Bicarbonate of soda, also known as sodium bicarbonate or baking soda, is classified as a weak base. When dissolved in water, it can accept protons (H+ ions) from acids, which is characteristic behavior of a base.
In a chemical context, bases are substances that can neutralize acids. Bicarbonate of soda reacts with acids, producing carbon dioxide gas, water, and salt. This property is why it’s commonly used in baking; it reacts with acidic ingredients to create bubbles of carbon dioxide that help baked goods rise.
While it does have some acidic properties due to its ability to act as a weak acid (releasing carbonic acid under certain conditions), its primary classification is as a base. In household terms, if you test it with litmus paper, it will turn red litmus blue, indicating its basic nature.