[KitCh]

Why is it that some compounds must always be written as the second reactant? For example, MgSO₄ and BaCl₂ must be written as BaCl₂ + MgSO₄ and NOT MgSO₄ + BaCl₂?

Another example - anything with sodium must (always?) be written as the second reactant as in this case - MgCl₂ + NaOH and not the other way around?

What about compounds with PO₄ or NO₃ etc.? Should they also always be written as the second reactant?

Thank you.

[Borek]

I am not aware of such rule, order of reactants in the reaction doesn't matter. Who told you it does?

[Arctic-Nation]

Though it is mostly a rule-of-thumb and not formal in any way, a reaction is usually written in the sense of 'reactants' + 'reagents' equals products. When you mix two compounds that react together and a distinction between reagent and reactant cannot be made (for example, HCl + NaOH), no rule exists. In the example you gave, barium chloride can be used to test for the presence of sulfate, making the latter the reactant. More commonly, sulfate is added to a solution to test for the presence of barium, making sulfate the reagent. However, except maybe for some chemical formalism nazis, most chemists couldn't care less.