It releases carbon dioxide near the fire.
Explanation
Sodium bicarbonate ($NaHCO_3$), commonly known as baking soda, is widely used in dry chemical fire extinguishers. Its effectiveness relies on its chemical decomposition when exposed to high temperatures.
Detailed Analysis:
- Thermal Decomposition: When sodium bicarbonate is heated by the fire, it undergoes a decomposition reaction:
$2NaHCO_3 \xrightarrow{\text{Heat}} Na_2CO_3 + H_2O + CO_2$ - Role of Carbon Dioxide: The reaction releases carbon dioxide ($CO_2$) gas. Since carbon dioxide is heavier than oxygen and does not support combustion, it forms a blanket over the fire. This effectively cuts off the supply of atmospheric oxygen to the fuel, thereby extinguishing the flame (smothering effect).
- Comparison with other options:
- It releases oxygen when heated.: Incorrect. The reaction releases carbon dioxide, not oxygen. Releasing oxygen would intensify the fire.
- It absorbs heat from the fire.: While the decomposition reaction is endothermic (absorbs heat), the primary and most significant mechanism for extinguishing the fire in this context is the release of non-combustible gas ($CO_2$) to displace oxygen.
- It forms a solid layer over the fuel.: Incorrect. While the residue ($Na_2CO_3$) is solid, the primary extinguishing action is not the formation of a solid layer but the gaseous smothering.
Key Takeaway:
Sodium bicarbonate functions in fire extinguishers primarily by undergoing thermal decomposition to release carbon dioxide, which displaces oxygen and starves the fire.