If I soak 5 grams of iron filings in 125 mL of water, does the mass of iron go up or down?
The Iron will turn to iron oxide (rust) thus increasing its mass by taking oxygen from the water.
The mass of iron will remain the same. It is possible that some of the iron will rust and form iron oxide (by reacting with oxygen that was dissolved in the water) but that does not change the amount of iron.
