The gas used in fire extinguishers is carbon dioxide.

Fire extinguishers are devices used to control or extinguish small fires in emergency situations. They work by removing one or more of the three elements necessary for a fire to burn: heat, fuel, and oxygen. Different types of fire extinguishers use different extinguishing agents to achieve this.

Carbon dioxide (CO2) is the gas commonly used in fire extinguishers.

Reasons for using carbon dioxide in fire extinguishers:

1. Non-conductive and non-corrosive: Carbon dioxide is non-conductive and non-corrosive, making it suitable for use on electrical fires and fires involving sensitive equipment. Since it does not conduct electricity, it can be safely used on live electrical equipment without the risk of electrical shock.

2. Displaces oxygen: Carbon dioxide is heavier than air and can quickly displace oxygen, suffocating the fire. When released from a fire extinguisher, it forms a blanket of gas over the fire, preventing oxygen from reaching the fuel and extinguishing the flames.

3. Does not leave residue: Carbon dioxide does not leave behind any residue after use, which is beneficial in situations where cleanup and damage control are important. It does not damage surfaces or leave behind any harmful or corrosive substances.

4. Effective on Class B and C fires: Carbon dioxide is particularly effective on fires involving flammable liquids (Class B fires) and electrical fires (Class C fires). It can quickly suppress these types of fires by removing the oxygen necessary for combustion.

5. Environmentally friendly: Carbon dioxide is a naturally occurring gas and does not contribute to ozone depletion or global warming. It is considered a more environmentally friendly option compared to other fire extinguishing agents.

In conclusion, carbon dioxide is the gas commonly used in fire extinguishers due to its non-conductive and non-corrosive nature, ability to displace oxygen, lack of residue, effectiveness on specific fire classes, and environmentally friendly properties.