# Why does copper oxidise if it is considered a “noble metal”?

I'm an electrical engineer and not a chemist, so please excuse me if this is obvious.

When soft-soldering electronic parts, we’re normally taught to clean copper of oxides (like using flux and so on) before soldering. Also I’ve observed that when heating copper to $$\ce{\approx 200 ^\circ C}$$ copper changes colour and builds an oxide layer. So why is copper considered a noble metal? Shouldn’t it be resistant to oxidation?

If the definition of noble metal is “less oxidation” instead of “no oxidation": Does that mean that gold and silver also build a small oxide layer in the $$\pu{25 ^\circ C} - \pu{200 ^\circ C}$$ temperature range?

• Nobel metal is a very woolly concept without much scientific basis. Silver and mercury are pretty reactive too. I would not give much time to the concept. – Waylander Apr 18 '20 at 9:22
• Even Pt oxidises, who's Cu? – Zenix Apr 18 '20 at 9:34
• In context of chemistry of metals, the adjective noble means more or less they do not dissolve in non oxidating acids. But copper slowly dissolves even in vinegar in oxygen presence. – Poutnik Apr 18 '20 at 10:35
• To my understanding "noble metal" meant that those metals frequently found in the metallic form in nature rather than an ionic form. Gold silver platinum and copper were all discovered as they can be readily obtained straight from the ground without smelting an ore. Most modern copper is mined as an ore and smelted to the metallic form, but substantial amounts of the metallic form do naturally occur. Many metals resist bulk oxidation by forming a protective surface oxide layer. – Max Power Apr 18 '20 at 21:50
• Pure gold(24 karat gold) does not tarnish at all but if that is alloyed with other metals, it tends to tarnish(onecklace.com/tips/does-real-gold-tarnish). As for silver, it is easily tarnished from oxygen and moisture(ncbi.nlm.nih.gov/pmc/articles/PMC5111099) and is common knowledge that silver reacts with sulfur in air to form black $\ce{Ag2S}$. See: chemistry.stackexchange.com/questions/78882/… – Nilay Ghosh May 2 '20 at 5:21