I know high pressure allows us to boil vegetables more quickly, and this is obvious when a pan lid is fitted tightly. But I can鈥檛 figure out why the pressure is still increased when the lid only partially covers the pan, leaving a 1 or 2-centimetre gap. It seems to me that air and steam can pass easily through this space and circulate freely, yet even a partially covered pan of liquid seems to boil more quickly. Why?
鈥� Partially covering the pan disrupts key mechanisms by which boiling water loses heat, namely evaporation and convection. A lid, close-fitting or otherwise, will reduce the convective cooling by preventing fresh cooler air from accessing the heated liquid.
The same applies to the evaporated water, which will condense almost immediately on the underside of the lid, returning much of the latent heat of evaporation back to the liquid in the pot.
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However, this is not the complete answer because an old proverb points to the quantum features of the pot: if continuously observed, we know it will never boil.
Neil Barry, By email, no address supplied
鈥� I have always assumed that the rapid boiling of water in a pan covered partly by a lid is the result of heat retention, rather than marginally increased pressure.
The small aperture left by a loosely covering saucepan lid will mean less heat loss, therefore the energy in the partially closed system can climb higher. This may produce a stable but more active boiling liquid than when the lid is off. The alternative is that 鈥渢oo much鈥� energy is retained and the simmering liquid becomes over stimulated, resulting in a rapid, catastrophic loss: the dreaded boil over.
Clive Tiney, York, UK
鈥� Water boils faster with a lid on the saucepan, but it is not because of increased pressure. The lid is condensing steam and returning it to the pan, along with the heat it would otherwise carry away. This will be most effective with the lid fully on, but will still work with the lid only partly covering the pan. The more steam that escapes, the greater the heat lost.
Guy Cox, Australian Centre for Microscopy and Microanalysis, University of Sydney, Australia
This article appeared in print under the headline 鈥淧ut a lid on it鈥�