5. Identify the oxidation state of Ca in CaF2 (aq).

Ca (s) + 2 HF (aq) → CaF2 (aq) + H2 (g)

+2

Explanation:

Chemical equation:

Ca + 2HF → CaF₂ + H₂

The oxidation state of Ca on left hand side is 0 while on the right hand side when it form CaF₂ its oxidation state becomes + 2 because it loses two. These two electrons are accepted by fluorine and complete its octet while Ca loses its two out most electron to complete the octet.

In this reaction oxidation of calcium takes place and calcium itself is reducing agent.

Reducing agents:

Reducing agents reduced the other element are it self gets oxidized.

Oxidizing agents:

Oxidizing agents oxidize the other elements and itself gets reduced.

Consider another examples.

4KI + 2CuCl₂ → 2CuI + I₂ + 4KCl

the oxidation state of copper is changed from + 2 to + 1 so copper get reduced.

CO + H₂O → CO₂ + H₂

the oxidation state of carbon is + 2 on reactant side and on product side it becomes + 4 so carbon get oxidized.