The police force said said 10 people, including the boy, were complaining of stinging or itchy eyes and skin, but "responded well to treatment" at the scene.
Members of the public detained the boy and store staff gave first aid treatment until ambulance crews arrived along with police officers and firefighters.
Avon and Somerset Police said the liquid is believed to be “a type of home-made ‘chilli spray’ with no strong acid or alkaline ingredients”.
The teenager has been arrested on suspicion of attempting to cause grievous bodily harm and administering a noxious substance, and remains in police custody.
“A juvenile was arrested on suspicion of attempting to cause grievous bodily harm and administering a noxious substance, and remains in police custody.
