Assuming that there is no phase change. Assuming a particular phase for convenience rather than from the problem description.
A well-written chemical equation should always indicate the particular phase of the compounds – solid (s) , liquid (l) , gas (g) or aqueous solution (aq) . However, it that information is not provided, looking an element up in the Periodic Table, or searching for a particular compounds usual state from a reference should be done. General familiarity with common compounds should be acquired by a student, as specific references may not always be available when needed.
In solutions, the primary error is not recognizing when a product compound becomes a precipitate (solid). The formation of a precipitate or the evolution of a gas from a set of compounds in solution is one of the key methods to determine the direction and products of a reaction. If no actual “reactions” occur between ions in solution, there is no useful chemistry taking place.