# Boiling aluminum in high-alkalinity water

This question ultimately relates to brewing beer, but more specifically to water treatment before brewing.

Though I can't get a water quality report, I suspect I have high levels of bicarbonate alkalinity in my tap water because boiling it causes a white precipitate (which I think is calcium carbonate) to form. I base this assumption on this formula, in the book Brewing:

$$\ce{Ca(HCO3)2 ->[heat] CaCO3 + CO2 + H2O}$$

I wanted to sanitize some aluminum foil so I boiled it in fresh tap water for about 15 minutes. I noticed a few things:

• The precipitate formed much faster than it normally would without foil in it.
• The foil began to blacken in certain parts.
• I'm not positive, but the precipitate seems a bit finer in texture than it normally is.

I thought at first maybe the foil was just providing nucleation points for the carbonate to form around. But then I thought I remembered reading somewhere that aluminum competes with calcium in certain situations (I think it was regarding osteoporosis, maybe). The question I have, then, is whether or not it's feasible that aluminum could be affecting a different reaction than what I'm assuming normally occurs in this situation?