Vol. III · Water Plate 2 of 11boiling, vapour pressure, boiling point, simmer, lid, evaporationHT

Boiling.

A pot at a rolling boil and a pot at a gentle bubble are at the same temperature. The extra flame only makes steam.

Heat a pot of water and its temperature climbs steadily until bubbles of steam form throughout it — and then it stops climbing. At 100 °C (at sea level) water's vapour pressure equals the pressure of the air above it, so steam can form inside the liquid; every extra joule from the burner turns more water into steam rather than raising the temperature. A rolling boil and a gentle one are both at the boiling point; one just loses water faster.

That flat line is one of the most useful facts in the kitchen. It makes boiling a thermostat: food in boiling water is cooked at a known temperature without any control at all. It also means a lid does not make boiling water hotter, but it does bring the pot to the boil sooner and saves the energy that would otherwise leave as steam. The workshop heats a pot of water on a burner.

Below: the temperature of a pot of water from the tap, with and without a lid, and the water it loses in twenty minutes.

What you will learn
Workshop
A pot of water from 15 °C on a 3 kW gas burner (about 1.2 kW reaching the pot), with or without a lid (stove bench; evaporation from the open surface grows with vapour pressure). The flat line is the boiling point at the altitude chosen.

A thermostat for free

Three litres of tap water on a domestic gas burner reach the boil in 17 min with the lid on and 18 min without — the open pot loses heat as vapour from its surface even before it boils. Once boiling, the temperature stays at 100.0 °C however high the flame: the extra heat only boils the water away, 1,252 g in an hour uncovered. Six litres take 33 min to boil, which is why pasta water is put on first.

A simmer is water just below or barely at the boil, with a few small bubbles — around 85–95 °C in practice — and a poach lower still (XVI.1). For braises, stocks and custards those few degrees matter; for pasta and green vegetables a full boil matters, because a big pot recovers its temperature quickly when cold food goes in (XXI.6). Either way, boiling water is a thermostat you do not have to set: no food in it gets hotter than the boiling point.

At the stove

Put a thermometer in a pot of water at a gentle simmer, then turn the burner to full. Watch the reading as the bubbles get bigger. It will not rise more than a fraction of a degree; then turn the heat back down and save the gas.

Boiling is so constant that it was used to define the temperature scale: 100 °C was the boiling point of water at standard atmospheric pressure until the scale was redefined in terms of absolute temperature in 1954.

Terms

Boiling point
The temperature at which water's vapour pressure equals the surrounding pressure; 100 °C at sea level.
Vapour pressure
The pressure of water vapour in equilibrium with liquid water; it rises steeply with temperature.
Simmer
Water held just below or at the boil, with small, occasional bubbles.
Rolling boil
Vigorous boiling that continues when stirred; at the same temperature as a gentle boil.
Boiling water cannot get hotter than its boiling point; more flame only makes more steam. A lid saves time and energy, not degrees.

Sources

  1. IAPWS, Revised Release on the IAPWS Industrial Formulation 1997 for the Thermodynamic Properties of Water and Steam (IAPWS-IF97), region 4 (saturation).
  2. ISO 2533:1975, Standard Atmosphere (the troposphere barometric formula).
  3. A teaching value chosen for the course: the right order of magnitude and shape, not a measurement of a particular food.