The Physics of Boiling Water

I believe one needs the internal pressure to rise in order to help it boil faster. That's why pressure cookers can be dangerous and explosive if used wrong.

With the lid on, less heat escapes so it should take less time. Unless you have a pressure cooker the boiling point is unlikely to be raised significantly, but even if it is, the water will have actually reached 100 deg faster assuming this is the required "cooking temperature". i.e. "boiling point" is an irrelevance, "cooking temperature" is the significant issue.

no doubt about it --- Lid On

It's also very important that you not watch the water - that's the same or even worse as having the lid off.

And keeping the lid on stops you watching it ... it has nothing to do with physics !

Factors That Affect the Boiling Point

Pressure: when the external pressure is:
less than one atmosphere, the boiling point of the liquid is lower than its normal boiling point.
equal to one atmosphere, the boiling point of a liquid is called the normal boiling point.
greater than one atmosphere, the boiling point of the liquid is greater than its normal boiling point.
The following graph shows the boiling point for water as a function of the external pressure. The line on the graph shows the normal boiling point for water.


Closed lid = greater pressure inside pot then outside. Water boils faster.

No lid = pressure outside is = to pressure inside pot. Water boils at 100 degrees Celsius.

Lady Diane keeps a lid in the drawer under the micro-wave, she says it's for medicinal purposes (and brownies)

snort

Around here it boils at 202f. Keep your funny numbers. That's 5,000 feet, by the way

Misinterpretation of graph, actually. Increasing pressure increases the temperature at which water boils. All else being equal (that is, if we ignore the fact that heat escapes with steam/vapor as water is heated with the lid off), it will take longer to reach the boiling point given an equivalent heat source, because the boiling point is higher. The real point of the pressure cooker (or the autoclave, in medical and scientific circles) is that water gets hotter without boiling at higher pressures, thereby making tender meat faster and killing some of the nastier sporulating bacteria that like to make messes of surgeries and experiments.

Nonetheless, I concur with the lid on crowd, because you lose much less heat (and water) to convection. In fact, I'm about to go check on the potatoes that are boiling under a lid at this very moment. (Since the lid is glass, however, I am still able to watch if so inclined.)

Yea, theoretically water should reach the boiling point faster with the lid off.

But it would hard to check that unless you have a glass pot so you can see the bubbles forming at the bottom. Otherwise, you'll remove the pressure when you take the lid off.

Unless the lid is that of a pressure cooker, the internal pressure will not increase a significant amount, and will not affect the point at which the water boils.

The thermal efficiency gained by having the lid on outweighs any effect a lid might have on the boiling point.

The pressure under the lid can only equal atmospheric pressure + (weight of lid)/(surface area of lid). A four-ounce lid on an 8 inch diameter pot can create a pressure differential of .005 lbs/square inch. Sea-level atmospheric pressure is something like 15 lbs/square inch.

Oh yea, very true. But with my pot lids the pressure would also have to overcome friction between the lid and the pot... there's about a half inch vertical lip on the lid that contacts the pot and creates a seal... not a pressure cooker seal, but I avoid closing the lid and turning the heat up because of what will happen when I lift the lid.

Minimal. Especially since it's being lubricated by the condensation of the steam.

As for the effect of opening the lid, the gas inside is almost pure steam.

Isn't steam normally called a vapour, not a gas? I know a vapour is a kind of gas, but it is normal to call the gaseous phase of a substance (e.g. water) "vapour" if we wish to emphasize that we are talking about the gaseous phase, even though for some reason the condensed phase is considered "normal". Anyhow, what is above the boiling water in a saucepan is tiny droplets of condensed water vapour.

OK, this thread is making a common mistake in discussions about science. You are comparing a significant effect with a very slight effect and considering them equal.

1) Yes, it is absolutely true that increasing the pressure will raise the boiling point of water. You would see this happen if you welded the lid on the pot (or used a pressure cooker).

2) Putting a lid on a pot increases the pressure very very slightly which is why the lid doesn't come off. The raise in pressure is so slight that the raised boiling point is insignificant compared to the effect below.

3) Putting a lid on a pot keeps heat energy from escaping. This means that more energy stays in the water making it hotter quicker (and less is put in the air). This is a significant effect.

Without question water will boil much faster with a lid on.

I seem to have read this somewhere above.

I doubt the difference in boiling time is very significant either way, but there's an easy way to tell for sure... try it with two equal amounts of water in similar containers and see what happens.

Depends on whether or not you're a pedantic Brit, I suppose.

In the U.S., the gaseous phase of a substance is called a gas.