what is sodium carbonate and citric acid?

Sodium carbonate, Na2CO3, is a white powdery compound which soluble in water used in the manufacture of baking soda, sodium nitrate, glass, ceramics, detergents, and soap. It is also used in processing wood pulp to make paper, in refining aluminum, in water softening, and in many other applications.
Citric acid, C6H8O7, is a colorless translucent crystalline acid, principally derived by fermentation of carbohydrates or from lemon, lime, and pineapple juices and used in preparing citrates and in flavorings and metal polishes. Citric acid is a relatively strong organic acid, and is very soluble in water. It occurs in relatively large quantities in citrus fruits. Today, essentially all of the commercial citric acid is produced by fermentation. Citric acid is widely used in the food and pharmaceutical industries. In foods it is used primarily to produce a tart taste and to complement fruit flavors in carbonated beverages, beverage powders, fruit-flavored drinks, jams and jellies, candy, sherbets, water ices, and wine. It is also used to reduce pH in certain canned foods to make heat treatment more effective, and in conjunction with antioxidants to retard enzymatic activity.

圖片參考：http://cp.gsm.com/apps/images/structures/001/citricac.gif

I hope this can help your understanding.

sodium carbonate is alkaline and citric acid is acid