State Symbols

Why chemical equations include states

A chemical formula identifies a substance, but it does not necessarily tell you its physical state. State symbols add this information to a chemical equation. They are written in round brackets immediately after each formula.

The four symbols you need are:

• $(s)$ for solid
• $(l)$ for liquid
• $(g)$ for gas
• $(aq)$ for an aqueous solution

For example, $H_2O(l)$ represents liquid water, while $CO_2(g)$ represents carbon dioxide gas. State symbols belong to the whole substance, so they are placed after its complete formula rather than after an individual element symbol.

Aqueous does not simply mean liquid

The distinction between $(l)$ and $(aq)$ is especially important. The symbol $(l)$ means that the substance itself is a liquid. The symbol $(aq)$ means that the substance has been dissolved in water to make a solution; “aqueous” is related to the Latin word for water.

For instance, pure liquid water is written $H_2O(l)$. Hydrochloric acid is hydrogen chloride dissolved in water, so it is written $HCl(aq)$. In an aqueous sodium chloride solution, sodium chloride is the solute and water is the solvent; the sodium chloride is written $NaCl(aq)$.

A substance does not have one unchanging state symbol. Its state depends on the conditions and how it is being used. Pure ethanol at room temperature is $C_2H_5OH(l)$, but ethanol dissolved in water is described as aqueous and may be written $C_2H_5OH(aq)$.

Reading a complete equation

Consider the reaction between copper carbonate and hydrochloric acid:

$CuCO_3(s) + 2HCl(aq) \rightarrow CuCl_2(aq) + CO_2(g) + H_2O(l)$

The equation communicates both chemical change and physical state:

• copper carbonate is a solid;
• hydrochloric acid is an aqueous solution;
• copper chloride remains dissolved in water;
• carbon dioxide is produced as a gas;
• water is formed as a liquid.

This extra information helps you picture what happens: a solid reacts with a solution, gas bubbles are released, and a dissolved salt remains in the mixture.

Choosing appropriate symbols

Use information in the question, your knowledge of the specified reaction and its conditions. Words such as powder, crystals or precipitate usually indicate $(s)$; gas or evidence such as bubbling may indicate $(g)$; and solution, dissolved or the name of an acid used in water usually indicates $(aq)$. A named pure liquid is labelled $(l)$.

Solubility matters when deciding between $(s)$ and $(aq)$. A soluble salt dissolved in the reaction mixture is aqueous, whereas an insoluble product that forms a precipitate is solid. Do not label something $(aq)$ merely because it is in contact with water: it must be dissolved in that water.

Adding states to a balanced equation

First write the correct formula for every reactant and product. Balance the equation by placing coefficients in front of formulae; never alter a chemical formula to make the atom totals match. Then place the appropriate state symbol directly after every formula.

For example:

$2Al(s) + 3H_2SO_4(aq) \rightarrow Al_2(SO_4)_3(aq) + 3H_2(g)$

The coefficients show the reacting proportions. The state symbols provide different information: aluminium is solid, sulfuric acid and aluminium sulfate are in aqueous solution, and hydrogen is gaseous. Together, the formulae, coefficients and state symbols form a complete description of the reaction.

Recall the four symbols

• solid: $(s)$
• liquid: $(l)$
• gas: $(g)$
• dissolved in water: $(aq)$

Write each symbol in round brackets immediately after the complete formula.

Make the key distinction

$(l)$ means the substance itself is liquid. $(aq)$ means it is dissolved in water. Therefore, water is $H_2O(l)$, while hydrochloric acid is $HCl(aq)$.

Select and check

Use the stated conditions and reaction information. Look for clues such as solid, precipitate, liquid, gas, solution or dissolved in water. Check that every substance has a state symbol when requested and that the equation is also balanced.

Example:

$CuCO_3(s) + 2HCl(aq) \rightarrow CuCl_2(aq) + CO_2(g) + H_2O(l)$