A chemical equation illustrates the transformation of reactants into products. While the numerical ratios of molecules (stoichiometry) are important, the physical context of the reaction is equally important. State symbols provide this necessary physical information, indicating the physical form of each substance as the reaction occurs. Determining these states accurately helps predict the conditions and outcomes of a reaction.
Understanding the Standard State Symbols
Four primary notations are utilized directly after a chemical formula to denote its physical state. The symbol (s) indicates a solid phase, possessing a definite shape and volume. Conversely, (l) represents the liquid phase, where the substance has a definite volume but takes the shape of its container. The letter (g) is used for substances existing as a gas, lacking both a fixed shape and a fixed volume.
The fourth symbol, (aq), stands for “aqueous,” which is distinct from a pure liquid state. An aqueous substance has been dissolved in water, meaning the molecules or ions are fully dispersed throughout the solvent. This distinction is important because an ionic solid changes its nature from a crystal lattice (s) to separated, solvated ions (aq) upon dissolving.
Identifying Inherent States of Elements and Simple Compounds
When a substance is not part of a solution, its state is determined by its nature at standard laboratory conditions (around 25 degrees Celsius and one atmosphere of pressure). Most pure elemental metals, such as iron, copper, and sodium, exist as solids under these conditions. The notable exception is mercury, which is a liquid metal at room temperature.
Many non-metals exist as diatomic gases, including hydrogen (\(\text{H}_2\)), nitrogen (\(\text{N}_2\)), oxygen (\(\text{O}_2\)), fluorine (\(\text{F}_2\)), and chlorine (\(\text{Cl}_2\)). Noble gases, like helium and neon, also exist naturally as single-atom gases. Simple molecules like water (\(\text{H}_2\text{O}\)) and bromine (\(\text{Br}_2\)) are commonly liquids, while iodine (\(\text{I}_2\)) and carbon (C) are typically solids.
The position of an element on the periodic table provides a quick reference for its default state. Elements on the left are overwhelmingly solids, while the small cluster of elements on the far right, including the halogens and noble gases, are often gases.
Applying Solubility Rules to Determine Aqueous States
The determination of a compound’s state becomes more complex when dealing with ionic substances mixed in a water solvent, particularly in double displacement reactions. The substance will either remain dissolved as an aqueous species (aq) or separate from the solution to form a solid precipitate (s). Solubility rules are general guidelines used to predict which outcome will occur based on the compound’s constituent ions.
Highly Soluble Ions
A starting point is that all compounds containing the alkali metal ions (e.g., lithium, sodium, and potassium) are soluble in water. Similarly, any ionic compound containing the ammonium ion (\(\text{NH}_4^+\)) is highly soluble. The nitrate ion (\(\text{NO}_3^-\)), acetate ion (\(\text{C}_2\text{H}_3\text{O}_2^-\)), and perchlorate ion (\(\text{ClO}_4^-\)) also form compounds that are completely soluble. These “always soluble” ions guarantee an (aq) state for that compound.
General Solubility Rules
The rules become more nuanced when considering other common ions.
- Halide ions (chloride, bromide, and iodide) are mostly soluble, but exceptions include compounds formed with silver (\(\text{Ag}^+\)), lead (\(\text{Pb}^{2+}\)), and mercury(I) (\(\text{Hg}_2^{2+}\)) ions. For example, silver chloride (\(\text{AgCl}\)) is an insoluble solid.
- Sulfate ion (\(\text{SO}_4^{2-}\)) compounds are mostly soluble, but the sulfates of barium (\(\text{Ba}^{2+}\)), strontium (\(\text{Sr}^{2+}\)), and lead (\(\text{Pb}^{2+}\)) are insoluble.
- Ions that are generally insoluble include carbonate (\(\text{CO}_3^{2-}\)), phosphate (\(\text{PO}_4^{3-}\)), and sulfide (\(\text{S}^{2-}\)). The vast majority of compounds containing these ions are insoluble solids.
- Hydroxide ions (\(\text{OH}^-\)) generally form insoluble compounds. The primary exceptions are the highly soluble alkali metal hydroxides and the moderately soluble hydroxides of calcium, strontium, and barium.
Note that the “always soluble” rule takes precedence over the insoluble rules. For instance, calcium carbonate is insoluble, but sodium carbonate is fully soluble because of the presence of the alkali metal ion.
Predicting States Based on Reaction Type
The physical states of products can often be predicted by the inherent mechanism of the chemical reaction itself, particularly for non-ionic compounds or gases. In an acid-base neutralization reaction, a strong acid reacts with a strong base to produce a salt and water. The water product is invariably written as a liquid (\(\text{H}_2\text{O}(l)\)) under standard conditions.
Combustion reactions involve a fuel reacting rapidly with an oxidant, typically atmospheric oxygen (\(\text{O}_2(g)\)). For the combustion of hydrocarbons, the products are consistently carbon dioxide and water. Carbon dioxide is usually a gas (\(\text{CO}_2(g)\)) due to its low boiling point. The water product’s state often depends on the heat generated and is frequently represented as a gas (\(\text{H}_2\text{O}(g)\)) due to the exothermic nature of the process.
Certain decomposition reactions yield products whose states are fixed. For example, when carbonic acid (\(\text{H}_2\text{CO}_3\)) is formed, it immediately decomposes into water (\(\text{H}_2\text{O}(l)\)) and carbon dioxide gas (\(\text{CO}_2(g)\)). Similarly, the formation of sulfurous acid can lead to the immediate generation of sulfur dioxide gas (\(\text{SO}_2(g)\)) and water.