Soot is the black powder that is left after something is burned.
Soot can make people think of dirty things, like old chimneys or fireplaces.
Examples:
The fireplace was covered in soot.
The soot from the fire made everything dirty.
The soot got on my hands and made them black.
The cat got into the chimney and came out covered in soot.
The dragon's breath made so much soot that it covered the whole town.
The soot from the fire made the whole room black.