Formation, Measurement & Real-Life Effects of Hard Water
Hard water is defined as water, which contains the salts of both magnesium and calcium principally as chlorides, bicarbonates, and sulfates. Often present and oxidised to ferric form is ferrous iron, which appears as a reddish-brown stain on enamel surfaces and washed fabrics. Since boiling transforms the bicarbonate into insoluble carbonate, the hardness of water caused by calcium bicarbonate is referred to as temporary. And, the hardness from the other salts is referred to as permanent.
Ions in Hard water
The higher fatty acids of soap combine with the magnesium and calcium ions in hard water to create an insoluble gelatinous curd, resulting in soap waste. This particular objectionable reaction does not occur with modern detergents.
Hard Water in Boilers
In boilers, the magnesium and calcium in hard waters form an adherent and hard scale on the plates. As a result of the scale's poor heat conductivity, fuel consumption can be increased, and also, the boiler rapidly deteriorates through the external overheating of the plates. If present, sodium carbonate hydrolyzes to form free alkali that causes caustic embrittlement and failure of boilerplates. Also, water is softened on a small scale by the addition of borax, or trisodium phosphate, ammonia, together with the sodium carbonate (otherwise called washing soda).
The calcium in water is then precipitated as carbonate, and the magnesium is precipitated as hydroxide. And, on a wide scale, water is softened by adding only enough lime to precipitate calcium as carbonate and magnesium as hydroxide, after which the sodium carbonate can be added to extract the remaining calcium salts. Home water softeners, which use natural or artificial zeolite mineral properties, are used in places where the water is rough.
Origins
Snow, rainwater, and other sources of precipitation usually contain low levels of multivalent cations including Magnesium and Calcium. They can contain small concentrations of ions such as Chloride, Sulphate, and Sodium derived from the wind action over the sea. Whereas precipitation falls in the drainage basins that are formed of impervious, calcium-poor, and hard rocks, only very low concentrations of the multivalent cations can be found, and the water is known as Soft water. Examples in the UK include the Western Highlands in Scotland and Snowdonia in Wales.
Areas that have complex geology can produce differential degrees of the hardness of water over short distances.
Permanent Hardness
The permanent hardness of water can be determined by the concentration of multivalent cations present in the water. The positively charged metal, that complexes with a charge greater than 1+, is referred to as a multivalent cation. In general, the cations hold a charge of 2+. Common cations found in the hard water include Mg2+ and Ca2+. These particular ions enter a water supply by leaching from the minerals within an aquifer. Gypsum and Calcite are given as the common calcium-containing minerals. A common magnesium mineral is given as dolomite (which also holds calcium). Distilled water and rainwater are soft because they have fewer ions.
The below equilibrium reaction describes the formation and dissolving of calcium carbonate and calcium bicarbonate:
CaCO3 (s) + CO2 (aq) + H2O (l) ⇌ Ca2+ (aq) + 2 HCO3 (aq)
The reaction can go in either of the directions. Rain that contains dissolved carbon dioxide can react with the calcium carbonate compound and carries the calcium ions away with it. The calcium carbonate can be re-deposited as Calcite as the CO2 is lost to the atmosphere, sometimes forming stalagmites and stalactites.
Effects
With the hard water, solutions of soap form a white precipitate (otherwise called soap scum) instead of producing the lather, due to the reason the 2+ ions destroy the surfactant properties of the soap by producing a solid precipitate (which is the soap scum). Calcium stearate is a primary component of such scum that arises from sodium stearate, the major component of soap:
2C₁₇H₃₅COO⁻(aq) + Ca²⁺(aq) ➝ (C₁₇H₃₅COO)₂Ca(s)
As a result, hardness can be defined as a water sample's soap-consuming capacity or soap precipitation capacity as a water property that prevents soap from lathering. At the same time, synthetic detergents do not form that kind of scum.
Since soft water contains a few calcium ions, the soaps' lathering action is not inhibited, and no soap scum can form during normal washing. In the same way, soft water produces zero calcium deposits in water heating systems.
Softening
Often, it is more desirable to soften the hard water. Most of the detergents contain ingredients that counteract the hard water effects on the surfactants. For this specific reason, softening of water is much often unnecessary. Where softening of water is practised, often it is recommended in hard water softener that only the water that is sent to domestic hot water systems so as to either prevent or delay damage and inefficiencies due to the scale formation in water heaters. A common method for water softening includes the ion-exchange resin use, which replaces ions such as Ca2+ with twice the number of mono cations such as potassium or sodium ions.
FAQs on Hard Water: What It Is, Why It Matters, and How to Treat It
1. What is hard water from a chemical standpoint?
From a chemical perspective, hard water is water that contains a high concentration of dissolved mineral ions. Primarily, these are positively charged calcium (Ca²⁺) and magnesium (Mg²⁺) ions, which enter the water as it percolates through deposits of limestone, chalk, or gypsum.
2. What is the core difference between temporary and permanent hardness in water?
The core difference lies in the types of salts dissolved in the water.
- Temporary hardness is caused by the presence of soluble bicarbonates of magnesium and calcium. It can be removed by simple methods like boiling.
- Permanent hardness is caused by the presence of chlorides and sulphates of magnesium and calcium. It cannot be removed by boiling and requires chemical treatment methods.
3. Why does soap fail to produce a good lather in hard water?
Soap, which is a sodium salt of a long-chain fatty acid (like sodium stearate), reacts with the calcium and magnesium ions in hard water. This reaction forms an insoluble, sticky precipitate known as scum (calcium or magnesium stearate). Lather is only produced after all the hardness-causing ions have been precipitated out as scum.
4. How can the temporary hardness of water be removed by boiling?
Boiling removes temporary hardness by decomposing the soluble bicarbonate salts into insoluble carbonates. For example, calcium bicarbonate (Ca(HCO₃)₂) decomposes upon heating to form insoluble calcium carbonate (CaCO₃), which precipitates out of the water, making it softer. The same principle applies to magnesium bicarbonate.
5. What are the main disadvantages of using hard water in industrial boilers?
In industrial settings, particularly in boilers, hard water is highly problematic. The continuous heating of hard water leads to the deposition of insoluble salts (like calcium carbonate and calcium sulphate) on the inner surfaces, forming a thick layer called 'boiler scale'. This scale is a poor conductor of heat, reducing the boiler's efficiency, increasing fuel consumption, and potentially causing the boiler to overheat and explode.
6. Explain the principle behind Clark's method for water softening.
Clark's method is used to remove temporary hardness. It involves adding a calculated amount of slaked lime (calcium hydroxide, Ca(OH)₂) to the hard water. The slaked lime reacts with the soluble bicarbonates to form insoluble calcium carbonate and magnesium hydroxide, which precipitate out and can be filtered off.
7. How does the ion-exchange method soften permanent hard water?
The ion-exchange method uses a material, often a synthetic resin or natural zeolite, that contains mobile sodium ions (Na⁺). When hard water passes through this material, the resin captures the hardness-causing Ca²⁺ and Mg²⁺ ions and releases its own Na⁺ ions into the water in exchange. Since sodium salts do not cause hardness, the water becomes soft.
8. Is it harmful to drink hard water?
No, drinking hard water is generally not considered harmful to health. In fact, according to the World Health Organization (WHO), it can be a beneficial dietary supplement for essential minerals like calcium and magnesium. The main issues with hard water are related to taste, its effect on appliances, and its interaction with soap, not health risks.
9. How is the degree of hardness in a water sample measured and expressed?
The degree of hardness is typically measured by titrating the water sample with a solution of EDTA (ethylenediaminetetraacetic acid). The hardness is then expressed in units like parts per million (ppm) in terms of an equivalent amount of calcium carbonate (CaCO₃), as it provides a uniform standard for comparison.
