Hard water contains a significant quantity of dissolved minerals, such as calcium and magnesium. In general, hard water is not harmful for health. In fact, it may confer some benefits because it is rich in minerals and reduces the solubility of potentially toxic metal ions such as lead and copper. However, there are some industrial applications where hard water may lead to inefficiencies or damage to containers and pipes. In such instances, water softening methods are used. When water is softened, metal cations are exchanged for sodium ions.

Comparison chart

Hard Water versus Soft Water comparison chart
Edit this comparison chartHard WaterSoft Water
Contains Minerals such as calcium and magnesium Sodium
Reaction with soap Film Suds
Problems Leaves deposit called “scale” None
Lather formation Doesn't form lather with detergents Forms lather with detergents
Removed by permutit process, by exchange of ions none


The most common technique to determine the hardness of water is by looking at sud formation with soap. There is less lather formation when soap is used with hard water compared to soft water. Instead a white precipitate (soap scum) is produced. Another way in which water manifests its hardness is scaling i.e. forming deposits through calcification that clog plumbing. These scales are usually white because calcium and magnesium are the most common sources of hardness in water. In swimming pools, a cloudy or milky appearance characterizes hard water.

What causes hardness in water?

A high concentration of multivalent cations (i.e. charge greater than 1+) leads to hardness in the water. Calcium and magnesium ( Ca2+ and Mg2+) are the most common sources. Water usually collects these minerals from the ground as it flows. Rainwater and distilled water are soft.

Temporary vs Permanent Hardness of Water

Hardness that cannot be removed by boiling the water is called permanent hardness. Temporary water hardness is caused by the presence of bicarbonate minerals (calcium bicarbonate and magnesium bicarbonate). Boiling causes the formation of carbonate from the bicarbonate and precipitates it out, leaving soft water upon cooling. Permanent water hardness is usually caused by chlorides or calcium and magnesium sulfates. Note that permanent hardness is not really permanent in that it can also be softened (just not by boiling).

Hard and Soft Water Classification

A single-number scale cannot describe hardness of water accurately because the behavior of hardness depends upon a variety of factors such as minerals in the water, pH and temperature. The U.S. Geological Survey uses the following ranges of measurements to classify water into hard and soft water:

Classification of Hard and Soft Water according to measurements of hardness in mg/L, mmol/L and dGH/°dH.

Effects of Hard Water

While hard water does not have an adverse effect on human health, it can leave spots and film on dishes and bathtubs and be more damaging to household appliances. It can leave deposits, called “scale,” that clog plumbing and impair the flow of heat in boilers, leading to overheating.

Limescale buildup in a PVC pipe.
Limescale buildup in a PVC pipe.
Limescale in a pipe reduces the flow of water.
Limescale in a pipe reduces the flow of water.
Limescale clogging up a showerhead
Limescale clogging up a showerhead

Drinking Water

Hard water is not considered to be dangerous to one's health, and it is perfectly healthy to drink. However, the minerals found in hard water can be detected in the taste, and so some people may find that it is slightly bitter, whereas soft water has a very pure, although occasionally very slightly salty taste. Some studies have shown that water hardness up to 170 mg/L can reduce the risk of cardiovascular disease in men but the WHO has reviewed the evidence and not found it to be conclusive and other studies have shown the correlation to be weak.[1]

The hard water residue on a cat water dish
The hard water residue on a cat water dish

Impact on hair and skin

Hair washed in hard water can feel sticky and look dull. Studies also suggest that hard water can cause an increase in eczema in children. This is because the minerals in hard water can dry out skin and hair. Hard water will cause treatments such as perms and dyes to fade faster and can cause a flaky scalp and hair breakage.

Soft water, however, can leave hair feeling oily and unclear, and hair washed in soft water has little volume.

Hard Water Treatment: How is water softened?

Hard water can be “softened” by reducing the concentration of calcium, magnesium and other minerals. Temporary hardness of water can be treated either by boiling it or by adding lime (calcium hydroxide). Permanent hardness of water can also be treated with ion-exchange resins in which hardness ions (Ca, Mg and other metallic cations) are exchanged for sodium ions. Chemicals such as (chelators) may also be used as water softeners. Citric acid is used in soap, shampoo, and laundry detergents to soften water.


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"Hard Water vs Soft Water." Diffen.com. Diffen LLC, n.d. Web. 18 Feb 2019. < >