(1) The outermost layer of the calcium atom loses 2 electrons, forming an ion with 2 positive charges. The calcium ion is expressed as: Ca2;

(2) By tag It can be seen that the pH value of the mineral water is 7.5~8.5, so the mineral water is alkaline;

(3) The mass fraction of silicon element in metasilicic acid = 2878×100≈35.9.

So the answer is: (1) Ca2; (2) Alkaline; (3) 35.9.