While you’ve likely heard of hard water and soft water, you may not know what these terms mean, how to tell whether your water is hard or soft, or which type of water is better. Here’s a look at these two types of water and what you need to know about them.
The Difference Between Hard Water and Soft Water
Hard water is water with a high mineral content, while soft water has a low mineral content.
- Soft water contains less than 17 parts per million calcium and magnesium ions.
- Slightly hard water has 17 to 60 parts per million of these cations.
- Moderately hard water has 60 to 120 parts per million of the cations.
- Hard water has 120 to 180 parts per million divalent cations.
- Very hard water has more than 180 parts per million calcium and magnesium.
But, the difference has to do with the type of minerals the water contains and not just the amount. Mostly, hard water contains calcium (Ca2+) and magnesium ions (Mg2+), but other dissolved metals also contribute, including aluminum, strontium, iron, zinc, manganese, barium, and lead. These metals exist as divalent cations. “Divalent” means they have an electrical charge of 2, while “cation” means these ions have a positive charge. While the minerals that cause hard water dissolve to yield both cations and anions (ions with a negative charge), the anion does not contribute to the water’s hardness. Monovalent cations (single positive charge), such as H+ (hydrogen) and Na+ (sodium), do not contribute to water hardness.
The minerals that make water hard include carbonates and non-carbonates. While only calcium and magnesium are included, other metals play a role:
|Carbonate hardness compounds||Non-carbonate hardness compounds|
|Calcium carbonate (CaCO3)||Calcium sulfate (CaSO4)|
|Magnesium carbonate (MgCO3)||Magnesium sulfate (MgSO4)|
|Calcium bicarbonate [Mg(HCO3)2]||Calcium chloride (CaCl2)|
|Magnesium bicarbonate [Mg(HCO3)2]||Magnesium chloride (MgCl2)|
|Calcium hydroxide [Ca(OH)2]|
|Magnesium hydroxide [Mg(OH)2]|
Examples of Hard and Soft Water
You encounter both hard water and soft water in daily life.
Examples of hard water include:
- Mineral water
- Spring water
- Most well water
- Most public water supplies
- Water from a water softener
Note that rain and snow fall from the sky with few minerals, but as soon as that water soaks into the ground, it picks up minerals and becomes harder. While most public water is hard, some places have naturally soft water. This happens when the water comes from land that has hard, calcium-poor rocks.
How to Tell If Water Is Hard or Soft
There are a few ways of telling whether your water is hard or soft.
- If you use public water, read the utility’s water quality report. This report includes a value for water hardness, as well as useful information about levels of contaminants. Note that this report is general, for all customers, so your water hardness may differ slightly.
- Test the water with a water hardness kit from a home improvement store. Basically, you dip a test strip in water and compare its color with a chart that tells how hard it is.
- Fill a clean, empty bottle about a third full of water. Add a few drops of pure liquid (Castille) soap. You want real soap, without dyes or perfumes (not liquid detergent). Cap and shake the bottle. If you have soft water, expect a lot of bubbles over clear water. If you have hard water, you won’t get many bubbles and the water may appear cloudy or scummy.
- Take a shower with soap. Use real soap because detergent works well in both hard and soft water. If you have hard water, soap won’t lather particularly well, but it rinses off and leaves you feeling “clean.” On the other hand, soft water lathers soap well, but leaves a slippery feeling on your skin when you rinse it off. After you dry off, consider how your skin feels. Many people find that hard water leaves their skin feeling drier than soft water.
Pros and Cons of Hard and Soft Water
Both hard and soft water have their uses, advantages, and disadvantages.
- Most people prefer the taste of hard water over soft water.
- Hard water may confer health benefits, particularly for cardiovascular health, mainly because it contains magnesium ions. That being said, too much magnesium leads to diarrhea and kidney problems. There are also studies indicating calcium and magnesium in hard water reduces the risk of certain cancers.
- Hard water tends to be alkaline because the minerals that make it hard are bases. Optimal skin and hair pH is acidic, so hard water can cause dry, itchy skin and hair.
- Hard water may be more likely to contain contaminants, such as heavy metals.
- Soap does not dissolve or lather well in hard water. But, it’s easier to rinse off your skin. Detergent works in both hard and soft water.
- Hard water can discolor laundry and leave spots on dishes.
- Scale and other deposits form when hard water has prolonged contact with metal. This is important in industry, but less of a concern for homes. Older homes with metal pipes may experience a problem.
- Soft water tends to taste flat or bland. The exception is water from some water softeners, which tastes (and is) salty.
- Soft water doesn’t positively effect health (except that proper hydration is important). But, most people get their minerals from food anyway.
- Some water softeners add sodium or potassium ions to the water, which can pose health risks.
- Soap dissolves and lathers in soft water.
- Soft water doesn’t discolor fabric or leave any residue.
- Soft water doesn’t leave deposits on pipes and machinery.
Permanent vs Temporary Hardness
All hard water is not created equal. The chemical nature of its hardness determines the best method for softening the water.
Permanent hardness depends on the level of multivalent cations in water. In other words, these are ions with a positive charge greater than +1, such as calcium (Ca2+) and magnesium (Mg2+). The minerals that cause permanent hardness also tend to release chloride (Cl–) or sulfate (SO42-) anions. Usually, boiling does not remove the permanent hardness of water. A water softener or ion-exchange column softens this type of hard water.
Temporary hardness depends on the concentration of dissolved bicarbonate minerals in the water, such as calcium bicarbonate and magnesium bicarbonate. When these minerals dissolve the level of carbonate (CO32-) and bicarbonate (HCO3–) anions increase. Boiling water with temporary hardness often decreases its hardness. Adding lime (calcium hydroxide) is another effective softening method. The reason is that boiling or adding lime precipitates the carbonates out of solution, leaving softer water. However, this also means that boiling water with temporary softness leaves residue on kettles and pots.
- Pocock, S.J.; Shaper, A.G.; Packham, R.F. (1981). “Studies of water quality and cardiovascular disease in the United Kingdom.” Sci. Total Environ. 18: 25–34. doi:10.1016/S0048-9697(81)80047-2
- Sengupta, Pallav (August 2013). “Potential Health Impacts of Hard Water.” International Journal of Preventive Medicine. 4 (8): 866–875.
- Water School Science (October 22, 2019). “Water Properties Information by Topic.” USGS – U.S. Geological Survey Office of Water Quality.
- Weingärtner, Herman (2006). “Water: Properties, Analysis, and Hydrological Cycle.” Ullmann’s Encyclopedia of Industrial Chemistry. Weinheim: Wiley–VCH. doi:10.1002/14356007.a28_001
- Yang, C.Y.; et al. (1998). “Calcium, magnesium, and nitrate in drinking water and gastric cancer mortality.” Jpn J Cancer Res. 89:124–30. doi:10.1111/j.1349-7006.1998.tb00539.x