Wife got burn her legs in.kitchen with boiled temp water it was just red but now.next day it become dark.skin and bubbles please tell prevention plz?

Cleanse+silvadene : It is best to se a physician. Second degree burns can become third degree burns if they get infected. Keeping them clean and moist will speed up healing to minimize healing which is usually done with cetaphil and silvadene (silver sulfadiazine) twice a day. Antibiotics are prescribed if infection is suspected. Leg elevation* gentle compression. No sun/tobacco.

About burns: Scald burns are very frequently full thickness burns. The fact that it has become dark is not a good sign. If it is small( < quarter) in size it wil probably heal. You want to watch for increasing redness around the burn or red streaks up the legs. You should have her evaluated by a general or plastic surgeon. Silvadene (silver sulfadiazine) cream is useful. Surgery may be needed. These may be trouble.

See doc: Sounds like a partial thickness, or second degree burn. You should really let a doc see it, but until then you could apply a triple antibiotic ointment like neosporin and a non adherent dressing like xeroform, or silvadene (silver sulfadiazine) cream until then.