While trying to write the chemical formula for a compound (a neutral molecule), one must identify and exchange the charge of the cation with that of the anion to become the subscript of one other. For example
Aluminium oxide has Aluminium (Al) and oxygen (O); since Al has a charge of 3+ (the cation) and O has a charge of 2- (the anion), the compound would have it's charges as Al³⁺O²⁻ and when the charges are exchanged to there subscripts, it would form Al₂O₃; thus there would be two cations of aluminium for every three anions of oxygen in order to have a neutral molecule.
This same explanation can be given to Aluminium sulfite. Aluminium sulfite has Aluminium (Al) and sulfite (SO₃). Al has a charge of 3+ (cation) while sulfite has a charge of 2- (anion), with the compound having it's charges as Al³⁺(SO₃)²⁻ and when the charges are exchanged to there subscripts, it would form Al₂(SO₃)₃ and would thus have 2 cations of aluminium (Al³⁺) for every 3 anions of sulfite (SO₃³⁻) in order to have a neutral molecule.
when Cl2 is dissolved in water it generates ClO which is a good oxidizing agent since it gives out atomic oxygen. but dissolution of F2 cannot generate atomic oxygen since F is more electronegative than O
That would be choice D which might be called a double decomposition reaction - an exchange of ions where no electrons are lost or gained. The silver chloride appears as a white precipitate.
The composition of sea water, smoke, and soft drinks are not consistent, and you can separate the parts that make up each. Sodium has a definite and consistent composition, so it is a pure substance.