Boiling tap water for about 15 to 20 minutes is enough to drive off free chlorine. Chlorine gas becomes less soluble as water temperature rises, and once water reaches a rolling boil, the dissolved chlorine escapes rapidly into the air. For most people, the real question is whether their water contains free chlorine or chloramine, because the answer changes everything.
Why Boiling Works for Free Chlorine
Chlorine is a dissolved gas, and like all dissolved gases, it becomes less soluble as water gets hotter. CRC Handbook data shows that chlorine’s solubility drops by about 50% between 10°C (50°F) and 40°C (104°F). By the time water hits 100°C, very little chlorine can remain dissolved. The vigorous agitation of a rolling boil accelerates the process further by exposing more water surface area to the air.
A short boil of 1 to 5 minutes removes a significant portion of free chlorine, but letting the water boil for a full 15 to 20 minutes ensures a more thorough removal, especially if your water sits at the higher end of allowable chlorine levels. The EPA permits up to 4 milligrams per liter of chlorine in drinking water, so starting concentrations can vary quite a bit depending on where you live and how close you are to a treatment plant.
Chloramine Is a Different Problem
Many water utilities have switched from free chlorine to chloramine, a compound made by combining chlorine with ammonia. Chloramine is more stable, which is exactly why utilities like it: it lasts longer in the pipes. But that stability also means boiling doesn’t work. American Water states plainly that boiling is not an effective method for removing chloramines from water. While free chlorine dissipates within a day or two if you simply leave water sitting out, chloramine can take weeks to break down on its own.
If your water is treated with chloramine, you’ll need a different approach (more on that below). Your annual water quality report, sometimes called a Consumer Confidence Report, will tell you which disinfectant your utility uses. You can usually find it on your water provider’s website or request a copy.
What Happens to Byproducts When You Boil
Chlorine and chloramine both produce disinfection byproducts, including a group of compounds called trihalomethanes. These form when disinfectants react with natural organic matter in the water. Boiling actually reduces most of these byproducts. Research published in Water Research found that boiling chlorinated water for 1 minute reduced trihalomethane concentrations by 64 to 98%, depending on the specific compound. Most other common byproducts dropped by at least 90% after just 1 minute of boiling.
There’s one wrinkle with chlorinated water specifically: during the first minute of boiling, chlorine is still reacting with organic material and creating new byproducts even as existing ones evaporate. That’s why chloroform levels dropped only 34% after a 1-minute boil in chlorinated water but 75% in chloraminated water, where less active chlorine was available to form new compounds. Boiling longer gives the byproducts more time to volatilize after the chlorine itself has been driven off.
The Concentration Tradeoff
One important thing boiling doesn’t fix: as water evaporates, anything that can’t escape as a gas becomes more concentrated in the remaining liquid. Heavy metals like lead, mercury, and arsenic don’t volatilize at boiling temperatures. Nitrates behave the same way. If your water contains elevated levels of these contaminants, prolonged boiling makes the problem worse, not better. This is worth keeping in mind if you’re on well water or in an area with known lead service lines.
Faster Alternatives to Boiling
Boiling works, but it’s slow and energy-intensive. If you regularly need chlorine-free water for brewing beer, filling a fish tank, or watering sensitive plants, other methods are more practical.
- Letting water sit uncovered. Free chlorine will dissipate from a gallon of tap water left out at room temperature within 24 hours. Sunlight and a wider container speed things up. This doesn’t work for chloramine.
- Activated carbon filters. Pitcher filters and faucet-mount filters with activated carbon remove both free chlorine and chloramine on contact. They’re the simplest solution for daily drinking water.
- Vitamin C (ascorbic acid). A small amount neutralizes both chlorine and chloramine almost instantly. Aquarium hobbyists and homebrewers commonly use this method. Tablets sized for water treatment are widely available.
- Campden tablets. Another favorite among brewers, these contain potassium metabisulfite and neutralize chlorine and chloramine in seconds. One tablet treats about 20 gallons.
How to Tell What’s in Your Water
Inexpensive test strips can confirm both the type and concentration of disinfectant in your tap water. Look for strips that measure both free chlorine and total chlorine. If the total chlorine reading is significantly higher than the free chlorine reading, your water contains chloramine. Knowing which one you’re dealing with determines whether boiling is even worth your time.
For free chlorine removal, a 15 to 20 minute rolling boil in an uncovered pot is reliable. For chloramine, skip the stove entirely and use a carbon filter or chemical neutralizer instead.