The pumpkin pie recipe I use has stiffly beaten egg whites in it. So, of course, it puffs up dramatically in the oven, coming out looking like a great big orange pillow. As it cools, it falls.
It’s the perfect physics lesson to explain the Ideal Gas Law! And my daughter fell right into it when she asked, “Why does it puff like that?”
If you’re not familiar with the Ideal Gas Law, here it is:
Where P=pressure, V=volume, n=amount, R=ideal gas constant, and T=temperature of the gas.
From this equation, we can clearly see that, as the air in the bubbles of egg white heats up, the volume of the air will increase (assuming, of course that the bubbles themselves can expand and maintain a relatively constant pressure, which egg white does beautifully), causing the pie to puff. As it cools, the air decreases in volume, and the pie falls.
Proving that understanding thermodynamics is easy as pie!