How should solder behave upon contact with the tube?

The correct behavior of solder upon contact with the tube is that it should melt on contact. When solder is properly heated, it transitions from a solid state to a liquid state, allowing it to flow and fill gaps, thus creating a strong joint between the materials being joined. This melting is essential for the solder to adhere effectively to the tube, ensuring a proper bond and sealing out any potential leaks.

Additionally, solder needs to reach a specific temperature, known as its melting point, which is typically lower than that of the materials being joined. This characteristic allows the solder to melt when it comes into contact with the heated surface of the tube, facilitating a good joint. The process involves the heat from the tube transferring to the solder, causing it to liquidize and thereby effectively fill any joint spaces.

The other behaviors suggested do not accurately represent the correct physical properties and outcomes associated with soldering. For example, vaporization would indicate extreme heat, which is not typical during the soldering process. Remaining solid or bubbling would not contribute to forming a good joint, as these states do not allow the solder to penetrate or adhere properly to the surfaces being joined.