The element in soap which makes it soft on skin is Potassium.

Soap is a cleansing agent that is used for various purposes, including personal hygiene. It is made through a process called saponification, which involves the reaction between fats or oils and an alkali. The alkali used in soap-making can be either sodium hydroxide (NaOH) or potassium hydroxide (KOH). Both sodium and potassium hydroxides are strong bases that contribute to the formation of soap molecules.

1. The role of alkali in soap-making:
- Alkalis are necessary for the saponification process, which converts fats or oils into soap.
- During saponification, the alkali reacts with the fatty acids present in the fats or oils, resulting in the formation of soap molecules.
- The alkali breaks down the ester bonds in the fats or oils, releasing the fatty acids and glycerol.
- The alkali then reacts with the fatty acids, forming soap molecules through a process called neutralization.

2. Sodium vs. Potassium:
- Both sodium hydroxide (NaOH) and potassium hydroxide (KOH) can be used as the alkali in soap-making.
- Sodium hydroxide is commonly used for making solid soap bars, while potassium hydroxide is used for making liquid soaps.
- The choice between sodium and potassium hydroxide depends on the desired properties of the soap.
- Potassium hydroxide produces a softer soap compared to sodium hydroxide.
- Potassium soap has a higher solubility in water, making it suitable for liquid soaps and shaving creams.
- The softness of potassium soap makes it more gentle and moisturizing for the skin.

3. Benefits of Potassium soap:
- Potassium soap is known for its ability to retain moisture and provide a soft, smooth feel on the skin.
- It is less likely to strip the skin of its natural oils, making it suitable for individuals with dry or sensitive skin.
- Potassium soap also has a lower pH compared to sodium soap, which can help maintain the skin's natural acidity and prevent dryness or irritation.

In conclusion, the element in soap that makes it soft on the skin is potassium. Potassium hydroxide is used in the saponification process to produce softer soaps that are gentle and moisturizing for the skin.