Bioethanol is a fuel made from plants, and it stores less energy per volume than petroleum-based fuels. When you burn it alongside gasoline, diesel, and kerosene — holding a test tube of water above each flame — bioethanol produces the smallest temperature rise of the four. Gasoline heats the water the most, though it burns out fastest at just 41 seconds. Diesel takes the opposite approach: it burns the longest at 89 seconds but heats the water less than gasoline does.

Energy density determines how much heat a fuel can produce from a given amount. In this experiment, gasoline heats the water the most, while bioethanol produces the smallest temperature rise — showing that fuels with higher energy density release more heat when burned.

When you burn bioethanol, gasoline, diesel, and kerosene and record the highest water temperature each flame produces, you're measuring how much of the fuel's stored chemical energy actually becomes useful heat. Gasoline heats the water the most, while bioethanol produces the smallest temperature rise of the four. That gap in peak temperature shows that bioethanol converts less of its chemical energy into heat than gasoline, diesel, or kerosene do.