Water turns into ice: this situation is called supercooling. The condition of water freezing is not only that the temperature is low enough, but also that there are condensation nuclei. Pure water easily forms supercooled water without vibration.

[Preparation and Operation]

60g sodium acetate crystal (NAAC·3H2O) was put into a 250ml beaker and injected with 40ml distilled water.

Next, heat it in a water bath until the crystal is completely dissolved, then continue for a few minutes, take out the beaker with the solution, let it stand and cool it to room temperature.

Gently rub the cup wall or stir the solution with a glass rod or put sodium acetate crystal into the solution as a "seed crystal". Suddenly, the needle-like crystals will precipitate in the solution and quickly spread all over the bottom of the beaker, as if forming an "ice cube".

Principle: This is an experiment about the instability of supersaturated solution. When the hot saturated solution of sodium acetate is cooled undisturbed, crystallization often does not occur. This kind of solution is called supersaturated solution, and it is a metastable system (unstable system, but it can still exist). When the solution is stirred or the "seed crystal" of solute is added, the crystals of excess solute can be precipitated.

[Notes]

During the cooling process, the solution must not shake or stick with dust, and sodium acetate should be recycled.

If sodium nitrate and sodium sulfate crystals (NaSO4 10H2O) are used, the same experiment can be carried out. When sodium thiosulfate (commonly known as hypo: na2s2o3 5h2o) is heated directly and carefully, the experimental phenomenon is more obvious when its crystal water is dissolved into supersaturated solution.