Salt (sodium chloride) is highly soluble in water. This common chemical compound readily dissolves because of its chemical structure and the unique properties of water. Salt is the main substance responsible for the salinity of the ocean and the extracellular fluid in the bodies of many organisms. Understanding this dissolution begins with defining what it means for a substance to be soluble.
Defining Soluble and Insoluble
Solubility describes the ability of a substance, called the solute, to dissolve in another substance, the solvent, forming a uniform solution. A substance is considered soluble if it can disperse completely into the solvent, creating a homogeneous mixture where the solute particles are no longer visible. For example, sugar is soluble in water because it vanishes when stirred into a cup of tea.
Conversely, a substance is classified as insoluble if it does not dissolve to any significant degree in a particular solvent. The majority of the material remains undissolved. Sand in water is a familiar example of an insoluble substance, as the solid particles settle to the bottom instead of dispersing.
The Molecular Process of Dissolution
The high solubility of table salt, or sodium chloride (\(\text{NaCl}\)), is explained by the strong attraction between its ions and the polar water molecules. Salt is an ionic compound, meaning it is held together by strong electrostatic forces, called ionic bonds, between positively charged sodium ions (\(\text{Na}^+\)) and negatively charged chloride ions (\(\text{Cl}^-\)). Water (\(\text{H}_2\text{O}\)) is often called the “universal solvent” because its molecular structure is highly polar.
In a water molecule, the oxygen atom pulls electrons closer to itself, giving it a partial negative charge, while the two hydrogen atoms have partial positive charges. When salt is introduced into water, this polarity allows the water molecules to engage in a molecular tug-of-war with the salt crystal. The partial negative end of the water molecules is attracted to the positive sodium ions, and the partial positive end is attracted to the negative chloride ions.
These attractions are strong enough to overcome the ionic bonds holding the salt crystal together, pulling the \(\text{Na}^+\) and \(\text{Cl}^-\) ions away from the solid structure. Once separated, the individual ions become completely surrounded by a sphere of water molecules, known as a hydration shell. This shell effectively shields the ions from one another, preventing them from rejoining and allowing them to remain dispersed throughout the liquid as a stable solution.
Limits to Salt Solubility
While salt is highly soluble, there is a distinct limit to how much can dissolve in a given amount of water at a certain temperature. When a solvent has dissolved the maximum amount of solute possible, it is known as a saturated solution. At this point, any additional salt added will simply fall to the bottom of the container, remaining as undissolved solid.
For sodium chloride, the solubility limit is approximately 357 grams per liter of water at \(25\) degrees Celsius. Temperature generally affects the solubility of solids, with higher temperatures increasing the amount that can dissolve. However, the solubility of \(\text{NaCl}\) does not increase dramatically with temperature, only rising slightly to about 384 grams per liter at \(100\) degrees Celsius.