A student puts a tiny piece of metal into a test tube filled with water. She then puts a cap on the test tube. Which change provides evidence that a chemical reaction occurred?

A)the water bubbles
B)the mixture gets heavier
C)the water level increases
D)the metal sinks to the bottom

Cr(oh)3 -> cr(3+) + 3oh-

Answer is: hydrogen is being oxidized.chemical reaction: co₂ + h₂ → co + h₂o.oxidation half reaction: h₂ → 2h⁺ + 2e⁻.reduction half reaction: c⁺⁴ + 2e⁻ → c⁺².oxidation is increase of oxidation number, hydrogen change oxidation number from neutral charge (in molecule of hydrogen) to oxidation number +1 (in water).