Sodium chloride, commonly known as table salt, consists of sodium and chloride ions arranged in a crystal lattice. When this ionic compound is placed in water, the polar water molecules interact with the ions, causing the crystal structure to break apart through a process called dissociation.
Water is a polar molecule due to the unequal sharing of electrons between oxygen and hydrogen atoms. Oxygen is more electronegative, pulling electron density toward itself, creating a partial negative charge. The hydrogen atoms develop partial positive charges. This polarity creates a dipole moment, making water an excellent solvent for ionic compounds.
When sodium chloride dissolves in water, the ions become surrounded by water molecules in what are called hydration shells. The partially negative oxygen atoms in water molecules are attracted to the positive sodium ions, orienting themselves to maximize this electrostatic attraction. Conversely, the partially positive hydrogen atoms in water molecules surround the negative chloride ions. This ion-dipole interaction is the driving force behind the dissolution process.
The dissociation process begins when water molecules approach the sodium chloride crystal. The polar water molecules exert attractive forces on the surface ions, gradually weakening the ionic bonds that hold the crystal together. As more water molecules surround each ion, the hydration energy becomes sufficient to overcome the lattice energy, causing the ions to separate from the crystal and become fully solvated in solution. This process can be represented by the equation: solid sodium chloride yields aqueous sodium ions plus aqueous chloride ions.
In the final dissolved state, sodium and chloride ions move freely throughout the solution, each surrounded by their respective hydration shells. This creates an electrolytic solution with several important properties. The mobile ions allow the solution to conduct electricity, as demonstrated by the current flow between electrodes. The presence of dissolved ions also affects colligative properties, lowering the freezing point, raising the boiling point, and creating osmotic pressure. These properties make salt water fundamentally different from pure water and explain many of its practical applications.