Science, Tech, Math › Science Heat of Fusion Example Problem: Melting Ice Calculate the Energy Needed to Change a Solid Into a Liquid Share Flipboard Email Print Leonid Ikan / 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 November 09, 2019 Heat of fusion is the amount of heat energy required to change the state of matter of a substance from a solid to a liquid. It's also known as enthalpy of fusion. Its units are usually Joules per gram (J/g) or calories per gram (cal/g). This example problem demonstrates how to calculate the amount of energy required to melt a sample of water ice. Key Takeaways: Heat of Fusion for Melting Ice Heat of fusion is the amount of energy in the form of heat needed to change the state of matter from a solid to a liquid (melting.)The formula to calculate heat of fusion is: q = m·ΔHfNote that the temperature does not actually change when matter changes state, so it's not in the equation or needed for the calculation.Except for melting helium, heat of fusion is always a positive value. Example Problem What is the heat in Joules required to melt 25 grams of ice? What is the heat in calories? Useful information: Heat of fusion of water = 334 J/g = 80 cal/g Solution In the problem, the heat of fusion is given. This isn't a number you're expected to know off the top of your head. There are chemistry tables that state common heat of fusion values. To solve this problem, you'll need the formula that relates heat energy to mass and heat of fusion:q = m·ΔHfwhereq = heat energym = massΔHf = heat of fusion Temperature is not anywhere in the equation because it doesn't change when matter changes state. The equation is straightforward, so the key is to make sure you're using the right units for the answer. To get heat in Joules:q = (25 g)x(334 J/g)q = 8350 JIt's just as easy to express the heat in terms of calories:q = m·ΔHfq = (25 g)x(80 cal/g)q = 2000 calAnswer: The amount of heat required to melt 25 grams of ice is 8,350 Joules or 2,000 calories. Note: Heat of fusion should be a positive value. (The exception is helium.) If you get a negative number, check your math.