The axid shower melts the Beebo doll but does nothing to the chair it's sitting in.
Axel comments that he has been trying to impress his father his whole life. When he was first introduced on the series, the original Trickster (Mark Hamill) revealed that he was his father.
Axel throws acid on Ralph, Cisco says "it's got a pH level in the negative 50s". pH level are strictly between 0 to 14, 7 being neutral, 7-14 being bases, so any acid's pH Level can only be 0-7, highest being 0. There is no "negative" pH level.