why are physical states of the reactants and products often indicated when writing a chemical equation?
Some substancescan react only in certain forms. For example, when we mix solid sodium chloride with solid silver nitrate, the reaction between these compounds doesn’t occur. These substances can react only when they both are in a solution, so the silver chloride would form and precipitate out (we write it as a solid; it helps explain why it's not written in separate ions like it would were it soluble and in a solution). Combustion, also, occurs with products and reactants in the gas stage only.