Tartaric acid reacts with baking soda.
When baking soda comes into contact with an acid such as vinegar or citrus such as lemon juice or even an alkaline food such as coco powder or dark chocolate it creates CO2 gas.
Both acid and alkaline foods react with baking soda by releasing carbon dioxide gas which causes doughs used to make cakes and cookies expand and what gives them the texture and shape while baking.
The difference is liquid acids like citrus react quickly and dry ingredients such as Cream of Tartar need moisture to react.
Tartaric acid - Comes from the fermentation of grapes.It's a by product of fermentation process.
Cream of Tartar - Recrystallized tartaric acid from grape fermentation sometimes used in certain food preparations where texture and stability is required.Sometimes used in place of salt to ensure product quality.