Causes of scale formation in the kettle. How to avoid its appearance?

The article discusses the causes of the formation of solid scale on the surface of kitchen equipment in which water is boiled: kettles, pots, etc. Which method of boiling and which water treatment systems to use in order to avoid or significantly reduce the rate of scale formation. The article also focuses on the possible significant influence of the chemical composition of water on the taste of cooked food, even if this water meets the requirements of the SanPiN “Drinking Water”.

When the water that meets the requirements for drinking water for cooking is boiling, the solid precipitate released from the water can only be calcium carbonate (CaCO₃). The precipitation of calcium sulfate and magnesium hydrate can occur only under conditions that are quite difficult to obtain in an ordinary kitchen. Perhaps, except for one case. When you constantly pour a little water into the kettle and boil it intensively. In this case, the water in the kettle will be saturated with salts present in the source water and, over time, extremely difficult to remove sulfate scale may fall out.

Now let us talk about it in detail. To begin with, consider an ordinary electric kettle. What happens to the water that has a chemical composition that meets the requirements for drinking water?

Let’s write down the standard chemical composition of drinking water typical for most of the full-flowing rivers of Russia.

Cations:

Calcium (Ca) – 1.0 – 3.5 mg-eq/l

Magnesium (Mg) – 0.5 – 2.5 mg-eq/l

Sodium (Na) – 0.5 – 1 mg-eq/l

Anions:

Bicarbonates (HCO₃) – 1.5 – 3.0 mg-eq/l

Sulfates (SO₄) – 0.5 – 1.0 mg-eq/l

Chlorides (Cl) – 0.5 – 1.0 mg-eq/l

All cations and anions included in the chemical composition of water do not undergo changes during boiling, except for one, bicarbonate (HCO₃). Bicarbonate ion in the process of boiling water gradually decomposes into two chemical compounds. These are carbon dioxide (CO₂) and carbonate ion (CO₃). The carbonate ion interacts with water and hydrate (OH) is formed. This leads to an increase in the pH value of the water.

(НСО₃)₂ = Н2О+СО₂ + СО₃

CO₃ + H₂O = OH + HCO₃

It is easy to verify this by boiling drinking water from the tap and finding that its pH has increased compared to the initial pH value of unboiled water. The longer you boil the water, the higher the pH value of this water will be. This suggests that carbonate ions begin to appear in the water and in the presence of calcium ions in the water they precipitate in the form of the well-known light scale (CaCO₃) on the inner surface of the kettle.

This process occurs due to the fact that carbon dioxide is removed with the steam from the water when it is boiled.

How to deal with it?

Obviously, if you remove calcium ions from the water, then there will be no scale. And there is a certain misunderstanding about this process. If you use Na – cation to replace all the calcium cations contained in the water with sodium cations, you will get water that contains only sodium as cations. This will lead to the fact that even with a short boiling period of such water, its pH value will increase significantly. In fact, you will get water which would have a such qualities as if you yourself  specifically added a little caustic soda (NaOH). I think it will significantly affect the taste of tea or coffee. The pH value of such water can be significantly higher than 9.0 units of pH, which immediately excludes it from the category of drinking water (the pH norm for drinking water is 6.0 – 9.0).

That is, everything is very simple. When carbon dioxide is released from the water, carbonate (CO₃) is automatically formed from bicarbonate (HCO₃). Further, the carbonate will either react with calcium and precipitate in the form of scale (CaCO₃), or it will remain with soluble sodium and as a result of hydrolysis (interaction with water molecules) it forms sodium hydrate (caustic soda – NaOH) and increases the pH value of water.

Nature is wise and certainly has provided for the possibility of boiling water without a strong increase in its pH value. That is, part of the carbonates precipitates, and part increases the pH. Because there is both calcium and sodium in the source water. If the source water had only sodium (softened water), then the high pH value of boiling water would significantly spoil the taste of food. If there were only calcium, then such water could not be attributed to drinking at all, because its pH would be significantly lower than 6.0 units of pH. Due to the content of a large amount of carbon dioxide in it to maintain carbon dioxide equilibrium.

Therefore, it is very good that there are both calcium and sodium in the source drinking water. You just need to follow the simple rules of boiling water in a kettle. First of all, if you want to drink tea, you need to pour out the rest of the water from the kettle every time and pour fresh drinking water. If you do not do this and add fresh water to the remainder of the already alkaline and scum-prone water, then you will very quickly get scum in the kettle and a high pH of boiled water, i.e. all the troubles at once. Therefore, the entire volume of fresh water should be boiled only once. The second time it is necessary to boil only a new portion of fresh water. And secondly, is it really necessary to brew tea with boiling water? This is of course a matter of taste. But if the water in the kettle is heated to 70-80 ⁰C and fresh water is poured for each boiling, then there will be no scale in the kettle for drinking water at all and the pH value of boiled water will practically not increase relative to the pH of the source water.

Now let’s look at what happens when you are cooking soup. The main difference from a kettle is that in a saucepan, when cooking soup, one volume of water boils for a long time. If you cook with completely softened water, you can get anything but not a delicious dish. Prolonged boiling removes carbon dioxide and, accordingly, the pH value increases. Of course, organic acids from vegetables will compensate for the excess pH value of the soup water, but this process is clearly not provided by the cooking technology and rather plays the role of a lucky-unlucky factor. Therefore, if possible, when preparing soup, it is better not to boil of water in a saucepan, especially for a long time, and do not use the original softened water. If in the kettle, when using simple techniques, it is possible to avoid scale or significantly reduce the rate of its formation, then it does not work in a saucepan. But in a saucepan, when using drinking water, scale forms quite slowly, probably due to the presence of organic acids of some vegetables. Therefore, a rare periodic washing of pots with citric acid should not cause difficulties.

In conclusion, I want to say that the chemical composition of natural drinking water is quite balanced and it is better not to change it by increasing or decreasing one or another ion. And if it is necessary to reduce the content of only hardness ions, it is better to use reverse osmosis. In this case, the content of all ions will decrease, but their approximate proportion will remain. Because the reverse osmosis membrane still has a slightly greater selectivity with respect to divalent ions. For commercial use, it is possible to use a pre-mix after osmosis to adjust the chemical composition.

If the source water does not meet the requirements for drinking water, then it is desirable that a specialist should be engaged in bringing such water to drinking standards. It will be necessary to take into account a lot of aspects, and poor-quality water significantly worsens  the quality of life.