A salt is an ionic compound formed from the electrostatic attraction between positively charged ions (cations) and negatively charged ions (anions). These ions arrange into a rigid, ordered structure called a crystal lattice, held together by ionic bonds. Solubility measures a substance’s ability to dissolve completely in a solvent, typically water, forming a uniform solution. This process requires the solid crystal to break apart, allowing the ions to disperse throughout the liquid. This article explains insoluble salts and the chemical principles that cause some ionic compounds to resist dissolving in water.
Defining Insoluble Salts
The term “insoluble salt” is relative, as no ionic compound is completely unable to dissolve in water. Chemists use this term for salts that exhibit a very low degree of solubility, meaning only a minuscule amount will dissolve in a given volume of water. The standard threshold for an “insoluble” salt is defined as one that dissolves less than 0.1 gram of solute per 100 milliliters of water at room temperature. For comparison, a highly soluble salt like table salt can dissolve over 35 grams in the same amount of water.
When an “insoluble” salt is placed in water, the vast majority remains as a visible solid, called a precipitate. A tiny, measurable amount of the solid breaks apart into ions, creating an equilibrium between the solid phase and the dissolved ions. Because the quantity of ions that escape into the solution is so small, the salt is considered practically insoluble for most purposes.
The Chemical Forces That Determine Solubility
The fate of a salt is determined by a competition between two opposing energy forces. The first force is lattice energy, which is the energy released when gaseous ions form the solid crystal lattice structure. This energy measures the strength of the ionic bond holding the salt together in its solid form. A high lattice energy indicates the ions are strongly bound, making the solid stable and difficult to break apart.
The second force is hydration energy, which is the energy released when water molecules surround and stabilize the separated ions in solution. Water molecules are polar, having a slightly negative side that attracts cations and a slightly positive side that attracts anions. This attraction provides the energy needed to pull the ions away from the crystal lattice.
A salt is soluble when the hydration energy is large enough to overcome the lattice energy holding the crystal together. For an insoluble salt, the lattice energy is significantly greater than the hydration energy. The attractive forces within the solid structure are too strong to be broken apart and stabilized by the surrounding water molecules. High ionic charge and small ion size generally contribute to a stronger lattice energy, resulting in lower solubility.
Common Examples and Real-World Relevance
Insoluble salts appear frequently in nature and have many practical applications. Calcium carbonate is a common example, making up geological structures like limestone, chalk, and marble. Its near-insolubility allows these rock formations to remain intact despite constant exposure to water.
Another well-known insoluble compound is barium sulfate, used in medical imaging as a contrast agent known as a “barium swallow.” Barium ions are toxic if absorbed, but because barium sulfate is highly insoluble, it passes safely through the digestive tract. Calcium oxalate is the primary component of most kidney stones, where this insoluble salt precipitates out of the urine.
The formation of an insoluble salt is also the basis for a precipitation reaction. This occurs when two soluble salts are mixed, and their ions combine to form a new, insoluble product. This technique is used in water treatment to remove unwanted ions, such as lead or other heavy metals, by converting them into an insoluble form that can be filtered out of the water supply.