Limescale Chemistry

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Limescale is a substance you’ve undoubtedly encountered, be it clogging up your kettle, or building  up on your bathroom surfaces. But how does it get there in the first place, and how do limescale removers work to get rid of it? Those are the questions this graphic aims to answer.

Water Hardness

To be able to discuss how limescale is formed, we first need to discuss the concept of water hardness. Hard water is water which contains a large concentration of mineral ions – these are usually calcium and magnesium ions. There’s also the concept of permanent hardness and temporary hardness to complicate matters a little.

Temporary hardness is caused by dissolved calcium and magnesium bicarbonates in the water, and can be removed by boiling. On the other hand, permanent hardness, which is caused mainly by dissolved calcium and magnesium sulfates, can’t be removed by boiling. Despite the ‘permanent’ part of the name, however, we’ll see that there are still ways in which this hardness can be removed as well.

How is limescale formed?

The formation of limescale is largely a consequence of the presence of soluble calcium bicarbonate in water. This compound can decompose when heated to form insoluble calcium carbonate, which makes up the large part of limescale. This is what removes temporary hardness from water. Boiling doesn’t work for permanent hardness because the calcium sulfate doesn’t decompose in the same way when heated, so both ions remain in solution.

The calcium and magnesium ions in the water can also react with compounds found in soaps to produce the soap scum that plagues bathrooms. This is due to the reaction of these ions with compounds such as stearates, producing insoluble stearate salts.

How can water be ‘softened’ to prevent limescale?

There are a number of ways in which water hardness can be softened. The first, ion exchange columns, are commonly used by dishwashers to prevent scale build-up. These work by containing what’s known as an ion exchange resin, which is formed into beads. The beads have other ions attached to them, usually sodium ions, but sometimes hydrogen ions instead. When the water passes through the ion exchange column, the calcium and magnesium ions in the water swap places with the ions in the ion exchange column, helping to remove hardness.

Many laundry detergents also contain water softening agents. These can be in the form of citric acid, sodium sesquicarbonate, or other compounds. Again, these work by grabbing, or ‘chelating’ the calcium and magnesium ions in the water and preventing them from being able to form limescale deposits.

How can limescale be removed?

Limescale removers usually make use of acids. Acids will react with the limescale to produce soluble metal salts which can then simply be washed away. For toilets, stronger acids such as hydrochloric acid are used, whereas for kitchen appliances such as kettles, citric acid, lactic acid, or formic acid are common components of limescale removal powders.

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References & Further Reading