Limewater is a clear, colorless solution of calcium hydroxide dissolved in water. It has been used for centuries in kitchens, laboratories, and factories, and its most famous party trick is turning milky white when carbon dioxide is bubbled through it. Making it is straightforward: you dissolve calcium hydroxide powder in water, let the excess settle, and pour off the clear liquid on top. But the backstory of where that calcium hydroxide comes from, why limewater behaves the way it does, and the surprisingly wide range of things people do with it is worth a closer look.
The Production Chain From Rock to Clear Solution
Limewater sits at the end of a three-step chain that starts with ordinary limestone, a sedimentary rock made mostly of calcium carbonate. The first step is calcination: heating limestone to high temperatures, typically above 900 °C, which drives off carbon dioxide gas and leaves behind a white, powdery solid called quicklime, or calcium oxide. This thermal decomposition of calcium carbonate into calcium oxide and carbon dioxide is one of the oldest industrial chemical processes in human history, dating back thousands of years.1Materials Science Forum. Thermal Decomposition and Solid Characterization of Calcium Oxide in Limestone Calcination
The second step is slaking. When you add water to quicklime, it reacts vigorously, releasing a large amount of heat and producing calcium hydroxide, commonly called slaked lime or hydrated lime. The reaction is exothermic enough to boil the water if you add too much quicklime at once, and the heat and reaction speed are actually used as quality indicators for quicklime in industries like steelmaking.2Ironmaking & Steelmaking: Processes, Products and Applications. Factors influencing reactivity of quicklime from zero-emission electrified calcination processes
The third step is the simplest. You take that calcium hydroxide powder and stir it into water. Calcium hydroxide does not dissolve very well, so most of it will sink to the bottom. You wait for the solution to settle, then carefully pour off or siphon the clear liquid from the top. That clear liquid is limewater. If you skip the settling step and keep the undissolved particles suspended, you get a milky slurry sometimes called milk of lime, which is a different product used in industrial settings.
Why Limewater Is Always Dilute
One of the distinctive things about limewater is that it is inherently a weak solution. Calcium hydroxide has low solubility in water, only about 1.5 grams per liter at room temperature. That means even a fully saturated solution, which is the strongest limewater you can make, contains very little dissolved material. Despite being so dilute, limewater is strongly alkaline. A saturated solution at room temperature has a pH of roughly 12.5, making it caustic enough to irritate skin and mucous membranes.3PubMed. In vitro susceptibility of oral Candida albicans strains to different pH levels and calcium hydroxide saturated aqueous solution
An unusual property of calcium hydroxide is that it becomes less soluble as the temperature rises. Most solid substances dissolve more readily in hot water, but calcium hydroxide bucks that trend. If you heat limewater, some of the dissolved calcium hydroxide actually comes out of solution. This inverse solubility relationship means that making limewater with cold water gives you a slightly more concentrated product than making it with warm water.4Journal of Chemical Education. Dissolution of Calcium Hydroxide in Water: A Guided Inquiry in University and High School Chemistry Laboratories
The Classic Carbon Dioxide Test
If you have taken a chemistry class, you have probably seen the demonstration where someone blows through a straw into a beaker of limewater and the clear liquid turns milky. That cloudiness appears because the carbon dioxide in your breath reacts with the dissolved calcium hydroxide to form calcium carbonate, a white solid that is essentially the same mineral as the limestone the whole process started with. The tiny particles of calcium carbonate are insoluble and remain suspended, scattering light and giving the solution its characteristic milky appearance.
This reaction is what makes limewater useful as a simple, reliable detector for carbon dioxide, and it has been a standard laboratory test for the gas for over a century. In medical settings, the same reaction was historically used in closed-circuit anesthesia systems to scrub exhaled carbon dioxide from breathing circuits. The white precipitate forming visibly in a clear solution makes it one of the most intuitive chemical tests around: you can literally watch the gas being removed.
There is a lesser-known follow-up, though. If you keep bubbling carbon dioxide through the milky limewater well past the point where it turns cloudy, the solution gradually clears again. The excess carbon dioxide reacts with the calcium carbonate precipitate to form calcium bicarbonate, which is soluble. This two-stage behavior, clear to milky to clear again, is a classic exam question in introductory chemistry courses, and it catches students off guard because they expect the milkiness to just keep getting thicker.
How Limewater Pulls Carbon Dioxide Out of the Air
The same reaction that turns limewater milky in a test tube works on a much larger scale in the open atmosphere. Calcium hydroxide naturally absorbs carbon dioxide from the air, forming calcium carbonate over time. This process, called carbonation, is why an open container of limewater develops a thin white crust on its surface if left out: it is literally pulling carbon dioxide from the room and converting it into a chalky film.
Researchers have explored this reaction as a potential tool for removing carbon dioxide from the atmosphere on a large scale. A 2025 study tested how quickly thin layers of both quicklime and slaked lime absorbed atmospheric carbon dioxide under ambient conditions. Spreading thin layers of less than 10 millimeters every five to ten days produced the highest removal rates per unit area, above two tons of carbon dioxide per hectare per day.5PubMed. Atmospheric carbon dioxide removal using layers of lime The practical challenge, as the study notes, is cost: producing lime with zero carbon emissions is expensive, so the economics depend on balancing production costs against land availability.
A related industrial application uses calcium hydroxide solutions to capture carbon dioxide from factory flue gas. When carbon dioxide-rich exhaust meets a calcium hydroxide solution, it reacts to form precipitated calcium carbonate, a commercially valuable material used in paper, paint, and plastics. One process simulation based on a cement plant showed that this approach could capture over 4,500 kilograms of carbon dioxide per hour with high efficiency, producing precipitated calcium carbonate as a useful byproduct.6CrossRef API. Techno-Enviro-Economic Analysis of Precipitated Calcium Carbonate Production from Carbon Dioxide in Cement Industry Flue Gas and Calcium Hydroxide So the same fundamental chemistry behind a high-school lab demonstration scales up to industrial carbon capture.
Limewater in Food
Calcium hydroxide has a long history in food preparation, and when people in culinary contexts talk about “lime water” or “cal water,” they are referring to the same chemistry. The most important culinary use is nixtamalization, the ancient Mesoamerican technique of soaking dried corn kernels in an alkaline solution made with calcium hydroxide. This process softens the tough outer hull of the corn, changes the texture of the starch, and makes certain nutrients more available, particularly niacin, which is otherwise locked inside the grain in a form the human body cannot absorb easily. Without nixtamalization, populations that rely heavily on corn risk niacin deficiency.
The calcium hydroxide concentration used in nixtamalization also affects the final product in ways food scientists continue to study. Research has shown that increasing the lime concentration during nixtamalization substantially reduces acrylamide, a potentially harmful compound that forms when starchy foods are cooked at high temperatures. Tortilla chips made from corn processed with higher lime concentrations had up to about half the acrylamide content of chips made with standard concentrations.7Elsevier / LWT – Food Science and Technology. Effect of added calcium hydroxide during corn nixtamalization on acrylamide content in tortilla chips
Beyond corn, calcium hydroxide solutions appear in various food traditions around the world. In Southeast Asian cooking, a dilute limewater solution is sometimes used to soak fruits and vegetables before pickling, which helps them stay crisp by strengthening the cell walls. Chinese-style century eggs (preserved duck eggs) use a paste containing calcium hydroxide as part of the alkaline curing mixture. In sugar refining, calcium hydroxide is added to raw cane juice to neutralize acids and cause impurities to coagulate and settle out of the liquid, a step known as defecation or clarification. This has been standard practice in the sugar industry for well over a century.
Water Treatment and Construction
Municipal water treatment plants are among the largest consumers of calcium hydroxide. In the lime-softening process, calcium hydroxide is added to hard water to raise its pH. This causes dissolved calcium and magnesium ions, the minerals that make water “hard,” to precipitate out as calcium carbonate and magnesium hydroxide, which can then be removed. Researchers continue to study the behavior of these precipitate particles, particularly how their surface charges interact with natural organic matter, silicates, and clay particles during treatment, because those interactions affect how well the particles settle and how clean the treated water ends up.8PubMed Central. Electrokinetic study of calcium carbonate and magnesium hydroxide particles in lime softening
Water treatment plants also use limewater or calcium hydroxide slurries to adjust the pH of acidic water supplies, protect pipes from corrosion, and help with flocculation, the process of clumping together fine particles so they can be filtered out. It is one of the cheapest and most widely available chemicals for these jobs, which explains its enduring popularity in municipal infrastructure even as newer treatment technologies have emerged.
In construction, calcium hydroxide has been a key ingredient in mortar and plaster for millennia. Traditional lime mortar works by a slow carbonation process: the calcium hydroxide in the mortar gradually absorbs carbon dioxide from the air over months and years, converting back to calcium carbonate and hardening in the process. This is essentially the same reaction that forms a skin on an open jar of limewater, just happening inside a wall joint instead of at a liquid surface. Lime mortars are still preferred for heritage building restoration because they remain slightly flexible and allow moisture to pass through, unlike rigid modern Portland cement mortars that can damage old masonry.
Safety and Handling
Because even a dilute, saturated limewater solution has a pH around 12.5, it is not something to handle carelessly. At that alkalinity, it can cause chemical burns to skin on prolonged contact and serious damage to eyes. The calcium hydroxide powder used to make limewater is more dangerous than the solution itself, because inhaling the fine dust irritates the respiratory tract and getting it in your eyes can cause severe injury. Standard safety measures include gloves, eye protection, and working in a ventilated area when handling the dry powder.
For food applications, the concentrations used are far lower than a saturated solution, and the alkaline treatment is usually followed by thorough rinsing, so the finished food does not carry a dangerously high pH. Calcium hydroxide is approved as a food additive by regulatory agencies in the United States, the European Union, and many other jurisdictions. In the context of food use, the risks are minimal as long as you follow established recipes and concentrations. Eating a handful of dry calcium hydroxide powder, on the other hand, would be a medical emergency.
Making Limewater at Home
If you want to make limewater yourself, the process is genuinely simple, though a couple of details matter. You need food-grade calcium hydroxide, which is sold at many grocery stores (especially Latin American and Asian markets) under names like “cal,” “pickling lime,” or “slaked lime.” Add a tablespoon or two to a quart of clean water, stir well, and let it sit undisturbed for several hours or overnight. The undissolved powder will settle to the bottom, and the clear liquid above it is your limewater.
Pour or siphon the clear liquid into a clean container, being careful not to disturb the sediment. Seal the container tightly. Remember that limewater absorbs carbon dioxide from the air, so every time you open the container, a little bit of the dissolved calcium hydroxide reacts with atmospheric carbon dioxide and forms a thin chalky film on the surface. If you see that white crust, it does not mean your limewater has gone bad; it means it has been exposed to air. You can filter it off and the remaining solution is still usable.
Store your limewater in a sealed container away from light and heat. Because calcium hydroxide is less soluble at higher temperatures, storing it somewhere cool keeps the solution at its strongest. The leftover sediment at the bottom of your mixing jar can be reused: just add more water, stir, and let it settle again. A single batch of calcium hydroxide powder can yield multiple batches of limewater before it is fully depleted.
Common Mix-Ups and Confusions
The word “lime” causes endless confusion because it refers to at least three unrelated things. In chemistry and industry, lime means calcium oxide (quicklime) or calcium hydroxide (slaked lime). In everyday life, a lime is a green citrus fruit. And “lime” is also a term used in agriculture for various calcium-containing soil amendments, which may or may not be pure calcium hydroxide. When a recipe calls for “lime water,” context is everything. A bartending recipe calling for lime water probably means water infused with citrus. A recipe for hominy or tortillas calling for lime water means the calcium hydroxide solution.
Another common point of confusion involves the difference between limewater and milk of lime. Limewater is the clear solution you get after settling and decanting. Milk of lime is the opaque, milky suspension you get when excess calcium hydroxide powder is still mixed in. They contain the same chemical, but at very different concentrations and in different physical forms. Industrial processes like water treatment and sugar refining typically use milk of lime because they need larger quantities of calcium hydroxide than a dilute saturated solution can provide. Laboratory tests and food recipes more commonly call for the clear solution.
People sometimes also confuse calcium hydroxide with calcium chloride, another white powder used in food processing, particularly in cheese-making and canning. These are entirely different chemicals with different properties. Calcium chloride is a salt, not a base; it does not make water alkaline the way calcium hydroxide does. Substituting one for the other in a recipe would produce completely wrong results and could be unsafe, so it is worth double-checking the label before you buy.
Limewater in Dentistry and Medicine
Calcium hydroxide shows up in dental practice in ways most people never see. Dentists use calcium hydroxide pastes and solutions as antimicrobial agents inside root canals during treatment. The extremely high pH of a saturated calcium hydroxide solution, around 12.5, creates an environment hostile to many bacteria and fungi. Research has confirmed that oral strains of the common fungus Candida albicans are susceptible to saturated calcium hydroxide solutions, and the high pH plays a direct role in that antimicrobial effect.3PubMed. In vitro susceptibility of oral Candida albicans strains to different pH levels and calcium hydroxide saturated aqueous solution Calcium hydroxide is also used as a lining material placed over exposed dental pulp to encourage the formation of a protective hard-tissue barrier.
In broader medicine, calcium hydroxide solutions have historically been used as antacids and in wound-care preparations, though modern alternatives have largely replaced them. The substance still appears in some traditional medicine practices, particularly in parts of South and Southeast Asia, where slaked lime paste is chewed alongside betel leaf and areca nut, a habit with its own set of well-documented health risks including oral cancer. The medical consensus is that while calcium hydroxide has genuine antimicrobial properties, its high pH makes it unsuitable for casual or unsupervised use on the body.