The effect of a lid on pot of boiling water

Matthew Alice: It’s time to settle an ongoing debate between my brother and I which started one evening when we were making up a batch of spaghetti for our kids. The pot of water was on the stove at full flame but didn’t seem to ever be getting to a rolling boil. I put a lid it, and soon we had a full boil. He contends that it would have happened anyway and that, in fact, putting a lid on the pot actually increases the amount of time it takes for water to boil. In my experience, water boils faster when you start with the lid on. Which is true, and why? — DanReznick@intuit.com

See, if everybody’d just paid attention in class, I’d be out of a job. Generations of goof-offs and daydreamers — the world’s best employment insurance.

Water only seems to boil sooner when it’s covered, because with the lid on, you stop staring into the pan and waiting for the bubbles, and you go off and do something else for a while to distract yourself. Time flies when you’re not watching water boil.

Water is at a full rolling boil when all the water molecules in the pan have acquired enough energy (heat) to counteract air pressure and water pressure and push their way to the surface. At sea level, this happens when the water temperature reaches 212 degrees. Anything that makes the molecules work harder to push their way up (increased air pressure, or salt, sugar, etc., dissolved in the water) will slow the boiling process because it will take more heat. Even before the water boils, surface molecules escape in the form of steam. Trap the steam under a lid, you increase the air pressure inside the pot, the water molecules have to push harder against the pressure to boil, so more heat and more time are required. Air pressure changes due to altitude or weather also affect the amount of heat required to boil water. And once water’s boiling, it doesn’t get any hotter. So don’t order three-minute eggs in Nepal. They’ll be pretty raw.

July 27 update

Matthew Alice’s evil twin authored last week’s bulletin from the M.A. Home for the Bewildered. One minute yours truly and the elf squad are toiling away, tapping out the facts, the next minute we’re bound and gagged and stuffed in a storage locker in Mira Mesa. A half hour later, Matthew Alice’s evil twin had wiped out the entire pressure cooker industry. Maybe it was the memory of too many of Ma Alice’s one-pot steamed squirrel and okra dinners. Anyway, the Howzzat? Team, the Matthew Alice Pan Club, appointed Bob Moision of Vista to set us straight. I’ll let Dan Reznick and his brother sort things out for themselves, assuming they’re still speaking to one another. The question Dan asked was whether water boiled faster covered or uncovered. Sez Bob, “True, the temperature at which a liquid boils is dependent on the atmospheric pressure, the amount of energy (and thus time) necessary to bring water to a boil is also dependent on the amount of evaporation from the liquid. When a molecule evaporates from a liquid, it takes energy from the liquid, thus lowering the temperature of the liquid. This is why sweating causes cooling of our bodies. Covering a pot minimizes evaporation and the cooling it causes.”