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Difference between опухать & распухать?

It's convertible terms. But usually we will use опухать when man look bad (he woke up, for example, after booze and can't opened his eyes because his eyelids were swollen). And распухать when man ill. For example he fell and broke his leg. It swelled. Or when his teeth hurts, his cheek will swell.

"Распухать" мы говорим когда заметно меняется размер того, что распухло.
"Опухать" мы говорим когда размер особо не изменился, т.е. распухание без особого изменения размера. Обычно "опухло/опухла/опухли" говорят о частях тела, которые изменили своё состояние в результате болезни или употребления алкоголя.

It means the same, but "распухать" is used only in conversational language.

Опухать is used for a local place on one's body. For example a bee stung somebody and this place (an arm, a chick etc) a bump appeared (опухло).
If somebody for example injured his leg and one leg became bigger than another that is распухать. Not a local place but the whole leg.
Also we can use опухать when we speak about a face. If you sleep too much or drank before all your face became bigger and does not look good it is also опухать.
in general, опухать means swell a bit or more. Распухать can mean a person need a doctor (a surgery) immediately. Too much swell.

The difference is insignificant or there is no difference at all depending upon the context.