Water bubbles are a common sight, whether rising in a glass of tap water or bursting in a boiling pot. A bubble is a pocket of gas suspended within a liquid, and its formation is not governed by a single cause. The gas inside can originate from a phase change of the liquid, the release of previously dissolved gases, or the intentional addition of gas. Understanding why bubbles appear requires looking closely at the physics and chemistry that control how gas and liquid interact.
Bubbles from Heat: The Science of Boiling
When water is heated, thermal energy causes its molecules to move more rapidly, increasing the internal pressure of the water vapor. This internal force, known as vapor pressure, constantly pushes against the external atmospheric pressure acting on the water’s surface. As the temperature rises, the vapor pressure increases until it equals the external pressure, allowing the liquid to change phase into a gas throughout the entire volume. This phase change creates bubbles of pure water vapor, or steam, which expand as they rise to the surface. These bubbles are the primary visual evidence of true boiling, which is the rapid conversion of liquid water into a gaseous state.
Bubbles from Pressure Changes: Dissolved Air
Before water reaches the boiling point, a different type of bubble forms from atmospheric gases, primarily nitrogen and oxygen, dissolved in the water. The solubility of these gases is inversely related to temperature; as the water warms, it holds less dissolved gas. This phenomenon also explains why cold tap water sometimes appears cloudy. Water is held under higher pressure in pipes, allowing more air to dissolve, a relationship described by Henry’s Law. When the water is dispensed, the pressure drops instantly, decreasing gas solubility and causing excess dissolved air to rapidly come out of solution as tiny bubbles that soon dissipate.
Bubbles from Infusion: Carbonation and Reactions
Another significant source of bubbles involves the intentional infusion of gas, most commonly seen in carbonated beverages like seltzer water. In this process, carbon dioxide (CO2) gas is forced into the liquid under high pressure. This high pressure maximizes gas solubility, keeping the CO2 trapped until the container is opened. When the seal is broken, the pressure above the liquid drops drastically, causing the supersaturated CO2 to escape rapidly as bubbles, creating the familiar fizz. Bubbles can also be created through chemical reactions, such as electrolysis, where passing an electric current through water splits the H2O molecules, producing separate bubbles of hydrogen and oxygen gas.
The Starting Point: Why Bubbles Need a Surface
Regardless of how a bubble is formed, it rarely forms spontaneously in the middle of clean, undisturbed water. This is because water molecules cling tightly due to surface tension, creating an energy barrier that must be overcome to form a new gas-liquid interface. To minimize the energy required, bubbles rely on microscopic imperfections known as nucleation sites. These sites are typically tiny scratches, crevices, or trapped air pockets found on the container surface or on dust particles suspended in the water. The imperfection provides an existing gas-liquid interface, allowing the gas to collect and expand more easily than it could in the pure liquid, which explains why streams of bubbles often appear to come from the same few points.