From my understanding (I might be very wrong), the "earth" in alkaline earth metals means non-metallic, insolube 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 Jan 16 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. $\endgroup$ – user3602727 Jan 16 at 1:30

"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 earth's, far exceeding anything else with similar properties in their common occurrence. They have become the namesake of the all the "alkaline earth metals" in Group 2.


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