4 Solubility Rules
Elenor Waas
These solubility rules can help you predict if a precipitate will form when two ionic compounds are mixed in a solution. If all combinations of ions are soluble, then no precipitate will form, and you will often be unable to isolate a single compound.
The solubility rules in this article are in water at room temperature. Some compounds can have very different solubilities in hot or cold water. For example, potassium bromate is quite soluble in hot water, but only slightly soluble in very cold water.
Even the most insoluble ionic compounds will dissolve into ions to a very small degree. The solubility product constant, known as the Ksp value, allows you to calculate how much will actually dissolve.
In a flask of water, you put in solid Fe(NO3)2 and AgOH and mix thoroughly. After letting the solution settle and reach equilibrium, you notice a precipitate forming at the bottom. What is the most likely composition of the salt? In other words, what cation-anion combination is the least soluble in the system?
In a flask of water, you put in solid KClO3, NH4NO3 and PbCl2 and mix thoroughly. After letting the solution settle and reach equilibrium, you notice a precipitate forming at the bottom. What is the most likely composition of the salt?
In a mixture of hydrofluoric acid and nitrous acid (HF and HNO2). You have four salts available: MgCl2, Ag2SO4, FePO4, and KI. Which salt would you add to your acid solution to selectively precipitate out the nitrite anions? What about to precipitate out the fluorides?
Solubility is a substance's ability to be dissolved. The substance that is dissolved is called a solute, and the substance it is dissolving in is called a solvent. The resulting substance is called a solution. Generally, the solute is a solid and the solvent is a liquid, such as our salt in water example above. However, solutes can be in any state: gas, liquid, or solid. For example, a carbonated beverage is a solution where the solute is a gas and the solvent is a liquid.
A solute is considered insoluble when they are unable to dissolve at a ratio greater than 10000:1. While many compounds are partially or mostly insoluble, there is no substance that is completely insoluble in water, meaning that it can't dissolve at all. You will see in the solubility rules that many compounds that are labeled as insoluble have exceptions, such as carbonates. This is partly why it's important to follow the solubility rules closely.
When you are working on chemical equations or building a hypothesis, solubility rules are helpful in predicting the end states of the substances involved. You will be able to accurately predict what combinations will lead to what results.
The solubility rules are only for ionic solids' ability to dissolve in water. While we can calculate the solubility by measuring each substance and following an equation, the solubility rules allow us to determine the solubility of a substance before you attempt to create it.
It is very important that the rules on this list are followed in order, because if a rule seems to contradict another rule, the rule that comes first is the one that you follow. Substances on this list are given by their elemental names. Referencing the periodic table below will help you work through the elemental names and groups.
Note: Letter e is an example of using the order of the rules to determine solubility. Rule 4 says that silvers (Ag) are frequently insoluble, but rule 3 says that chlorates (Cl) are soluble. Since Ag ClO3 is a silver chlorate, and rule 3 comes before rule 4, it supersedes it. This compound is soluble.
As we see from our solubility rules, some substances are very soluble, while some are insoluble or have low solubility. Let's take a look at how solubility works to better understand the solubility rules.
Generally, solutes are soluble in solvents that are most similar to them molecularly. Polar solutes will dissolve better in polar solvents, and non-polar solutes will dissolve better in non-polar solvents. For example, sugar is a polar solute, and absorbs very well in water. However, sugar would have a low solubility in a nonpolar liquid like vegetable oil. In general, solutes will also be more soluble if the molecules in the solute are smaller than the ones in the solvent.
Other factors that affect solubility are pressure and temperature. In some solvents, when heated the molecules vibrate faster and are able to break apart the solute. Pressure is mainly a factor when a gas substance is involved, and has little to no effect on liquid substances.
When a solute is mixed with a solvent, there are three possible outcomes: If the solution has less solute than the maximum amount it is able to dissolve (the solubility), it is a dilute solution. If the amount of solute is exactly the same as the solubility it is saturated. If there is more solute than is able to be dissolved, the excess separates from the solution and forms a precipitate.
A solution is considered saturated when adding additional solute does not increase the concentration of the solution. Additionally, a solution is miscible when it can be mixed together at any ratio- this mainly applies to liquids, like ethanol, C2H5OH, and water, H2O.
Knowing and following the solubility rules is the best way to predict the outcome of any given solution. If we know that a substance is insoluble, it is likely that it would have excess solute, thus forming a precipitate. However, compounds that we know to be highly soluble, like salt, are likely to form solutions at various ratios; in this case, we will be able to determine how much solute and solvent is needed to form each solution, and if it's possible to form one at all.
Preparing for the AP Chemistry test? Study with our articles on every AP Chemistry practice test available and the ultimate AP Chem study guide. Taking IB instead? Start with our study notes for IB Chemistry.
Looking for more chemistry help? We walk you through the solubility constant (Ksp) and how to solve for it, explain how to balance chemical equations, and go over examples of physical vs chemical change here.
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The following chart shows the solubility of various ionic compounds in water at 1 atm pressure and room temperature (approx. 25 C, 298.15 K). "Soluble" means the ionic compound doesn't precipitate, while "slightly soluble" and "insoluble" mean that a solid will precipitate; "slightly soluble" compounds like calcium sulfate may require heat to precipitate. For compounds with multiple hydrates, the solubility of the most soluble hydrate is shown.
The sugar we use to sweeten coffee or tea is a molecular solid, in which theindividual molecules are held together by relatively weak intermolecular forces. Whensugar dissolves in water, the weak bonds between the individual sucrose molecules arebroken, and these C12H22O11 molecules are released intosolution.
It takes energy to break the bonds between the C12H22O11molecules in sucrose. It also takes energy to break the hydrogen bonds in water that mustbe disrupted to insert one of these sucrose molecules into solution. Sugar dissolves inwater because energy is given off when the slightly polar sucrose molecules formintermolecular bonds with the polar water molecules. The weak bonds that form between thesolute and the solvent compensate for the energy needed to disrupt the structure of boththe pure solute and the solvent. In the case of sugar and water, this process works sowell that up to 1800 grams of sucrose can dissolve in a liter of water.
Ionic solids (or salts) contain positive and negative ions, which are heldtogether by the strong force of attraction between particles with opposite charges. Whenone of these solids dissolves in water, the ions that form the solid are released intosolution, where they become associated with the polar solvent molecules.
We can generally assume that salts dissociate into their ions when they dissolve inwater. Ionic compounds dissolve in water if the energy given off when the ions interactwith water molecules compensates for the energy needed to break the ionic bonds in thesolid and the energy required to separate the water molecules so that the ions can beinserted into solution.
Discussions of solubility equilibria are based on the following assumption: Whensolids dissolve in water, they dissociate to give the elementary particles from which theyare formed. Thus, molecular solids dissociate to give individual molecules
The concentrations of these ions soon become large enough that the reverse reactionstarts to compete with the forward reaction, which leads to a decrease in the rate atwhich Na+ and Cl- ions enter the solution.
Eventually, the Na+ and Cl- ion concentrations become largeenough that the rate at which precipitation occurs exactly balances the rate at which NaCldissolves. Once that happens, there is no change in the concentration of these ions withtime and the reaction is at equilibrium. When this system reaches equilibrium it is calleda saturated solution, because it contains the maximum concentration of ions thatcan exist in equilibrium with the solid salt. The amount of salt that must be added to agiven volume of solvent to form a saturated solution is called the solubility ofthe salt.
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