What is the difference (if any) between a mineral salt, a dissolved mineral, and a mineral ion?

A salt, in chemistry, refers to any ionic compound (IUPAC, 1990). For example, table salt, sodium chloride, is an ionic compound composed of sodium (Na+) and chloride (Cl–) ions. A mineral refers to chemical compounds formed naturally on rocks or the Earth, not produced by life processes. Thus, mineral salts are naturally occurring ionic compounds, and they include table salt as well as many other salts found or used in brewing water, such as magnesium sulfate (Epsom salts).

A dissolved mineral refers to the mineral substance dissolved in water. When a salt is dissolved in water, it dissociates — its positive and negative ions separate completely. Sodium chloride dissolved in water no longer exists as a compound; it instead exists as individual sodium and chloride ions dissolved in water. A mineral ion refers to any one of these ions.

Are all mineral salts also compounds?

A compound is any substance formed of two or more chemical elements chemically bonded together. In solid mineral salts, the elements involved are bonded together with ionic bonds, a type of chemical bond, so these are considered compounds. When salts dissolve in water, the ionic bonds break and the ions dissociate, so the chemical elements are no longer considered a compound.

Table salts dissociate in water. Is that also true for mineral salts?

Yes, all salts dissociate completely when dissolved in water. Table salt is one example of a mineral salt.

Some salts are more soluble than others, however, so a less-soluble salt (such as calcium carbonate) might only partially dissolve. In this case, some of the ions will dissolve in water and fully dissociate, but some will remain a solid. Small particles of solid calcium carbonate may spread throughout the water and create a suspension, making the liquid cloudy. Or the remaining solid calcium carbonate may get deposited on surfaces, forming limescale.

Are mineral ions attached to individual H₂O molecules, or do they float freely between H₂O molecules?

When dissolved in water, ions are bonded to several water molecules by hydrogen bonds. Hydrogen bonds are fairly weak, so they easily break and reform. This allows the ion to move through the water, bonding to different water molecules as it goes.

How do ‘multivalent’ ions, such as calcium and magnesium ions, form bonds different from ordinary ionic bonds?

Multivalent ions have more than one charge — for example, a calcium atom loses two electrons to become an ion, and so it has two positive charges (written Ca²+). Multivalent ions form ionic bonds in the exact same way that monovalent ions such as sodium (Na+) do, except that each multivalent ion needs two negative charges to balance it out. Thus, in calcium chloride (CaCl₂), each calcium ion bonds to two chloride ions, as each chloride ion has only a single negative charge (Cl–).

When water dissolves magnesium and calcium from bedrock, do those elements enter the water as simply Mg2+ and Ca2+, or do they immediately make a bond with some other element?

Magnesium and calcium occur in the bedrock only as mineral salts — for example, as calcium carbonate and magnesium sulfate. In the mineral salts, these elements are present as cations (Mg2+ and Ca2+) bonded to different anions. in this case, those ions are chloride (Cl–) and sulfate (SO 4 2-). When water passes through the bedrock, some of these salts dissolve in the water and the ions dissociate. Both the calcium/magnesium cations and the respective anions dissolve in the water.

Is the ‘2’ in Mg²+ different from the ‘2’ in H₂O?

Yes. In chemistry, numbers written in subscript (e.g., in the abbreviation H₂O) tell you what number of atoms of that element are present in a compound. So, in the case of water, the ‘2’ tells you H₂O contains two hydrogen atoms and one oxygen atom.

Numbers written in superscript (e.g., in the abbreviation Mg2+) tell you the number of charges an ion has. Magnesium loses two electrons to form an ion, so it gains two positive charges. Chloride picks up one electron to become an ion, so it gains one negative charge (C–). Because the charges must balance in a salt, magnesium chloride consists of two chloride ions for every magnesium ion, so is written as MgCl₂.

Does permanent hardness have any potential to form scale, e.g., in the case of espresso machine steam boilers?

Limescale is mainly composed of calcium carbonate, which is formed by dissolved calcium bicarbonate that breaks down when water is boiled. When this happens, the insoluble calcium carbonate precipitates out of solution and forms a solid deposit called limescale.

Because boiling the water removes the calcium bicarbonate from the water, it is known as temporary hardness.

Permanent hardness refers to other calcium and magnesium salts dissolved in water that are not removed by boiling. Because these do not precipitate when the water is boiled, they do not contribute significantly to limescale.

What does the term ‘binding energy’ mean? Does it mean dissolving power?

Binding energy refers to the strength of a chemical bond; a high binding energy forms a bond that is hard to break. In his paper on the effect of cations on extraction (Hendon et al., 2014), chemist Christopher Hendon refers to how the dissolved metal ions Ca2+ and Mg2+ bind to flavour compounds in coffee. Because these ions are dissolved in water, binding strongly to the flavour compounds helps to pull them into the water ₂— the strong bonds that are formed with the dissolved ions help to break the weaker bonds attaching the flavour compounds to the coffee bean surface.

Is there a difference between general hardness and total hardness?

No, the terms and concepts are synonymous.

Hardness refers to the concentration of multivalent metal ions in water, usually expressed as calcium equivalents. In other words, it refers to the parts per million for this number of ions if you assumed they were all calcium. For a solution containing equal parts calcium and magnesium chloride, the TDS and GH measurements would differ slightly because the GH measurement would treat all the cations as if they were calcium, whereas the TDS should account for the different sizes of the magnesium and calcium ions.

Many other solids typically dissolved in water, such as sodium chloride, do not contribute to hardness — they would contribute to TDS but not to GH. As a result, in drinking water, the TDS measurement is typically quite different from the GH measurement.

I understand that carbonate hardness (KH) is not really a measurement of mineral content but, instead, the measurement of the buffering potential created by the negatively charged ions. Does that mean I can subtract my KH values from my GH values to estimate my water’s permanent hardness?

KH (carbonate hardness or temporary hardness) is a measure of the carbonate and bicarbonate ions in water. These ions are removed when water boils, forming calcium carbonate deposits (limescale). The remaining hardness is called permanent hardness. Permanent hardness and temporary hardness, added together, are the total hardness or general hardness (GH). Thus, the difference between total hardness (GH) and temporary hardness (KH) is exactly your permanent hardness.

How does the H+ cation bond with H₂O? Does it form its own ionic bond with the negative side of the oxygen part of the water molecule?

In water, hydrogen and oxygen atoms form covalent bonds. In this type of bonding, the hydrogen and oxygen atoms share two electrons, instead of one atom losing an electron and one atom gaining one. When an acid is dissolved in water, the hydrogen ions released from the acid bond covalently to water molecules to form H₃O+ ions.

In the H₂O molecule, each hydrogen atom contributes one electron to the bond; the other comes from the oxygen atom. In the H₃O+ ion, both electrons in the bond come from the oxygen atom, but they are shared between the oxygen and the hydrogen atoms, forming a coordinate covalent bond.

References

CH Hendon, L Colonna-Dashwood, and M Colonna-Dashwood. 2004. “The Role of Dissolved Cations in Coffee Extraction,” Journal of Agricultural and Food Chemistry. doi: 10.1021/jf501687c (https://pubs.acs.org/doi/pdf/10.1021/jf501687c)

International Union of Pure and Applied Chemistry (IUPAC), 1990. IUPAC Nomenclature of Inorganic Chemistry, 3d ed. (the Red Book), p.118. Royal Society of Chemistry. ISBN 0-632-02494-1