A white man made a sexual and racist comment at the black mother, 29, before throwing the substance at her back. She turned round to confront the offender, who then threw more liquid at her chest from a plastic container.
The woman was treated for burns and is now in a stable condition in hospital. Her baby was not injured.
The incident happened in a subway leading to Salford Shopping City in Salford, Greater Manchester, at around 2pm last Thursday. The offender, who is thought to be in his 40s and is described as having a stocky build with short, dark hair with a bald patch, ran off towards Broad Street, Pendleton in Salford.
Detective Constable Jim Harris, from Salford CID, said: “This is an appalling attack on a lady who was walking with her very young baby.
“Thankfully none of the corrosive liquid hit the baby or went on the victim’s face as the incident could have been a lot worse than it already is.”
The man was wearing a long-sleeved dark grey jumper with a round neck, black tracksuit bottoms with a grey and white stripe down each leg, and black Adidas trainers.
“The offender needs to be caught and held responsible for his actions which have caused a nasty injury to the lady and much distress,” said DC Harris.
“I would urge anyone who may have seen this man running away from the subway or may have witnessed the attack to contact police as soon as possible.”