When you combine baking soda (sodium bicarbonate) and vinegar (acetic acid), you trigger an acid-base chemical reaction. The two substances react to produce three new things: sodium acetate (a salt), water, and carbon dioxide gas — the CO₂ you see bubbling furiously out of the mixture. That gas is the invisible fire extinguisher.

Carbon dioxide is denser than air — about 1.5 times heavier. Unlike the smoke from a fire, which rises because it is hot and light, CO₂ sinks. It collects at the bottom of whatever container it is in. When you tip the jug toward the candle flame, the CO₂ spills out like an invisible liquid and flows downward, settling around and over the flame.

Fire needs three things to keep burning: fuel, heat, and oxygen. Remove any one of them and the fire stops. This is called the fire triangle. The CO₂ you pour onto the candle doesn't cool it down and doesn't remove the wax — it pushes the oxygen away. With no oxygen reaching the flame, the chemical reaction of combustion cannot continue. The flame goes out in an instant, even though nothing visible touched it.

The reaction between baking soda and vinegar is irreversible — the CO₂ escapes into the air and cannot be recombined back into the original ingredients. Once the gas is gone, so is your fire extinguisher. Which is why, if you try to relight the candle immediately, the flame catches — the CO₂ has dispersed and oxygen has returned.