Why do airplanes leave white trails in the sky?

Answer: Hot exhaust water vapor freezes into ice crystals

Hot exhaust water vapor freezes into ice crystals ✓ — Correct! These trails are called 'contrails' (condensation trails). When jet fuel burns, it produces water vapor as a byproduct. At cruising altitude where temperatures drop to -40°C or colder, this hot humid exhaust hits the frigid air and the water vapor instantly freezes into tiny ice crystals. It's the same thing that happens when you see your breath on a cold day—just happening 10 kilometers up. If the air is too warm or too dry, no contrail forms.

Jet fuel burns white and smoke is visible from ground — Wrong. Jet fuel exhaust is mostly invisible gases (carbon dioxide, nitrogen, water vapor). The white you see isn't smoke or the color of burning fuel. It's ice crystals that form when water vapor from the hot exhaust freezes in the extremely cold high-altitude air.

Planes spray chemicals to seed clouds for weather control — Wrong. Contrails are not chemicals being sprayed. They're simply ice crystals formed from water vapor in engine exhaust. The water comes from burning jet fuel (hydrocarbons + oxygen = carbon dioxide + water). This is basic chemistry, not any kind of spraying operation.