Water is often called the universal solvent because it dissolves more substances than any other liquid. When a substance (the solute) mixes evenly into the water, the resulting mixture is called a solution. Solubility defines the capacity of water to dissolve a solute, representing the maximum amount that can be dissolved under specific conditions. Understanding these limits requires defining what it means for water to become “saturated.”
The Equilibrium Point: Defining Saturated Water
Saturated water is a solution that has reached the maximum concentration of dissolved solute possible at a given temperature and pressure. The water can no longer accommodate any more of the substance in a dissolved state. If additional solute is added, it will simply settle to the bottom as an undissolved solid, liquid, or gas.
Saturation is not static; rather, it is a state of constant activity called dynamic equilibrium. While no net change in the amount of dissolved solute is observed, two opposing processes occur simultaneously at equal rates. Solute molecules continually dissolve into the water, while an equal number of dissolved molecules simultaneously precipitate out or crystallize back into their original form.
Imagine dissolving sugar into water until it begins to accumulate at the bottom. The solution above the undissolved sugar is saturated because the rate at which sugar molecules enter the solution is perfectly balanced by the rate at which they return to the solid pile. This balance determines the concentration of a saturated solution, which is known as the substance’s solubility.
Comparing Solutions: Unsaturated and Supersaturated
Saturation is a specific point on a spectrum of solution states, flanked by unsaturated and supersaturated conditions. An unsaturated solution contains less solute than the maximum amount the water can hold. If more solute is introduced, it will dissolve completely, indicating the water still has capacity.
In contrast, a supersaturated solution is an unstable state holding more dissolved solute than is possible at that temperature and pressure. This condition is typically achieved by manipulating a saturated solution. A common method involves heating the water to increase solubility, dissolving excess solute, and then gently cooling the solution without disturbance.
The excess dissolved solute in a supersaturated solution is easily prompted to precipitate out, returning the solution to its stable saturated state. This rapid crystallization can be triggered by a physical disturbance, such as shaking, or by introducing a tiny seed crystal of the solute. The seed provides a template for the excess dissolved molecules to rapidly crystallize, demonstrating the fragile nature of supersaturation.
External Factors That Change Saturation Levels
The concentration defining a saturated solution is highly dependent on external physical conditions. Temperature is the most significant factor influencing solubility, though its effect varies depending on the type of solute. For most solid substances, solubility increases as the water temperature rises, which is why more sugar dissolves in hot tea than in iced tea.
The opposite trend is observed for gases dissolved in water, such as oxygen or carbon dioxide. As water temperature increases, the solubility of these gases decreases, causing them to escape the solution. This explains why a warm carbonated drink goes flat faster than a cold one, and why aquatic life is stressed by thermal pollution.
Pressure is another factor, primarily affecting gaseous solutes. According to Henry’s Law, the solubility of a gas in a liquid is directly proportional to the partial pressure of that gas above the liquid. This principle is intentionally exploited in the production of carbonated beverages, where high pressure forces large amounts of carbon dioxide gas into the liquid. When the container is opened, the pressure is released, the water becomes supersaturated, and the excess gas bubbles out until a lower saturation level is reached.