Dilution

Adding water dilutes a solution. The moles of solute stay the same and spread through a bigger volume, so the concentration falls:

c₁V₁ = c₂V₂

$200\ \mathrm{mL}$ of $0.2\ \mathrm{M}$ CaCl₂ plus $800\ \mathrm{mL}$ of water makes $1000\ \mathrm{mL}$: $c_2 = \frac{0.2 \times 200}{1000} = 0.04\ \mathrm{M}$.

Same particles, five times the volume

V₂ is the total volume, not the water added. The ions follow: [Ca²⁺] = $0.04\ \mathrm{M}$, [Cl⁻] = $0.08\ \mathrm{M}$.