[ChrisW]

Hi, first time here and thanks for the welcome!
oh and, forgive me if this is posted in the wrong place

Questions following some experiments:

A Potassium hydroxide solution (or even a Sodium hydroxide solution) dissolved in cold water 50:50 concentration
mixed with...
a Calcium sulphate Hemihydrate (CaSO4.0.5H2O) powder results in nearly an instantaneous solid set mass.

Whereas;
a Calcium sulphate Hemihydrate (CaSO4.0.5H2O) powder mixed with plain cold water sets in around 10-15 minutes (Plaster of Paris, stucco etc.) at ambient temps.

I understand that the base reacts with the acid, but is it the highly exothermic reaction that accelerates the setting process?
Also does crystallisation still take place (fully, partly or at all) or is it simply ionic bonding?

Thanks in advance.
Chris

[AWK]

Both calcium sulfate CaSO₄ and calcium hydroxide Ca(OH)₂ solubility products are of the same order but solubility of calcium hydroxide is about 30 times greater. Hence precipitation in this equilibrium reaction (CaSO₄ + 2 NaOH) = Ca(OH)₂ + Na₂SO₄)you can achieve only because of the common ion effect caused by a high concentration of OH^- ions.

I understand that the base reacts with the acid

show acid and base in this reaction!

[ChrisW]

The diatomic anion OH- is usually derived from the dissociation of a base?

A salt and water is formed from the reaction between an acid and a base - neutralization reaction.
acid+ + base- --> salt + water

A positive ion from the base forms a salt with a negative ion from the acid.
Two moles of sodium hydroxide (NaOH) (base) combines with one mole of sulfuric acvid (H2SO4) to form two moles of water and one mole of sodium sulfate for example.

    2 NaOH + H2SO4 --> 2 H2O + Na2SO4

This reaction may be restricted to aqueous solutions, but given that the CaSO4.0.5H2O (a salt of sulfuric acid) powder is mixed with an hydroxide solution would qualify as an acid base reaction?

Chris