Hardness
The combined concentration of dissolved calcium and magnesium. Hardness is the single water property most people notice: it determines whether soap lathers, whether kettles scale, and whether the water tastes flat or full.
Key facts
- Unit
- mg/l as CaCO₃
- Metric
- Hardness
What it measures
The sum of divalent cations, overwhelmingly calcium and magnesium, conventionally expressed as the equivalent concentration of calcium carbonate. It is a composite measure, not a substance in its own right.
How it is measured
By titration with EDTA, or calculated from separately measured calcium and magnesium. The conversion is total hardness as CaCO₃ = 2.497 × calcium + 4.118 × magnesium, both in milligrams per litre.
How to read a value
| Value (mg/l as CaCO₃) | What it indicates |
|---|---|
| 0 – 50 | Soft. Lathers readily, no scaling, but more aggressive toward metal plumbing. |
| 50 – 100 | Moderately soft. |
| 100 – 150 | Slightly hard. Some scaling in kettles over time. |
| 150 – 200 | Moderately hard. Noticeable scaling; higher soap and detergent use. |
| 200 – 300 | Hard. Significant scale on heating elements and in kettles. |
| Above 300 | Very hard. Typical of groundwater from the chalk aquifers of south east England. |
Bands are conventional interpretation aids, not regulatory limits. Where a legal standard exists it is stated on the relevant standards page.
What causes it
- Water percolating through limestone, chalk and dolomite dissolves calcium and magnesium carbonate
- Gypsum-bearing strata contribute calcium sulphate, which produces permanent hardness
- Granite, sandstone and peat catchments yield soft water because there is little to dissolve
Why it matters
- Temporary hardness, associated with bicarbonate, precipitates as scale when heated; permanent hardness, associated with sulphate and chloride, does not
- Hardness has no adverse health effect. Epidemiological work has looked repeatedly for a protective cardiovascular association with hard water, and the evidence remains inconclusive
- Softening by ion exchange replaces calcium and magnesium with sodium, which raises the sodium content of the water
- Scale costs energy: a layer of limescale on a heating element measurably reduces its efficiency
Related intelligence
Sources
- World Health Organization — Guidelines for Drinking-water Quality. Public but restricted · CC BY-NC-SA 3.0 IGO
- National Institute of Standards and Technology — NIST Chemistry WebBook — thermophysical properties of water. Open · US Government work — public domain