​Why does one burn?

​You might even leave your whole s’more sitting near the fire to help melt the chocolate. In that moment, the campfire is transferring energy to the wood in the fire and to the chocolate, but they’re responding in very different ways. The wood is burning, while the chocolate in the s’more is melting. What’s the deal?

​Melting - Solid to Liquid

Melting is a phase change from the solid phase to the liquid phase. The molecules in a solid chocolate bar are packed tightly together and can only move in place. When energy is added to the solid chocolate—like thermal energy from a fire—the molecules gain energy and begin to move around more.

​Changing Phase Does Not Change The Substance

​When they have enough energy and freedom of movement to flow around each other, the chocolate becomes a liquid. It’s important to remember that melting doesn’t change substances into other substances. . The molecules that make up your chocolate bar haven’t changed; they’re just moving around more.

​Combustion

​The wood in your campfire is also receiving energy from the fire, but it definitely isn’t melting into a liquid. Instead, it’s burning. Burning, also known as combustion, is a chemical reaction that happens when some materials reach a certain temperature and their molecules react with oxygen. The fire transfers energy to the wood, raising its temperature.

​Now That, Is Hot!

​The fire transfers energy to the wood, raising its temperature. When the wood reaches about 150°C (302°F), its molecules begin to break down. Some change into the gas phase and escape into the air; others react with oxygen in the air and give off light and heat. Some parts of the wood don’t burn and are left behind as ash.

​What Do Fires Need to Burn?

​ The combustion reaction keeps repeating itself until it runs out of fuel (like wood) to burn, until its supply of oxygen is cut off, or until its temperature is no longer hot enough to keep breaking down the molecules in the wood. That’s why the fire goes out a little while after you stop adding new logs. Without fuel to burn, the reaction that causes the fire can’t continue.

​Melts vs Burns

​So the chocolate on your s’more melts, while the wood in the campfire burns. But why? Every material has a different temperature at which it melts (its melting point) and a temperature at which it burns (its flash point). How the material reacts to heat depends on which of those temperatures is lower.

​Flash Point

If a material’s flash point is lower than its melting point, that material will burn before it melts.

If a material’s melting point is lower than its flash point, it will melt before it burns.

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Wood’s flash point is lower than its melting point, so it burns. Chocolate’s melting point is lower than its flash point, so it melts.