Boiling, a type of phase transition, is the rapid vaporization of a liquid, which typically occurs when a liquid is heated to its boiling point, the temperature at which the vapor pressure of the liquid is equal to the pressure exerted on the liquid by the surrounding atmospheric pressure. Thus, a liquid may also boil when the pressure of the surrounding atmosphere is sufficiently reduced, such as the use of a vacuum pump or at high altitudes. Boiling occurs in three characteristic stages, which are nucleate, transition and film boiling. These stages generally take place from low to high surface temperatures, respectively.
Nucleate boiling is characterized by the incipience and growth of bubbles on a heated surface, which rise from discrete points on a surface, whose temperature is only slightly above the liquid’s saturation temperature. In general, the number of nucleation sites are increased by an increasing surface temperature. An irregular surface of the boiling vessel (i.e. increased surface roughness) can create additional nucleation sites, while an exceptionally smooth surface, such as glass, lends itself to superheating. Under special conditions, a heated liquid may show boiling delay when heated over its boiling point, by starting to boil suddenly and violently.
When the surface temperature reaches a maximum value, the critical superheat, vapor begins to form faster than liquid can reach the surface. Thus, the heated surface suddenly becomes covered with a vapor layer. Because of the vapor layer’s lower thermal conductivity, this vapor layer insulates the surface. This condition of a vapor film insulating the surface from the liquid characterizes film boiling.
Transition boiling may be defined as the unstable boiling, which occurs at surface temperatures between the maximum attainable in nucleate and the minimum attainable in film boiling.
Boiling in cookery
In cookery, 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.
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, amoeba, and other microbial pathogens. It thus can help prevent cholera, dysentery, and other diseases caused by microorganisms.
The temperature of a substance is constant as it undergoes a phase transition. Therefore, increasing the temperature of a liquid already boiling by increasing the rate of heat transfer is impossible: it will just boil more quickly. Once it has turned into steam, water will increase in temperature as heat is applied to it. 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. In Denver, Colorado, which is at an elevation of about one mile, water boils at approximately 95 C. Depending on the type of food and the elevation, the boiling water may not be hot enough to cook the food properly. The boiling point is defined as the temperature at which the vapor 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 a water soluble substance, such as salt or sugar also increases the boiling point. This is called boiling-point elevation. However, the effect is very small, and the boiling point will be increased by an insignificant amount. On the other hand, salt or ethylene glycol can cause significant freezing point depression. Due to variations in composition and pressure, the boiling point of water is almost never exactly 212 F / 100 C, but rather close enough for cooking. Boiling is when the vapor pressure exceeds the air pressure.
Foods suitable for boiling include:
- Farinaceous foods such as pasta
- Stocks and soups
- Older, tougher, cheaper cuts of meat and poultry can be made digestible
- It is appropriate for large-scale cookery
- Nutritious, well flavoured stock is produced
- It is safe and simple
- Maximum colour and nutritive value is retained when cooking green vegetables, provided boiling time is kept to the minimum
- There is a loss of soluble vitamins in the water
- It can be a slow method
- Foods can look unattractive
A wet pot takes longer to boil
The water on the outside of the pot actually increases the time it takes a pot of water to boil. The pot will heat at a normal rate once all excess water on the outside of the pot evaporates.