How to Identify States of Matter in a Chemical Formula

Chemical equations use chemical formulas to represent reactants that enter into a chemical reaction and products that form from a reaction. Chemical formulas are comprised of one or two letters to represent the type of atom present, as well as subscript numbers to indicate multiple atoms that form a molecule -- for example, the common formula H₂O represents two hydrogen atoms and one oxygen atom to form the water molecule. Molecules like H₂O can be in various states of matter, so you need some additional information to determine which state of matter was involved in the equation.

You can often discern the states of matter from the text. Look for contextual clues if no state of matter is written after the chemical formula. You might have a description of an equation, such as “two sodium atoms dissolve in water to produce aqueous sodium hydroxide and hydrogen gas.” Similarly, you can tell that gas was produced when the text describes observable bubbles forming after an acidic substance like vinegar reacts with a base like baking soda in a neutralizing reaction.

Learn “rules” about common molecules that will likely be in one state of matter if no parenthetical state is notated. For example, ions are unstable atoms or molecules that either have an extra or a missing electron. Ions with opposite charges bond and form an ionic compound like salt. If you know a substance is an ionic compound, you know it will likely be a solid because ionic compounds in their standard state, or 25 degrees Celsius and 1 atmosphere, are solids; an exception to this rule is the case when the ionic compound is dissolved in water. When two soluable salts react, an insoluable salt will form as a solid precipitate in the solution. In combustion reactions, if a hydrocarbon burns with sufficient oxygen supply, carbon dioxide gas and water vapor are released as products.

Look for the notation (s), (l), (g) or (aq) in a chemical formula, regardless of whether it is in the context of an equation. This text tells you the state of matter of the molecule -- solid, liquid, gas or aqueous solution. The term aqueous solution refers to a substance dissolved in water. For example, NaCl(aq) is table salt dissolved in water. The letter in parentheses will always be after the chemical formula. The empirical formula NaCL without notations represents the solid form of salt, like you see in a table shaker.