The substance used to prevent oxide formation on freshly cleaned copper before soldering or brazing is called what?

• A. Solder
• B. Acetone
• C. Flux
• D. Alcohol

Answer: (C)

When copper is heated for soldering or brazing, its surface tends to form oxide that blocks solder from wetting and bonding. The substance used to prevent that oxide from forming and to help the solder flow is flux. Flux cleans the copper surface, dissolves oxides, and creates a protective layer as heating occurs, which lets the solder wet the metal and fill the joint properly. In plumbing practice, flux often contains compounds that chemically clean and barrier-oxide the surface, improving capillary action and joint integrity. Solder is the filler metal, not the oxide-preventive agent. Cleaning solvents like acetone or alcohol remove oils and debris but don’t prevent oxide formation during heating.