Every fuel-burning experiment you run will give a value lower than the textbook. It's not technique—it's thermodynamics. Heat escapes to the air, warms the apparatus, and some fuel burns incompletely. Lower the efficiency and watch how the energy picture changes.
Set the experimental efficiency · 0.5 g ethanol burned releases 14.8 kJ · 100 g water
Heat escapes to the surroundings because air is a poor insulator—convection currents carry energy away faster than you can measure it. The apparatus itself (beaker, stirrer, thermometer) absorbs energy, warming up instead of just the water. Evaporation from the water surface carries latent heat away. Incomplete combustion produces CO instead of CO₂, releasing less energy per mole of fuel. All four factors stack, so even careful technique gives you 60–80% of the published value. Assessment tasks want you to list these specific mechanisms, not just say "heat loss."
Heat of combustion measures energy per gram or per mole of fuel, while efficiency is useful energy out divided by total energy in—experimental values are always lower because losses to surroundings, apparatus heating, evaporation, and incomplete combustion are unavoidable.