From my understanding (I might be very wrong), the "earth" in alkaline earth metals means non-metallic, insoluble in water, and resistant to heating, therefore the oxides and hydroxides of group 2 metals should be insoluble in water by definition of "earth".

However, doesn't the term "alkaline" by definition mean bases that are soluble in water? How can the two definitions be co-applicable?

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    $\begingroup$ Earth means just a soil. $\endgroup$
    – Mithoron
    Commented Jan 16, 2019 at 1:26
  • $\begingroup$ ok thank you, i did some research and about three sites said earth meant insoluble in water so i was sort of confused. $\endgroup$ Commented Jan 16, 2019 at 1:30
  • $\begingroup$ Alkaline simply means a pH above 7. The term for bases that are soluble in water is alkali, in which the alkali metals are appropriately named because the hydroxides that they make are soluble in water ,for example sodium hydroxide. $\endgroup$ Commented Sep 6, 2022 at 21:54
  • $\begingroup$ This may give you some context: chemistry.stackexchange.com/questions/66546/… $\endgroup$ Commented Sep 7, 2022 at 3:11

3 Answers 3


"Insoluble (in water)" is a relative term, especially for ionic compounds, and so is "alkaline". That relativeness gives us enough wiggle room for two of our most common bases to slip through.

Roughly speaking, it takes a thousand volumes of water to react with and then dissolve one volume of calcium oxide, so the solubility of lime in water would not be evident from casual observation. Yet if you put an indicator into the water it reacts in a way that indicates the water has turned alkaline. The solubility of lime in water is hard to see with the eye, but it overwhelms the tiny intrinsic autoionization of water, so lime gives evidence of being both an "earth" and "alkaline". Ditto (despite the still lower solubility) for magnesia.

Lime and magnesia, then, are the quintessential alkaline earths, far exceeding anything else with similar properties in their common occurrence. They have become the namesake of all the "alkaline earth metals" in Group 2.

  • $\begingroup$ Your name identifies you but does not define you or totally describe you. In a like manner the names of chemicals of groups of chemicals identify them and in some cases do specify some general properties because of the periodicity in electronic structure. The common names of chemicals arose from the familiar elements and in many cases were extended to similar elements when they were discovered. The origin of names is fascinating in its own right but let's treat each element as a unique entity as it deserves to be. $\endgroup$
    – jimchmst
    Commented Jan 3, 2022 at 0:33

This definition has a historical origin, as these names have been given in the $18$th and $19$th century. In the $18$th and $19$th century, oxides and hydroxides were not always separated. Hydroxides were often considered as hydrated oxides, like salts were known sometimes as anhydrous and sometimes hydrated. Metallic oxides and hydroxides were not considered really different.

In these old times, the oxides (or hydroxides) were known and studied before the elements were discovered. So first alkaline oxides (or hydroxides) were found to be very soluble and strongly alkaline. Then earths were similar to alumina (aluminum oxide) which is insoluble in water and neither basic nor acidic. But aluminum oxide (or hydroxide) neutralizes acids somewhat like an alkaline hydroxide. So there is a common character to alkaline hydroxides and earths : the reaction with acids, making salts.

Well, now, chalk, magnesia and baryta (oxides or hydroxides) are half way between alkaline hydroxides and earths : they are a little soluble, and this behavior is between soluble alkaline hydroxide and insoluble earths. Furthermore, chalk, magnesia and baryta react with acids like alkaline hydroxide and earths. So these oxides and hydroxides are intermediate between alkaline compounds and earths. This is why they are called "alkaline earths". Later on, the same name have been given to the metal, when the corresponding metal has been produced, usually by electrolysis.


An alkaline substance doesn't always have to dissolve in water(or at least very well), for example, copper(II) hydroxide, as shown in Wikipedia is negligible.

But then there are bases like sodium hydroxide and potassium hydroxide which are quite soluble.

Wikipedia says:

An alkali can also be defined as a base that dissolves in water. Alkalis are normally water-soluble, although some like barium carbonate are only soluble when reacting with an acidic aqueous solution.

More information below:

Sodium Hydroxide

Copper (II) Hydroxide



  • $\begingroup$ Copper hydroxyde is not an alkaline substance $\endgroup$
    – Maurice
    Commented Sep 7, 2022 at 5:57
  • $\begingroup$ It is considered a weak base. Also it's hydroxide. $\endgroup$ Commented Sep 7, 2022 at 18:00
  • $\begingroup$ Yes. Copper hydroxide is a weak base and a hydroxide. But it is not an alkali, even if Wikipedia states it. And barium carbonate is NOT an alkali, despite Wikipedia. $\endgroup$
    – Maurice
    Commented Sep 7, 2022 at 20:21

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