The middle equation in Figure 1-72 describes the reaction that is most closely related to the one used to make phosphoric acid on a large industrial scale (Volume 3). Agitating Ca3
leads to the formation of H3
and insoluble CaSO4
that can be separated by filtration.
The bottom equation in Figure 1-72 represents a special case. The silicic acid (H2
) produced in this reaction is a solid that is insoluble in water, yet it does not precipitate out in the conventional sense and cannot be isolated by filtration. The particles of the silicic acid formed in this reaction are tiny, forming a sol
, a colloidal solution
that looks like an opaque (cloudy) liquid (Figure 1-73). Such very small particles, traditionally called colloids
, have recently received a new fancy name "nanoparticles
". Colloids are very interesting and important substances that are studied in a special branch of chemistry, called Colloidal Chemistry.