Hard Water and Soft Water

What is Hard Water?

Hard water is a term that denotes water having a very high mineral content (the term is the opposite of ‘soft water’). As water percolates into deposits of calcareous, gypsum or chalk that are primarily composed of carbonates of magnesium or calcium, bicarbonates and sulfates, hard water is formed.

Drinking hard water can have certain benefits to human health. However, when it comes to industrial settings, hard water is highly undesirable and can pose several major problems. For example, water hardness is controlled in industries in order to prevent costly breakdowns in cooling towers, boilers, and other water handling equipment. Soft water is also demonstrated in domestic settings by a lack of foam forming when soap is agitated in water, and by limescale forming in water heaters and kettles. The adverse effects of hard water are usually mitigated by the process of water softening.

What Causes Hardness of Water?

The hardness of a given water sample is usually defined by the concentration of the multivalent cations that are present within the water. Multivalent cations are the metal complexes that hold positive charges and the magnitude of these charges is always greater than 1 +. The cations typically bear a positive charge of 2 +. In hard water, certain specific cations are very common. Such examples include cations like Ca2 + and Mg2 +. Such ions can enter a reservoir of water within an aquifer via the leaching from minerals. Common minerals which contain calcium are gypsum and calcite. Dolomite (another mineral which also contains calcium), is a common magnesium mineral. Rainwater and purified water are considered to be soft water samples, because they have only a few ions dissolved in them.

Classification of Hardness of Water

Temporary Hardness of Water

A type of water hardness caused by the presence of dissolved bicarbonate minerals (including ionic salts such as magnesium bicarbonate and calcium bicarbonate) is often referred to as the temporary hardness of water. These minerals, when dissolved in the water, are known to dissociate into calcium and magnesium cations (denoted by the symbols Ca2+ and Mg2+), and anions of the carbonate and the bicarbonate. The presence of cations of the metal is responsible for the hardness of the water sample.

Unlike the type of hardness known as permanent hardness, which is usually caused by sulfate and chloride compounds, this type of “temporary” hardness of water can be decreased by either boiling the water, or by adding lime (also known as calcium hydroxide) via the lime softening process. The boiling of the water facilitates carbonate formation from bicarbonate and precipitates calcium carbonate from its solution. Finally, when the temporarily hard water is cooled after boiling, what is left behind is soft water.

Permanent Hardness of Water

Permanent hardness of water (usually attributed to mineral content in the water) is normally difficult to extract by boiling. When this happens, it is typically caused by the presence in the water of certain salts such as calcium chloride or calcium sulfate, and/or magnesium chloride or magnesium sulphate. It can be noted that these are the salts that do not precipitate out as the temperature rises (and cannot be extracted by simply boiling the water). Using a water softener, or an ion exchange column are great ways to extract the ions that cause permanent water hardness and obtain soft water as the product.

The permanent hardness of water is attributed to sulfates or chlorides of calcium and/or magnesium. Therefore, the factors that contribute towards the permanent hardness of water can be understood with the help of the following equation:

Permanent hardness of the water sample = permanent hardness of the calcium component + permanent hardness of the magnesium component.

Soft Water

Soft water can be defined as surface water that contains relatively low ion concentrations, and is low in calcium and magnesium ions in particular. Soft water naturally occurs where rough, impermeable and calcium-poor rocks are responsible for the formation of the runoff and the drainage basin of rivers.

The term ‘soft water’ is also used often to describe water created by a water softening process (despite the fact that it is more practical to refer to these water samples as softened water). In these cases the water can also contain high sodium and bicarbonate ion levels.

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Frequently Asked Questions on Hard Water and Soft Water

Is it safe to drink hard water?

Hard water does not pose a health threat. Indeed, there exist several reports that suggest that hard drinking water usually contributes a small amount to overall human dietary needs in terms of calcium and magnesium requirements. Many researchers researched water hardness and its correlations with mortality from cardiovascular disease.

Why is water containing large amounts of dissolved calcium and magnesium salts referred to as “hard water”?

Hard water (also referred to as water hardness) is a very common characteristic associated with water which contains dissolved chemical compounds of magnesium and calcium, and sometimes, certain other trivalent and divalent metallic elements. Initially, the term hardness was applied to water that was difficult to wash in, due to soap wasting nature of hard water.

What are the disadvantages associated with hard water?

The primary disadvantage of hard water is that it results in the wastage of soap. In bathing activities, hard water does not freely lather with a soap solution and it develops gritty scum on the bath and body. In cooking activities, the boiling point of the water becomes elevated due to the presence of the dissolved hardness-producing salts. This may have an adverse effect on the taste of the food.

Why is hard water undesirable?

Water that is referred to as “hard” contains high concentrations of dissolved calcium and magnesium. Hard water is not a health risk but due to mineral deposition on plumbing fixtures and poor soap and/or detergent results, it is a nuisance. Water is a strong solvent, which quickly absorbs impurities.

What happens when hard water is boiled?

The hardness of water may either be either temporary or permanent. When it comes to “temporary” hardness, it may either be reduced by boiling the water, or by adding lime (calcium hydroxide). Hard boiling water precipitates the calcium carbonate out of the solution, leaving softer water when it cools down.

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