Boiling

Boiling is when a liquid turns quickly into a gas or vapor. This happens when the liquid is heated to its boiling point.

Two main types of boiling are nucleate boiling, where small bubbles form, and critical heat flux boiling, where a film of vapor covers the surface when it gets too hot.

The boiling point of water is 100 °C or 212 °F, but it is lower at higher altitudes. Boiling water is an important way to make it safe to drink because it kills most germs. Holding water at 100 °C for one minute is usually enough to do this. Boiling is also a common cooking method used in techniques like boiling, blanching, steaming, and poaching.

Types

Free convection

When liquid starts to boil gently, the warmest parts rise because they are lighter. This happens when the liquid is just a little above its boiling point.

Nucleate

When a liquid boils more actively, small bubbles form on the hot surface. The more uneven or rough the surface, the easier it is for these bubbles to form. Very smooth surfaces, like plastic, can sometimes delay boiling.

Critical heat flux

When a surface gets too hot while boiling liquid, it can suddenly stop moving heat away well. This causes the surface to overheat quickly because a layer of vapor forms.

Transition

This is a mix of gentle and very active boiling, happening at temperatures in between.

Film

If the heating surface is much hotter than the liquid, a thin layer of vapor forms between them. This vapor layer stops heat from moving well, keeping the liquid from boiling properly.

Main article: Leidenfrost effect

Influence of geometry

Pool boiling

Pool boiling is when a liquid heats up and turns into gas without any strong movement. The movement happens because some parts of the liquid are lighter than others.

Flow boiling

Flow boiling is when a liquid moves while it heats up and turns into gas. This can happen in pipes, often pushed by pumps or by natural movement because of weight differences. The mix of liquid and gas can change how well heat moves.

Confined boiling

Confined boiling happens in small spaces. The size of these spaces can change how the liquid boils.

Boiling is a way to cook food by heating it in water or other liquids. Simmering is a gentle form of boiling, while poaching is when the liquid moves a little but doesn’t bubble much.

The boiling point of water is usually 100 °C, but this can change with pressure or height above sea level. At higher elevations, water boils at lower temperatures, so cooking can take longer. Using a pressure cooker can raise the boiling point and cook food faster.

Boil-in-the-bag

Main article: Boil-in-bag

See also: Sous-vide

Boil-in-bag cooking involves heating sealed bags of food in boiling water. This makes cleaning easier because no pots or pans are needed. These meals are useful for camping or home cooking.

Contrast with evaporation

Evaporation is when tiny parts of a liquid on the surface get enough energy to turn into gas and float away. This only happens at the top of the liquid.

Boiling is different. When a liquid gets hot enough, it reaches its boiling point. Then, tiny bubbles of gas form inside the liquid all the way through. These bubbles rise to the top and burst into the air. If you heat the liquid even more, it just boils faster, but the temperature stays the same.

This difference only talks about liquid turning into gas. When a solid turns straight into a gas, we call that sublimation.