When the pressure of a confined gas is doubled at a constant temperature, the volume of the gas is reduced by half, this statement is an example of

Boyle's Law

Explanation:

Boyle's Law states that the volume of a gas is inversely proportional to the pressure of the gas at constant temperature so if the pressure goes up then the volume goes down and vice versa

When the metal can exist in more than one oxidation state

Explanation:

When metal exhibit more than one oxidation sate, we have to write the oxidation number of the metal in front of it to for a better clarity. Most of the transition metals exhibit more than one oxidation state due to presence of d electrons. For example the oxidation state of iron are "+2 and +3"

The oxidation state of copper are "+1 and +2"

The oxidation state of Mn are :"+2", +4, +7. so we generally write there oxidation state in roman For Mn compound in +2 oxidation state we write Mn(II) For Mn compound in +7 oxidation state we write Mn(VII)