You are born of acid, and have control over it.

Heart of Acid

At 1st level, whenever you start casting a spell of 1st level or higher that deals acid damage, acidic magic spills from you. This spillage causes creatures of your choice that you can see within 10 feet of you to take acid damage equal to half your sorcerer level (minimum of 1). You are also resistant to acid damage.

Acid Affinity

Starting at 6th level, when you cast a spell that deals acid damage, you can add your Charisma modifier to one damage roll of that spell. These spells ignore resistance, and immunity drops to resistance.

Acidomancer's Fury

Starting at 14th level, when you are hit by a melee attack, you can use your reaction to deal acid damage to the attacker. The damage equals your sorcerer level, and ignores resistance to fire damage.

Acid Soul

At 18th level, you gain immunity to acid damage. In addition, any spell or effect you create ignores resistance to acid damage and treats immunity to acid damage as neutral to acid damage. Likewise, resistance to acid becomes vulnerability.