What Makes Water Hard?

shower glass

Water is referred to be “hard” if it contains high levels of magnesium, calcium, and other alkaline minerals in it. Scientists use the pH scale to measure hardness in water by measuring the hydrogen-ion concentration in the liquid. Low pH indicates the water is acidic while high pH means water is alkaline i.e., able to neutralize acids.

The pH scale measures substances from pH 1 – 14, pH 7 being neutral. The US Environmental Protection Agency recommends that households use water ranging from a pH of 5.5 to 9. 

Common signs of hard water are using more soap to clean hands, dishes, and laundry. Certain types of fish die in your aquarium if you supply hard water. Alkaline deposits damage plumbing systems, dishwashers, and other appliances. You also notice mineral deposits on your dishes and an insoluble scum in your bathtub. The presence of magnesium and calcium ions do not pose health threats but make water unfit for many uses unless it is softened.

Water softeners replace calcium and magnesium in hard water with sodium chloride and potassium chloride in the softening unit.

Problems with hard water


Ionic reactions lead to the formation of mineral deposits. For example, heating hard water makes calcium ions reach with bicarbonate ions to form insoluble calcium carbonate, as shown.

The precipitate formed is called scale. It coats your heating appliances hence reducing heat transfer. Food cooks slowly and unevenly. In industries, boilers get covered with scale, reducing efficiency in the heating transfer. That skyrockets the electricity bills for heating. Cooking vessels become stained as well as dishes. The scale also builds in pipes, and they must be replaced.

Some strategies to “soften” hard water

glass of water

The lime-soda treatment is used for large scale water treatment to remove magnesium and calcium ions. Water can be treated by adding slaked lime, Ca(OH)2, and soda ash, Na2CO3. The results are CaCO3 and Mg(OH)2 insoluble precipitates. 

Ca(OH)2 is slightly soluble in water. It, therefore, dissociates in water to give Ca2+    ions and OH  ions for every unit of Ca(OH)2

Magnesium ions, Mg2+ react with 2 hydroxide ions, 2 OH  to form magnesium hydroxide.

Household water softeners use ion exchange to soften water. When water containing Ca2+ and Mg2+ ions is passed through the ion exchange column, these ions get attracted to the anion groups, e.g., COO. The Na+ ions in the beads are replaced as Na+ goes into the water.

Sources of hardness minerals in drinking water

Water is a universal solvent. It picks carbon dioxide to form a weak carbonic acid. Underground water dissolves minerals in soil and rocks in its solution. Calcium and magnesium are the most common minerals that make water hard. The degree of water hardness is determined by the level of concentration of magnesium and calcium.

Indications of hard water

water molecule

  • Clothes washed in hard water eel dingy and scratchy
  • Spots on dishes and glass upon drying
  • Visible film on glass doors, bathtubs, shower walls, sinks, etc.
  • Hair feels sticky and dull after cleaning in hard water
  • Deposits in pipes cause reduced water flow
  • Using more soap and detergent to clean clothes, hands, dishes, etc.
  • Visible sticky soap curd on utensils
  • Noticeable sticky soap curd on the skin after bathing
  • Irritation on the skin because resulting from soap curd on the skin
  • Dull hair upon cleaning with hard water
  • The fabric feels stiff and rough during cleaning
  • Sour odor develops on clothes
  • Graying of fabric and loss of brightness color
  • Continuous laundering shortens the lifespan of clothes
  • Soap curds deposits on bathtubs, utensils, showers, etc.
  • Inefficient operations when using appliances because of the buildup of scale
  • Red and black stains on dishwashers

Scale deposits are the typical indication of water hardness. Calcium, magnesium, and other divalent or trivalent metallic elements are the common causes of water hardness.

Potential health effects


Drinking hard water is not a health hazard. In fact, it contributes to supplying magnesium and calcium dietary needs. However, if you notice the presence of iron and manganese, ensure that you check for arsenic levels in your water. There is still ongoing research on the relationship between water hardness and cardiovascular disease mortality. 

Some studies suggest that ward water lowers the cardiovascular disease mortality rate while others do not indicate so. The National Research states that the current results are inconclusive, and further research must be done.

Water testing

People get water supply from their municipality and some from private water sources. The municipality is responsible for testing water hardness. If yours is from a private source, you can test it yourself. Some labs offer a water hardness test at a fee. They give a broad test and result in analytics o your water. To get an estimate, you can buy the DIY test kits and strips

Interpreting test results

Water hardness is given in grains per gallon or milligrams per liter (mg/l) or parts per million. One grain of hardness is equal to 17.1 mg/l or parts per million of hardness.

The Environmental Protection Agency classifies drinking water into Primary standards and Secondary Standards. Primary Standards are based on health considerations, while Secondary Standards are based on odor, taste, color, corrosivity, foaming, and staining water properties. The US Department of Interior and Water Quality Association uses this scale.


classification Ppm or mg/l Grains/gal
soft 0 – 17.1 0 – 1
Slightly hard 17.1 – 60 1 – 3.5
Average hardness 60 – 120 3.5 – 7.0
Hard 120 – 180 7.0 – 10.5
Very hard 180+ 10.5 and over


Drops of soap – a hardness water test

Drops of soap in 12 oz. Hardness – CaCo3/L Grains per gallon Classification
10 0 – 17.1 0 – 1 Sot
20 17.1 – 60 1 – 3.5 Slightly hard
30 60 – 120 3.5 – 7.0 Moderate hard
40 120 – 180 7.0 – 10.5 Hard
50 180 and over 10.5 and over Very hard




You can use a packaged water softener or the mechanical water softening unit. Packaged water softeners use chemicals and are classified as precipitating and non-precipitating.

Precipitating water softeners use borax and washing soda. They form insoluble precipitates with calcium and magnesium ions. The water becomes cloudy and builds upon surfaces. It, however, increases water alkalinity that causes damage to the skin and other materials.

Non-precipitating water softeners use complex phosphates to counter magnesium and calcium ions. They don’t precipitate of increase alkalinity. They dissolve curd for a limited period.

The mechanical water softening units are connected to your central plumbing system to filter magnesium and calcium continuously. It uses the ion exchange process. It has a chamber with supersaturated beads of sodium. When hard water is passed through, hardness minerals get attached to the resin beads, and sodium from resin is gradually released into the water. Recharging must be done by flushing brine through it. The ion exchange column adds sodium at the rate of 8mg/liter or every hardness grain that is removed

Some people are barred from drinking soft water because of the presence of sodium. It is also not good or watering plants.

Potassium chloride is sometimes used as an alternative to creating the brine. Potassium is used in the place o sodium for hardness mineral exchange. 

Test water hardness and iron content before selecting a mechanical water softener. When choosing a water softener, consider its capacity to remove water hardness, the regeneration control system, and iron limitations.  

There are three types of regeneration systems. These are the time-clock control, water meter control, and the hardness sensor control.

There are small capacity softeners and large capacity softeners. Ensure that you identify your daily water supply requirement to help you determine the best softener to use. Multiply your household water needs and the water hardness (grains/gallon).

Water treatment improves hardness issues

You need a water softening system for the whole house or building supply, not just a faucet. Use the following methods to soften your water for wholeness supply:

  • Cation exchange softening
  • Membrane separation filtration
  • Chemical softening


Hard water is not a threat to human health. You can approximate the hardness of water using a home water testing kit. Advanced and more accurate results can be obtained using a certified laboratory test. You can manage hardness in your water using the mechanical ion exchange softening unit and the packaged water softeners. You can also look out for sodium-free softeners to reduce moderate hardness only. 

sodium-free softeners