Soda-acid fire extinguishers are a type of portable fire extinguisher commonly used to extinguish small fires. They work by releasing a stream of water mixed with a chemical agent to suppress the fire. The chemical agent used in a soda-acid fire extinguisher is baking soda.

Explanation:
1. Soda-acid fire extinguisher composition:
- Soda-acid fire extinguishers consist of two main components: a container filled with water and a separate container filled with a mixture of baking soda (sodium bicarbonate) and a weak acid.
- The two containers are connected by a tube or hose, and when the extinguisher is activated, the acid is released into the water, causing a chemical reaction.

2. Chemical reaction:
- When the baking soda (sodium bicarbonate) reacts with the weak acid (typically tartaric acid or sulfuric acid), it produces carbon dioxide gas.
- The carbon dioxide gas generated rapidly expands, creating pressure inside the extinguisher, which propels the water and the gas mixture towards the fire.

3. Suppression mechanism:
- The water in the extinguisher cools down the fire, reducing the temperature below the point at which combustion can occur.
- The carbon dioxide gas helps to suffocate the fire by displacing oxygen, which is necessary for combustion.

4. Advantages of baking soda:
- Baking soda (sodium bicarbonate) is commonly used in soda-acid fire extinguishers because it is non-toxic, non-corrosive, and effective against certain types of fires, such as Class B (flammable liquids) and Class C (electrical) fires.
- It is also readily available and affordable, making it a practical choice for fire extinguishers.

In conclusion, baking soda is used in a soda-acid fire extinguisher because it reacts with a weak acid to produce carbon dioxide gas, which helps to suppress the fire by cooling it down and displacing oxygen. Baking soda is a safe and effective chemical agent for extinguishing certain types of fires.