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Mystery #4Tier 3chemistry / energy

Where does a reaction's heat actually come from, or go?

A hand-warmer pours out heat; a cold pack drinks it up. But heat can't appear from nowhere or vanish into nothing. So if the energy isn't being created or destroyed, where was it hiding before, and where does it actually end up?

Glowing orange sparks streaming from a campfire at night, energy stored in chemical bonds released as heat and light.
Photo: kallerna, Wikimedia Commons · CC BY-SA 3.0 · via Wikimedia Commons

Your guess

What's your hunch about where a reaction's energy is stored before it's released, and what it is stored IN?

There's no single magic word to type, have a go in your own words, then reveal the answer to see how close you got.

Make a guess to unlock the hints and the answer.