Why is it unnecessary to supply all 1½ parts of oxygen when burning acetylene?

The correct answer points to the fact that when burning acetylene, a portion of the oxygen required for the combustion process comes from the surrounding air. Acetylene combustion, like other types of combustion, typically requires an oxidizing agent, and while pure oxygen can enhance the process, the ambient atmosphere already contains about 21% oxygen. This means that fewer than the theoretical amount of oxygen needs to be supplied directly to achieve efficient combustion.

In practical applications such as welding or cutting with acetylene, the mixture of acetylene and air allows the gas to ignite and burn effectively without needing the full 1½ parts of oxygen to be supplied externally. This not only makes the process more efficient but also lessens the amount of pure oxygen needed, making handling safer and more economical.

The other choices, while indicating different concepts, do not accurately reflect the dynamics of acetylene combustion. For instance, the idea that oxygen can be recovered after use does not apply in this scenario, as the oxygen is consumed during the burning process. Similarly, suggesting that acetylene burns completely without oxygen is misleading, because combustion indeed requires an oxidizer. Lastly, while heat is critical for initiating combustion, it does not replace the need for an oxidizing agent like oxygen.