Water is considered “hard” when it contains a lot of dissolved mineral ions, mainly calcium (Ca²⁺) and magnesium (Mg²⁺).

What actually makes water hard?

  • The hardness of water is essentially the concentration of dissolved calcium and magnesium ions in it.
  • These ions usually come from rocks like limestone, chalk, and gypsum that the water has flowed through underground.
  • Other metal ions (such as iron, manganese, strontium, zinc, etc.) can contribute a bit, but calcium and magnesium are the main players.

In short, the more calcium and magnesium your water has picked up from surrounding rocks and soil, the “harder” it is.

How does water become hard?

  • Rainwater starts out soft (very low in minerals).
  • As it soaks into the ground and moves through soil and rock, it dissolves minerals like calcium carbonate from limestone, chalk, or gypsum.
  • This dissolution releases calcium and magnesium ions into the water, raising its hardness level.

Regions built on limestone or similar rocks are especially prone to hard water because those rocks are rich in calcium compounds.

Temporary vs. permanent hardness

Hardness can come in slightly different “flavors” depending on the form of the dissolved minerals.

  • Temporary hardness
    • Caused mainly by calcium and magnesium bicarbonates.
* When you boil this water, bicarbonates decompose and form insoluble calcium carbonate, which can then precipitate out, so hardness is partly removed.
  • Permanent hardness
    • Caused by calcium and magnesium sulfates and chlorides.
* Boiling does not remove these, so more advanced treatment (like ion exchange or special softening methods) is needed.

Why hard water leaves scale and feels “weird”

  • When hard water is heated (in kettles, boilers, coffee makers, water heaters), minerals like calcium carbonate come out of solution and form limescale deposits on surfaces.
  • These scale deposits can clog pipes, coat heating elements, and reduce efficiency in household and industrial equipment.
  • Hard water also reacts with soap, forming insoluble scum and making it harder to get a good lather, which is why your skin or hair can feel less “slippery clean.”

Is hard water bad?

  • For most healthy people, drinking hard water is not harmful and may even provide a small extra source of calcium and magnesium.
  • The real issue is practical: scale buildup, poor soap performance, and shorter lifespans for appliances and plumbing.

TL;DR:
What makes water hard is mainly the amount of dissolved calcium and magnesium ions it picks up while flowing through mineral-rich rocks like limestone, chalk, or gypsum, which then cause limescale, poor lathering with soap, and equipment issues when the water is heated.

Information gathered from public forums or data available on the internet and portrayed here.