Boiling

Boiling is a mode of vaporization in which a liquid changes to a gas when its temperature reaches the point at which its vapor pressure equals the surrounding pressure. As heat is supplied, bubbles of vapor form inside the liquid rather than only at the surface; these bubbles rise and release vapor when they reach the surface. Boiling involves the latent heat of vaporization, so the liquid’s temperature remains near the boiling point until much of it has converted to vapor.

The boiling point depends on ambient pressure. At sea level, water boils at 100°C. At higher altitudes, lower atmospheric pressure lowers the boiling point. Conversely, increasing the pressure raises the boiling point, a principle exploited in pressure cookers. Each pure substance has its own characteristic boiling point, and mixtures can alter boiling behavior through interactions and the presence of impurities.

Bubbles form through nucleation, typically at microscopic imperfections or irregularities on container surfaces. In nucleate boiling, many bubbles form and rise, making the liquid vigorously vibrate. If heat is very high or surface conditions change, a thin vapor layer may form on the surface, a regime known as film boiling, which is less efficient for heat transfer.

Boiling is distinct from evaporation, which occurs at temperatures below the boiling point and mainly at the surface. In practical terms, boiling is used for cooking, sterilization, distillation, and various industrial processes where rapid heat transfer and phase change are required. Safety considerations include the handling of hot liquids and steam, which can cause burns.