What precaution should be taken before using a stopcock?

Filling the stopcock completely before use is essential because it ensures that there are no air bubbles trapped inside the device, which could interfere with its operation and affect the accuracy of measurements or the flow of liquid. A properly filled stopcock also provides a more reliable seal, preventing leaks and ensuring that the liquid flows smoothly when dispensed.

The notion of filling the stopcock completely relates to maintaining a consistent and controlled flow, which is critical in laboratory procedures where precision is necessary. Using an incompletely filled stopcock might result in fluctuations in flow rate, inconsistent delivery of liquid, or contamination of samples due to air exposure.

In contrast, checking for leaks, heating the stopcock, or marking it with the liquid type are important safety and operational practices, but they do not directly address the immediate concerns of ensuring proper function and reliability of the stopcock at the onset of its use. These precautions can enhance safety and understanding but are secondary to ensuring a complete fill for correct operation.