 of 6 x p be the pressure inside the pan. The enthalpy of boiling water is 50.0 kJmol-1. 

In this example, it is simpler to insert values into Equation (5.5) and to rearrange later. Inserting values gives 

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