State Symbols in Equations
State symbols are abbreviations used in chemical equations to show the physical state of each reactant and product. They are essential for conveying the complete context of a reaction, including conditions and the nature of the substances involved.
Part of the ESAT Chemistry syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- The four standard state symbols are (s) for solid, (l) for liquid, (g) for gas, and (aq) for aqueous.
- A substance is in an aqueous state (aq) when it is dissolved in water to form a solution.
- The liquid state (l) refers to a pure substance in its liquid form, such as molten sodium chloride, NaCl(l), or pure water, H2O(l). This is distinct from an aqueous solution.
- State symbols help identify key reaction events, such as the formation of a solid precipitate (s) or the evolution of a gas (g).
- You must be able to deduce the correct state symbol from descriptions provided in a question, for example, 'a solution of' implies (aq).
Definitions
- State Symbol
- A one or two-letter code written in parentheses after a chemical formula to denote the physical state of that substance under the reaction conditions.
- Aqueous Solution
- A solution in which the solvent is water. Indicated by the state symbol (aq).
- Precipitate
- An insoluble solid that emerges from a liquid solution during a chemical reaction. It is always given the state symbol (s).
Worked example
When solid calcium carbonate is added to hydrochloric acid, a solution of calcium chloride is formed, along with liquid water and carbon dioxide gas. Write a balanced chemical equation for this reaction, including all state symbols.
- 1
Identify the reactants and their states from the description:
solid calcium carbonate is CaCO3(s) and hydrochloric acid (an acid in water) is HCl(aq).
- 2
Identify the products and their states:
a solution of calcium chloride is CaCl2(aq), liquid water is H2O(l), and carbon dioxide gas is CO2(g).
- 3
Write the unbalanced equation:
CaCO3(s) + HCl(aq) → CaCl2(aq) + H2O(l) + CO2(g).
- 4
Balance the equation.
There are two chlorine atoms and two hydrogen atoms on the right side, so we need a coefficient of 2 for HCl on the left.
- 5
The final balanced equation is:
CaCO3(s) + 2HCl(aq) → CaCl2(aq) + H2O(l) + CO2(g).
Answer: CaCO3(s) + 2HCl(aq) → CaCl2(aq) + H2O(l) + CO2(g)
Common mistakes
- ×Confusing (l) with (aq). Remember (l) is for a pure liquid (e.g. H2O(l), Br2(l)), while (aq) is for a substance dissolved in water (e.g. NaCl(aq)).
- ×Incorrectly assigning states to common substances. For example, acids like HCl or H2SO4 are typically used as aqueous solutions (aq) in school-level chemistry.
- ×Forgetting to include state symbols in the final answer when the question explicitly asks for them.
No-calculator tips
- ✓State symbols are determined by chemical knowledge and careful reading, not calculation. Look for keywords in the question.
- ✓Keywords like 'solution', 'dissolved in water' point to (aq).
- ✓Keywords like 'precipitate', 'solid', 'powder' point to (s).
- ✓Keywords like 'gas evolved', 'fizzing', 'effervescence' point to (g).