Writing Symbols of physical states

Physical states tell us about the visible forms of elements/compound. Above both the balanced equations do not tell us anything about the physical state of elements / molecules of reactant and product.

To make a chemical equation more informative, the physical states of the reactants and products are mentioned along with their chemical formulae. The gaseous, liquid, aqueous and solid states of reactants and products are represented by the notations (g), (l), (aq) and (s), respectively.

The word aqueous (aq) is used when reactant or product is soluble in water or dissolve in water.

Now applying these symbols of physical  state we have the balanced equation – I

3Fe (s) + 4H₂O (g) → Fe₃O₄ (s) + 4H₂(g)

// Here we see that H₂O is gaseous state it means it is water  vapour or steam. //

Writing medium/condition in presence of which reaction takes place

Sometimes we write the reaction conditions like temperature, pressure and catalyst etc. i.e. heat, sunlight, catalyst like nickel, palladium, chlorophyll etc.  

Examples:

(i)  

(ii) 

Precipitation Reaction: 

Any reaction that produces a precipitate can be called a precipitation reaction.

Na₂SO₄(aq) + BaCl₂(aq) → BaSO₂(s) + 2NaCl(aq)

(Sodium sulphate) + (Barium chloride) → (Barium  sulphate) + (Sodium chloride)

The white precipitate of BaSO₄ is formed by the reaction of SO₄ and Ba. The other product formed is sodium chloride which remains in the solution.

This white precipitate BaSO4 is insoluble in water; 

Precipitate: This insoluble substance formed is known as a precipitate.