Have you ever wondered whether it’s safe to reboil water? Do you even know what reboiling water means? Reboiling water it when you boil water, let it cool below its boiling point, and then boil it again. For example, you reboil water if you boil it in the microwave, forget about it, and then boil it again before taking it out. To some extent, you also reboil water when you top off the tea kettle, although that water is a mixture of fresh water and boiled water.
Here’s a look at the chemistry and physics of reboiled water and whether it’s safe to drink.
What Happens When You Reboil Water
When you reboil pure water its chemistry remains unchanged. You start with pure water and end with pure water, no matter how many times you boil it. Distilled water and reverse osmosis water are pure enough that they are unaffected by reboiling.
Reboiling drinking water, mineral water, and spring water does change its chemistry because the water contains other compounds. This is because when you boil water some gets lost to the atmosphere as steam, concentrating the remaining chemicals. This is not a health concern with clean drinking water, although reboiling hard water may lead to chalky white scale deposits inside of the kettle and hot water heater. Scale mainly consists of calcium carbonate (CaCO3), magnesium hydroxide (Mg(OH)2), and calcium sulfate (CaSO4). Most people find these minerals improve the flavor of the water and make it more healthful. However, reboiling water high in calcium (hard water and mineral water) may be a concern for people with gallstones, kidney stones, arthritis, or atherosclerosis.
Does Reboiled Water Cause Cancer?
Basically, no. Drinking reboiled drinking water does not cause cancer. But, it can increase your risk for cancer (from pesticides and other chemicals), methemoglobinemia (from nitrates in fertilizers), and arsenic toxicity (from arsenic, either natural or from waste). Lead and fluoride are also concentrated by reboiling. Lead comes from solder used in metal pipes, while fluoride either occurs naturally or is added to some public water supplies. Water companies test water, so you can learn about potential issues from them. If you use well water or are concerned about lead from plumbing, you can get your water tested. Keep in mind, reboiling does not significantly increase the concentration of undesirable chemicals already in water. Contaminated source water should be filtered before boiling once, much less twice. Uncontaminated water is safe to boil and reboil.
The Main Risk of Reboiled Water
One real health risk of reboiling water is that it may superheat and present a burn hazard. Normally, tiny gas bubbles in fresh water act as nucleation sites for the bubbles in boiling water so the water boils as expected. Reboiling water drives out dissolved gases in the water, making it “flat.” Superheating may occur, making the water hotter than its normal boiling point and causing it to explosively boil when disturbed. For this reason, it’s a bad idea to reboil water in a microwave.
Reboiling Water May Affect Flavor
Although not a safety consideration, using reboiled water may affect the flavor of coffee or tea. This is because fresh water contains lots of tiny air bubbles. When you heat the water, dissolved gases slightly increase the acidity of water, allowing for a better extraction of flavorful molecules. Some people also claim boiled water tastes flat. So, it’s best to make coffee or tea with fresh, cold water. It’s a tiny change, so it’s likely very few people can tell whether you used fresh water or whether you reboil water. The quality of the water (its source and dissolved minerals) and the coffee or tea play a much larger role in the flavor of the finished product. Also, it’s worth noting that the optimal temperature for brewing coffee and tea is below the boiling point of water, so reboiling water might not even enter into the equation.
The Bottom Line
Generally, there is no health risk if you boil water, let it cool, and reboil it. It’s best if you only reboil water once or twice before starting from scratch. Reboiling tap water does concentrate minerals and other chemicals in the water. So, if your source water is high in nitrates, arsenic, or other undesirable chemicals, avoid reboiling it. Similarly, if your water is high in calcium and you have health conditions affected by the mineral, it’s better to avoid reboiling water. It might even be worth switching to distilled or reverse osmosis-purified water. Reboiling pure water does not change its chemical composition at all.
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