Soap frosts, and there will always be some glycerin, salt and caustic soda in the soap. Soap will "sweat" and frost, which is a troublesome substance. Glycerol likes water very much. It will "capture" moisture from the air.

The content of glycerol in soap is much higher. When glycerin absorbs water, the surface of soap will "sweat", which will make the surface of soap sticky, affect the quality of soap and not washable. The salt content of ordinary soap is about 0.5%.

When the soap is left for a long time, the water evaporates, the salt is separated out, and a layer of frost forms on the soap. Another reason for frosting is that sodium hydroxide has undergone chemical changes: it absorbs carbon dioxide in the air, turns into sodium carbonate, and precipitates white crystals.

Extended data:

The component of soap is sodium stearate, which is ionized by water in water to form stearate ions and sodium ions. Generally, hard water contains a lot of calcium ions and magnesium ions, and stearate ions will combine with magnesium ions and calcium ions to generate magnesium stearate and calcium stearate, which are insoluble precipitates in water;

So if soap is put in hard water, it will precipitate.

There are no or trace magnesium ions and calcium ions in soft water, so if soap is put into soft water, there will be no precipitation, and the water is pure and transparent.