Question

When dal is cooked in a pressure cooker, the pressure cooker’s inner surface becomes black.

Answer 1

When pressure cooker is used for cooking dal :
 
--->Mostly the outer bottom surface gets blackened due to incomplete combustion reaction of fuel which results in soot.
---> If the inner surface gets blackened there could be two reasons:
1.Dal is cooked for more time .
2. Water might have been used up for cooking dal. Hence due to insufficient water, inner surface  part may turn black