1092. In his Lectures on the Elements of Chemistry, Joseph Black discussed his difficulty in understanding latent heat. He performed an experiment where water in a tube was brought below freezing without a phase change. The water remained in this equilibrium as long as the tube of water was not disturbed. When it was disturbed, the water instantly turned to ice, releasing enough heat to raise the temperature of the ice to 0° C. Please explain why the system remained in equilibrium until it was acted upon by some external motion. — EDH, Annapolis, MD
The water in Black's tube was in an unstable equilibrium state known as supercooled water. Supercooled water tends to spontaneously convert into ice. When part of this supercooled water does convert to ice, it releases enough latent heat energy to raise its temperature and that of the remaining water to 0° C, thereby terminating the phase transition before all of the water has become ice.
But in the experiment you describe, the supercooled water was having trouble nucleating the initial seed ice crystal on which the remaining water could crystallize. Given enough time, that water would have spontaneously formed a seed crystal and the growth of the ice crystal would have proceeded rapidly after that. However, Black accelerated the formation of the seed crystal by shaking the tube. A defect at the surface of the tube or a piece of dust then acted as the trigger and helped the seed ice crystal form. The water then crystallized rapidly around this seed crystal. After the ice had formed, the water was truly in equilibrium.