When a plumber finishes soldering a joint, what should be used to test it for leaks?

• A. Air
• B. Water
• C. Nitrogen
• D. Soap

Answer: (B)

When a plumber finishes soldering a joint, testing with water is preferred because it provides a safe, realistic pressure test that makes leaks easy to see. Water is essentially incompressible, so filling the system to a set pressure and watching for dripping or wet spots at each joint clearly reveals leaks without subjecting the fittings to dangerous or high-velocity gas forces. Using air can risk blowing apart weak joints or driving leaks that aren’t immediately visible, and nitrogen isn’t the typical medium for testing potable-water lines in this context. Soap is helpful as a leak-detection aid (you’ll see bubbles where there’s a leak) but it doesn’t supply the actual pressure test. So, the testing medium of choice is water.