Abottle is labeled as 2.5 m ca(oh)2. what is the concentration of ca2+?

2.5 mol·L⁻¹  

Explanation:

                        Ca(OH)₂(aq) ⟶ Ca²⁺(aq) + 2OH⁻(aq)

I/mol·L⁻¹ :               2.5                    0                    0

C/mol·L⁻¹:              -2.5                 +2.5               +5.0

F/mol·L⁻¹:                 0                    2.5                 5.0

Ca(OH)₂ is a strong electrolyte. It dissociates completely in solution to form ions

Initial

Assume you dissolve 2.5 mol of Ca(OH)₂ in enough water to make 1 L of solution.

In the instant before it dissociates, you have a 2.5 mol·L⁻¹ solution of Ca(OH)₂ and no Ca²⁺ ions.

Change

The Ca(OH)₂ is dissociates completely into ions. You lose 2.5 mol of Ca(OH)₂ and form 2.5 mol of Ca²⁺.

Final

You end up with no formula units of Ca(OH)₂ and 2.5 mol of Ca²⁺.

answer is: c. the experiment shows that the red substance experienced a chemical change.

this is chemical change (chemical reaction), because two new substances are formed (metalig liquid and gas), the atoms are rearranged and the reaction is followed by an energy change.

this is example of chemical decomposition.

chemical decomposition (analysis) is the separation of a single chemical compound into its two or more elemental parts or to simpler compounds.  

chemical decomposition is opposite of chemical synthesis.