The gas necessary for burning fire is oxygen.

Burning is a chemical reaction that occurs between a fuel source and a gas called oxygen. Oxygen is an essential element for combustion to take place. When a fire burns, it combines oxygen from the air with a fuel source to produce heat, light, and other byproducts.

Explanation:
Here is a detailed explanation of why oxygen is necessary for burning fire:

1. Combustion process:
- Fire is a result of a chemical reaction called combustion.
- Combustion is a rapid oxidation process that occurs when a fuel source combines with oxygen in the presence of heat or ignition source.
- During combustion, the fuel molecules break apart and recombine with oxygen, releasing energy in the form of heat and light.

2. Oxygen as an oxidizer:
- Oxygen is considered an oxidizer because it readily reacts with other substances, including fuels.
- In the presence of oxygen, fuels undergo oxidation, which is a chemical reaction that involves the transfer of electrons.
- Oxygen acts as an electron acceptor, effectively oxidizing the fuel molecules and releasing energy in the process.

3. Role of oxygen in fire:
- Oxygen is a key component of the fire triangle, which represents the three elements necessary for fire: fuel, heat, and oxygen.
- Without oxygen, the combustion process cannot occur, and the fire cannot sustain itself.
- Oxygen molecules provide the necessary reactant for the fuel to burn, allowing the fire to continue.

4. Availability of oxygen:
- The Earth's atmosphere contains approximately 21% oxygen, which is sufficient for supporting combustion.
- When a fire burns, it draws in surrounding air containing oxygen molecules to sustain the reaction.
- This is why fires require an adequate air supply to continue burning.

Conclusion:
In conclusion, the gas necessary for burning fire is oxygen. Oxygen acts as an oxidizer and combines with fuel molecules during the combustion process, releasing energy in the form of heat and light. Without oxygen, the fire cannot sustain itself, making it an essential component for the burning process.