Soot means carbon-rich fuel, like RP1, and a very fuel-rich mix. Most launches I ever saw had basically zero soot, and a clean exhaust of a well-balanced fuel / oxidizer mix.
Military rockets, and solid-fuel boosters like the kind the Shuttles used to use, indeed produce very visible exhaust, because they use heavy fuels, and sometimes heavier oxidizers, like nitric acid. This is because they need to be in the fueled state for a long time, ready to launch in seconds; this excludes more efficient but finicky cryogenic fuels used by large commercial rockets.
The large plumes that you usually see the first few seconds when a rocket is blasting off a launch pad are mostly water vapor. The launch pad would be destroyed by the exhaust were it not cooled during the launch by large amounts of water, which gets evaporated instead of the concrete.
hyperbrainer
The water also serves the purpose of reducing reflecting acoustic energy.
Military rockets, and solid-fuel boosters like the kind the Shuttles used to use, indeed produce very visible exhaust, because they use heavy fuels, and sometimes heavier oxidizers, like nitric acid. This is because they need to be in the fueled state for a long time, ready to launch in seconds; this excludes more efficient but finicky cryogenic fuels used by large commercial rockets.
The large plumes that you usually see the first few seconds when a rocket is blasting off a launch pad are mostly water vapor. The launch pad would be destroyed by the exhaust were it not cooled during the launch by large amounts of water, which gets evaporated instead of the concrete.