Salt is definitively a solute. The process of dissolving salt in water is a common demonstration of how substances interact at a molecular level to form a specific type of mixture. Understanding why salt fits the definition of a solute requires defining the vocabulary used to describe these mixtures in solution.
Defining Solutes, Solvents, and Solutions
A solution is a homogeneous mixture where one substance is completely and uniformly dispersed throughout another substance. The resulting mixture appears as a single phase, meaning it looks the same throughout, like saltwater.
The two components that make up a simple solution have distinct names based on their roles. The substance that is being dissolved is known as the solute. The substance that does the dissolving is called the solvent.
The solvent is typically the component present in the greater quantity, while the solute is the minor component. Solutions can involve different states of matter; for instance, the carbon dioxide gas in soda is a solute dissolved in a liquid solvent.
Why Salt Meets the Solute Criteria
Table salt, chemically known as sodium chloride (NaCl), is classified as a solute when mixed with water. This classification is based on its role and its relative quantity in the typical mixture of saltwater. When salt is added to water, it is the substance being dispersed and dissolved by the surrounding liquid.
The water, present in a much larger amount, serves as the solvent. Water’s ability to dissolve many different substances has earned it the nickname “universal solvent.” Because salt is the minor component being distributed evenly throughout the water, it meets the criteria for a solute.
How Salt Dissolves in Water
The reason salt acts as a solute in water is due to the chemical structures of both compounds. Sodium chloride is an ionic compound, meaning its structure is a crystal lattice held together by the strong electrical attraction between positively charged sodium ions (\(\text{Na}^{+}\)) and negatively charged chloride ions (\(\text{Cl}^{-}\)). Water (\(\text{H}_2\text{O}\)) is a polar molecule, meaning it has a slight positive charge near the hydrogen atoms and a slight negative charge near the oxygen atom.
When salt is introduced to water, the polar water molecules interact with the salt crystal. The slightly negative oxygen ends are attracted to the positive sodium ions. Simultaneously, the slightly positive hydrogen ends are drawn to the negative chloride ions. This attraction is strong enough to overcome the ionic bonds holding the salt crystal together.
The water molecules pull the individual sodium and chloride ions away from the crystal lattice, a process called dissociation. Once separated, the ions are surrounded by water molecules, which stabilize them and prevent them from rejoining. This surrounding layer of water molecules is known as a hydration shell, and it allows the ions to be dispersed uniformly, completing the dissolution.