Because of the pressure inside a popcorn kernel, water does not vaporize at 100
ID: 1991894 • Letter: B
Question
Because of the pressure inside a popcorn kernel, water does not vaporize at 100 degrees Celsius.Instead, it stays liquid until its temperature is about 175 degrees Celsius, at which point the kernel ruptures and the superheated water turns into steam. How much energy is needed to pop 95.0 g of corn if 14% of a kernel's mass consists of water? Assume that the latent heat of vaporization for water at 175 degrees Celsius is 0.90 times its value at 100 degrees Celsius and the kernels have an initial temperature of 175 degrees Celsius.Explanation / Answer
specific heat of water = 4.186 kJ/kgC
specific heat of steam = 2.1 kJ/kgK
heat of vaporization of water = 2256 kJ/kg
taking 0.9 of 2256 kJ/kg gives us 2030 kJ/kg
14% of 95g is 13.3g or 0.0133kg
energy to heat water to 175C from 25C
E1 = 4.186 kJ/kgC x 0.0133kg x 150C = 8.35 kJ
energy to boil water at 175C
E2 = 2030 kJ/kg x 0.0133kg = 27 kJ
Assuming, as you said, that the initial temp of the corn is 175C (which is pretty silly), then it takes 27 kJ
Assuming a more realistic initial temp of 25C, then it takes
E = 27+8 = 35 kJ