Question

How many elements are involved in the reaction of calcium with water?

Answer 1

Three - Calcium (Ca), Hydrogen (H) and Oxygen (O).

Explanation:

Every element has it's own symbol. This symbol represents one atom of the given element. For example in your equation, Ca just means one atom of Calcium.

`H_2O` means you have two atoms of Hydrogen (H) and one atom of Oxygen (O).

The 2 before the `H_2O` means you have two lots of `H_2O` (i.e. four hydrogen atoms and two oxygen atoms).

A molecule is a combination of elements (bonded together), i.e. `H_2O` and `Ca(OH)_2`.

Therefore in your equation you have three elements present and three molecules present. (Ca itself is not a molecule here as it is not bonded to anything - unlike `H_2` which is as two hydrogen atoms are bonded together).

Answer 2

Are there not three elements in total...?

Explanation:

`Ca(s) + 2H_2O(l) rarr Ca(OH)_2(s)darr + H_2(g)uarr`

And the three elements are calcium metal, hydrogen as its oxide, and oxygen as its hydride....the calcium is oxidized and the hydrogen is reduced.