If you want your soap to last longer, here is what to do:
1. Don’t buy cheap soap. Cheap soap is not really soap. It is mostly detergent and water. Sometimes the detergency is given an extra boost with sodium bisulfate, which dissolves the soap. The result can be effective at removing grime, but it does so in a way that is different from soap and much less gentle.
Here’s the simplest test for whether something is really soap: if you put it in a dish with water, does it form a layer of bubbles on top? If it does, it’s soap. If not, it’s detergent.
2. Keep your bar of soap dry between uses. A bar of wet soap will dissolve more rapidly than a bar of dry soap. This means that the part of your bar that stays dry will last longer than the part of your bar that gets wet (your hands).
3. If you have hard water, use soft water to wash with; otherwise soft water will accelerate the dissolving process.
4. Use gentler cleaning methods than washing with water to clean your hands: Try using an unscented liquid hand sanitizer or rubbing alcohol, instead of plain old water!