CaO and pH
Is calcium oxide (CaO), as opposed to slaked lime
(Ca(OH)2), an alkali in the true sense of the word?
I'd say CaO is a "true alkali".
The mere addition of water is considered a given, kind of a freebie.
Alkalinity or acidity are often meaningless and un-tested until water is
Certainly Na2O is little different in effects from the 2 NaOH it turns into.
Sister compound MgO is somewhat stable, often used as insulating powder in
and metal-sheathed thermocouples.
So it must not convert to Mg(OH)2 very fast, and may not be very alkaline.
Only 6mg/Liter dissolves in cold water.
You could almost think of it as a soft ceramic powder or insoluble metal
(Adding acid would still dissolve it in a hurry.)
Comparing the pH of Mg(OH)2 and Ca(OH)2,
Mg gives only pH=10.4, while Ca gives pH=12.4, a much stronger base.
Likewise I think CaO will convert to Ca(OH)2 faster when water is added.
CaO dissolves 200x as much in water: 1.3 gm/Liter.
So I'd tend to draw the imaginary border between these two.
By the way, what defines your sense of "true alkali"?
Only comparing the facts with your specific criterion can answer this kind
Both can be antacids (except the CaO is too alkaline for internal contact
before it gets some acid).
Both are high-melting: CaO ~2600, MgO ~2800 C.
Calcium oxide (CaO) reacts readily with water to form (Ca(OH)2) so it is a
ready precursor of an alkali -- if you define it as a source of hydroxyl
ion (OH[-1]) -- which is the common defintion. In this regard it is
similar to NaO that reacts readily with water to form NaOH: The reaction
being: Na2O + H2O ---> 2 Na(OH). So it becomes a matter of choice whether
one wants to consider the oxides "true" alkaline materials. In general, I
would say yes.
As Linus Pauling, for one, states:
The elements of Group I are Alkali Metals - lithium, sodium, potassium,
rubidium and cesium - and are soft, silvery-white metals with great chemical
The elements of Group II are Alkaline-earth metals - beryllium, magnesium,
calcium, strontium, barium and radium. These metals are much harder and
less reacitve than the alkali metals. The early chemists gave the name
"earth" to many nonmetallic substances. Magnesium oxide and calcium oxide
were found to have an alkaline reaction, and hence were called the alkaline
So, I would say no, CaO is alkaline-earth and not an alkali.
Hope this helps.
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Update: June 2012