Police say the 16-year-old girl was found after fleeing to California following the attack.
Authorities have not identified the teen girl because, so far, she is still being charged as a juvenile.
She was arrested in California and brought back to Chicago Saturday night. Police say the teenager did confess to the crime. Now, they're looking for another teenage girl and others. One is caught, the other still wanted.
Police say the two teenage girls seen on a surveillance videotape are responsible for throwing a caustic liquid on 47-year-old Medina, as she was getting into her car. The substance was so caustic that it caused third-degree burns over nearly one-third of her Medina's body.
Authorities say a nearby security camera caught the offenders arriving and leaving in the Logan Square neighborhood where the attack occurred. A break in the case came the same day when Cicero police contacted Chicago police.
"They reported they had checked a well-being call the night of this incident, in which they discovered a 16-year-old female suffering from acid burns on her chest and articles," said Chicago Police Cmdr. Joe Salemme.
According to police, the girl gave conflicting stories on how the burns occurred. The next day, others recognized her from the videotape.
"They stated that the person seen the night before was one of the people seen on the video," Salemme said.
The 16-year-old then fled to California, where she was arrested over the weekend.
Medina's daughters, who asked for public help the day after the attack, are relieved someone is caught but frustrated over the motive.
"I don't know why somebody would not like my mom. She gives what she has to other people," said Shirley Medina.
Esperanza Medina remained in stable condition at a hospital Sunday night. She had already undergone several very painful skin grafts.
On the day of the incident, Medina was ready to go to work as a social worker in Aurora. Her family members are convinced she must have been targeted in the attack.