Boiling

Boiling is cooking food in boiling water, or other water-based liquid such as stock or milk. Simmering is gentle boiling, while in poaching the cooking liquid moves but scarcely bubbles. Under special conditions, a heated liquid may show boiling delay when heated over its boiling point, by starting to boil suddenly and violently.

In places where the available water supply is contaminated with disease-causing bacteria, boiling water (and allowing it to cool) before drinking it is a valuable health measure. Boiling water for a few minutes kills most bacteria, amoebas, and other microbial pathogens. It thus can help prevent cholera, dysentery, and other diseases caused by these organisms.

Increasing the temperature of a liquid already boiling by adding heat is impossible. Pressure and a change in composition of the liquid may alter the boiling point of the liquid. For this reason, high elevation cooking generally takes longer since boiling point is a function of atmospheric pressure. The boiling point is defined as the temperature at which the vapour pressure of the substance equals the pressure above the substance. Increasing the pressure as in a pressure cooker raises the temperature of the contents above the open-air boiling point.

Adding salt also increases the boiling point. Due to variations in composition and pressure, the boiling point of water is almost never 100°C, but rather closes enough for cooking.