Hard Water
Definition and meaning of Hard Water in chemistry.
Hard water is water that contains high levels of dissolved calcium and magnesium ions, written Ca2+ and Mg2+. These ions usually enter groundwater as it flows through limestone, chalk, or dolomite rock. Hard water is not dangerous to drink, but it causes practical problems around the home.
In more detail
Calcium and magnesium ions are called divalent cations because each carries a plus-two charge. When hard water meets soap, these ions react with the fatty acid molecules in soap. This forms insoluble salts, commonly known as soap scum.
This scum reduces lathering and wastes soap, since some of it gets tied up in solid deposits instead of cleaning. Heating hard water causes a second problem. Dissolved calcium bicarbonate breaks down when heated, releasing carbon dioxide gas and leaving behind solid calcium carbonate.
This solid builds up as scale inside pipes, kettles, water heaters, and boilers, reducing their efficiency and eventually clogging them. Water hardness is usually reported as an equivalent concentration of calcium carbonate. Magnesium salts contribute too, but this convention gives a single standardized number for comparison.
Households and industries reduce hardness using ion-exchange water softeners. These devices pass hard water through a resin that swaps out calcium and magnesium ions for sodium ions. Sodium ions do not cause scale or scum.
A common misconception is that hard water is unsafe or unhealthy. In reality, hard water is generally safe to drink. It can even provide a small amount of dietary calcium and magnesium. Its downsides are mostly practical, involving cleaning efficiency and appliance maintenance rather than health.
Key facts
| Field | General Chemistry |
|---|---|
| Main ions | Ca2+ and Mg2+, sometimes Fe2+ |
| Hardness threshold | 121-180 mg/L as CaCO3 is classified hard |
| Very hard threshold | Above 180 mg/L as CaCO3 |
| Common softening method | Ion exchange with Na+ resin |
| Common effects | Soap scum and mineral scale buildup |
A groundwater supply with 150 mg/L of calcium and magnesium, expressed as calcium carbonate, falls into the hard range. The standard USGS hardness scale sets that range at 121 to 180 mg/L. In this water, soap lathers poorly and scale forms readily when the water is heated.
Frequently asked questions
What is the difference between temporary and permanent hardness?
Temporary hardness comes from dissolved bicarbonates and is removed by boiling, which drives off carbon dioxide and precipitates calcium carbonate. Permanent hardness comes from calcium and magnesium sulfates or chlorides and remains after boiling.
Is hard water unsafe to drink?
No. Hard water is generally safe and can even add small amounts of dietary calcium and magnesium. Its main drawbacks are reduced soap efficiency and scale buildup in plumbing.
How does a water softener work?
A water softener passes hard water through a resin that swaps calcium and magnesium ions for sodium ions. This prevents scale and soap scum without harming the water's ability to dissolve soap properly.