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Material World

Fuel

A fuel stores energy in chemical bonds or in a nucleus, and the two are separated by about six orders of magnitude. What they have in common is not a chemistry but an economics: each was, in its period, the cheapest way to obtain heat at the scale required.

Energy density is two numbers and confusing them is the commonest error in the field. Hydrogen has the highest energy per unit mass of any chemical fuel and among the lowest per unit volume, which is why it suits a rocket and is awkward in a car.

Each of these uses is established and cited on its own. Whether something counts as an ore, for instance, depends on grade, price and era — and those qualifications are shown alongside it.