Water becomes hard when it dissolves minerals, primarily calcium and magnesium, as it travels through soil and rock. The presence of these dissolved mineral ions, especially calcium (Ca²⁺) and magnesium (Mg²⁺), is what defines hard water.
The Role of Calcium in Water Hardness
As stated in the reference, as water moves through soil and rock, it dissolves very small amounts of minerals and holds them in solution. Dissolved calcium and magnesium are the two most common minerals that make water hard.
Specifically, calcium ions (Ca²⁺) are picked up by water when it flows over or through deposits like limestone (calcium carbonate, CaCO₃) and gypsum (calcium sulfate, CaSO₄). These calcium compounds are relatively soluble in water, particularly when the water contains dissolved carbon dioxide (forming carbonic acid), which enhances their dissolution.
How Mineral Content Affects Hardness
The reference clearly states: The degree of hardness becomes greater as the calcium and magnesium content increases. This means the more calcium ions (and magnesium ions) dissolved in the water, the harder the water will be. Hardness is typically measured in units like milligrams per liter (mg/L) of calcium carbonate or grains per gallon (gpg).
Minerals Causing Hardness
While calcium is the primary contributor, it's important to remember its partner:
- Calcium (Ca²⁺): The most significant contributor, often originating from limestone and gypsum.
- Magnesium (Mg²⁺): The second most common contributor, often found alongside calcium deposits.
Here's a simple overview:
| Mineral | Chemical Symbol | Common Sources | Impact on Water |
|---|---|---|---|
| Calcium | Ca²⁺ | Limestone, Gypsum | Increases Hardness |
| Magnesium | Mg²⁺ | Dolomite, Magnesite | Increases Hardness |
Practical Effects of Hard Water
The presence of dissolved calcium (and magnesium) ions leads to several noticeable effects:
- Soap Scum: Calcium ions react with soap molecules to form an insoluble precipitate, commonly known as soap scum. This reduces the effectiveness of soap.
- Scale Buildup: When hard water is heated, calcium carbonate (CaCO₃) can precipitate out and form a hard, white deposit called scale or limescale on surfaces like pipes, kettles, and water heaters. This can reduce efficiency and lifespan of appliances.
- Difficulty Lathering: Soap doesn't lather well in hard water because calcium ions tie up the soap molecules needed for lathering.
In summary, calcium makes water hard by being dissolved into it as the water interacts with rocks and soil. The concentration of these dissolved calcium ions (along with magnesium) directly determines the level of water hardness.
