Science, Tech, Math › Science Heat of Vaporization Example Problem How to Calculate the Energy Needed to Turn Water Into Steam Share Flipboard Email Print Adam Gault / Getty Images Science Chemistry Basics Chemical Laws Molecules Periodic Table Projects & Experiments Scientific Method Biochemistry Physical Chemistry Medical Chemistry Chemistry In Everyday Life Famous Chemists Activities for Kids Abbreviations & Acronyms Biology Physics Geology Astronomy Weather & Climate By Todd Helmenstine Todd Helmenstine is a science writer and illustrator who has taught physics and math at the college level. He holds bachelor's degrees in both physics and mathematics. our editorial process Todd Helmenstine Updated December 06, 2019 Heat of vaporization is the amount of heat energy required to change the state of a substance from a liquid into a vapor or gas. It is also known as enthalpy of vaporization, with units typically given in joules (J) or calories (cal). Heat of Vaporization Problem This sample problem demonstrates how to calculate the amount of energy required to turn a sample of water into steam: What is the heat in joules required to convert 25 grams of water into steam? What is the heat in calories?What you know: Heat of vaporization of water = 2257 J/g = 540 cal/g Note: You won't be expected to know enthalpy or heat values; they will be given in a problem or can be looked up in a table. How to Solve You can solve this problem using either joules or calories for heat. Part I: Use the formula q = m·ΔHv in which q = heat energy, m = mass, and ΔHv = heat of vaporization.q = (25 g)x(2257 J/g)q = 56425 JPart II: q = m·ΔHfq = (25 g)x(540 cal/g)q = 13500 cal Answer The amount of heat required to change 25 grams of water into steam is 56425 joules or 13500 calories. A related example illustrates how to calculate the energy when water changes from solid ice into steam.