When sodium chloride (NaCl), commonly known as table salt, is introduced to water (H2O), a molecule composed of two hydrogen atoms and one oxygen atom, a complex process begins. This interaction involves intricate scientific principles that explain why the salt appears to vanish.
The Science of Salt Dissolving
Salt is an ionic compound, consisting of positively charged sodium ions (Na+) and negatively charged chloride ions (Cl-) held in a crystal lattice structure. Water molecules are polar; they have a slightly negative charge near the oxygen atom and a slightly positive charge near the hydrogen atoms. This polarity is crucial for water’s ability to dissolve many substances.
When salt crystals enter water, the water molecules interact with the ions in the salt lattice. The slightly negative oxygen end of the water molecule is attracted to the positive sodium ions, while the slightly positive hydrogen ends are attracted to the negative chloride ions. These attractions are strong enough to overcome the ionic bonds holding the salt crystal together, pulling individual ions away from the lattice.
As ions separate from the crystal, water molecules surround them, forming what are called “hydration shells.” Each sodium ion is enveloped by water molecules with their oxygen atoms facing the ion, and each chloride ion is surrounded by water molecules with their hydrogen atoms facing inward. This process, known as solvation, allows the separated ions to disperse evenly throughout the water.
Changes to Water’s Characteristics
The presence of dissolved salt ions alters water’s physical properties. Saltwater becomes denser than pure freshwater due to the added mass of the dissolved ions. This increased density explains why objects can float more easily in saltwater.
Dissolved salt also impacts water’s freezing and boiling points. Water molecules form hydrogen bonds, which influence these phase change temperatures. When salt is dissolved, the ions interfere with the formation of these hydrogen bonds. More energy is required to break the bonds and allow water to boil, leading to an elevation of the boiling point. Conversely, the presence of salt ions disrupts the ordered structure required for water to freeze, causing a depression of the freezing point.
When Water Can’t Hold More Salt
Water can only dissolve a limited amount of salt, a point known as saturation. When water becomes saturated, no more salt will dissolve, and any additional salt added will settle at the bottom as a solid. For sodium chloride, water at room temperature (around 20-25°C) can dissolve approximately 36 grams per 100 milliliters, or about 357 grams per liter.
Even at saturation, dissolution does not entirely stop. Instead, a dynamic equilibrium is established. Salt crystals continue to dissolve into ions, while simultaneously, dissolved ions recombine and crystallize back into solid salt at an equal rate. This means the net amount of dissolved salt remains constant, with individual ions continuously moving between dissolved and solid states.