1. What is the white limescale that builds up inside pots and kettles made of?
If you use a kettle or pot for a long time, a white, rough substance builds up on the bottom and walls. This deposit, called limescale, is formed from minerals like calcium and magnesium that are present in tap water [6]. It is strange that these minerals were originally dissolved in the water. While dissolved, they are invisible, but when the water is boiled, they remain as a white solid.
It is tempting to think that the water simply evaporated, leaving the contents behind. Many home guides describe it as what remains after drying [6], but that is not the whole story. A reaction happening inside the water while it boils is involved. This same substance forms inside large boilers, leading to wasted fuel [9]. Let us first look at the reaction inside the pot.
2. What happens to the minerals in hard tap water when it is boiled?
In tap water, calcium is dissolved along with bicarbonate ions. This type of hardness can be removed by boiling, because calcium carbonate settles out of the water. This is why it is called temporary hard water [2]. Hardness caused by ions like sulfate, which cannot be removed by boiling, is called permanent hard water [2].
When water is boiled, carbon dioxide gas leaves the water. This shifts the balance between carbonic acid and bicarbonate ions in the water, causing calcium carbonate to settle out as a solid [4]. Some explanations say it hardens because it becomes less soluble when hot. However, the relationship between temperature and solubility alone cannot explain this reaction. This article focuses on the carbon dioxide leaving the water [4].
What is water hardness? Water hardness refers to the amount of calcium and magnesium dissolved in the water. The United States Geological Survey (USGS) classifies water as soft if it has 0–60 mg per liter of calcium carbonate, moderately hard at 61–120 mg, hard at 121–180 mg, and very hard if it exceeds 180 mg [1]. Is this white stone formed outside the pot as well?
3. Do cave stalactites form in the same way as limescale in a pot?
Stalactites are formed when calcium carbonate dissolved in groundwater becomes supersaturated and settles out as calcite crystals, often because carbon dioxide escapes into the air [5]. Supersaturation means the water holds more dissolved material than it normally can.
The chemical reaction described for stalactites involves calcium bicarbonate splitting into calcium carbonate, water, and carbon dioxide [3]. This is the same form of reaction as the one described for limescale in the pot [4]. In the pot, heat causes carbon dioxide to escape. In caves, groundwater emerges into an open space, and carbon dioxide escapes into the air. Although the trigger for releasing the gas is different, the result is the same: dissolved calcium bicarbonate forms solid calcium carbonate.
So, the white layer inside the kettle is, in its main component, the same substance as stalactites. However, the place and speed of formation are different. In a pot, this white layer is usually just a nuisance. But in devices that make large amounts of hot water, even a thin layer causes problems. What happens?
4. Why does a thin layer of scale in a boiler waste fuel?
On the water-side heat transfer surfaces of boilers, calcium, magnesium, and silica from the water can form continuous layers. This is called scale [8]. Scale is a poor conductor of heat, which lowers the boiler's thermal efficiency, leading to energy loss and potential damage to the heat transfer surfaces due to overheating [9].
Documents from the U.S. Department of Energy (DOE) indicate that the thermal conductivity of scale is an order of magnitude lower than that of bare steel. They also state that fuel consumption in fire-tube boilers can increase by up to 5% [8]. In the same document's table, "normal" scale of 1/32 inch (about 0.8 mm) causes a 2.0% fuel loss, while 1/16 inch (about 1.6 mm) causes a 3.9% loss [8]. When the thickness doubles, the loss also roughly doubles (3.9 ÷ 2.0 ≈ 1.95).
This table is for American fire-tube boilers without waste heat recovery. Since it is not measured in Japanese boilers, read these numbers as guidelines for "what happens under these conditions." To prevent this, the same document lists removing the minerals beforehand using water softeners, desalination, or reverse osmosis, adding chemicals, and performing blowdown (replacing the water inside the boiler) [8]. Yet, your home pot already has a white layer. How can it be removed?
5. Why can citric acid remove limescale from a pot?
Limescale in pots is an alkaline dirt. Therefore, it can be removed by neutralizing and dissolving it with acidic citric acid [6]. Citric acid is often used in home cleaning. Manufacturer pages for humidifier maintenance suggest that warm water (below 40°C) increases effectiveness [7]. However, this is for humidifier care, so for pots, it is best to follow the method in the pot's instruction manual.
It is natural for the calcium carbonate in limescale to dissolve in acid, as seen in the previous steps. The calcium that was dissolved in water became solid when boiled. Adding acid returns it to a dissolved state. So, how can you check this limescale with your own eyes?
6. How can you observe limescale in a pot and investigate it further?
What you can do today: Boil water, and after the pot has completely cooled, look inside in a bright place. To avoid burns, always wait until it is cool and do this with a family member. Check if there is white roughness, whether it is only on the bottom or also on the walls. Before touching anything, read the pot's instruction manual with a family member to find the cleaning method and safety advice. If you find the white substance, remember that the main component is calcium carbonate, the same as stalactites in caves [5].
If you want to investigate more: After using the pot for several days, observe the inside after it cools and write down the number of uses and the appearance of the white substance in a notebook. Compare how it looks in weeks with many boils versus weeks with few boils.
Water hardness can differ by region. Your local water utility may publish water quality information, which you can search for in Japanese. The USGS classification (0–60 mg/L for soft water) uses calcium carbonate equivalent, so before comparing it with local figures, check whether the water utility's numbers use the same units and standards [1]. The boiler number table is in reference [8].
SAFETY NOTE: Looking inside before the pot cools down poses a risk of burns. Always wait until it is completely cool and do this with a family member.