Keywords |
• Physics,
• Geology,
• Matter

# Hardness

The term hardness is used in various fields. It is essentially used to characterise water but is also used in mineralogy and physics.

## The hardness of water

The hardness of a particular water is its concentration in calcium (Ca2+) and magnesium (Mg2+) ions. Different degrees are used in different countries; in the UK, English degrees (°e) and in the USA, ppm, also called American degrees. 1°e is equivalent to 2.80 mg of calcium ions or 1.68 mg of magnesium ions per litre. 1 ppm is equivalent to 0.4 mg of calcium ions or 0.24 mg of magnesium ions per litre.

The higher the value, the harder the water.

Soft water has a low mineral content and a high capacity to dissolve minerals including heavy metals such as the lead from old water pipes.

## Mineralogy

In mineralogy hardness is the resistance of a mineral to scratching. The Mohs scale classifies hardness in relation to ten test minerals (10: diamond, 9: corundum, 8: topaz, 7: quartz, etc.).

## Physics

Measures the ability of a material to resist deformation.

The test methods to determine hardness are:

• the Brinell test: the penetration of a ball of diameter D under a load F into the material is measured by measuring the diameter D of the indent and calculating the area S of the spherical section; then the Brinell hardness = F/S;
• the Rockwell test: the successive penetrations (e) of a 120° conical diamond tip are measured under loads of F0 (e0), F1+F0 (e1) and F0 (e2) (persistence). e3 = e2 -e0, e = e3/0.002 is calculated and HRC (Rockwell hardness) = 100 - e ;
• the Vickers test: a pyramid-shaped diamond with a square base under a load F is applied for a given time; the diagonal of the indent (d) is measured and the surface area S of the indent is calculated, and HV (Vickers hardness) = F/S;
• the Shore test: a ball of mass m and diameter D is dropped from a height H and the rebound height h is measured. The energy of the fall is absorbed by a plastic deformation (without indent) of the material.

The classification of water hardness. © Wikipedia, CC by-sa

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