2 formatting fix edited Mar 15 '14 at 14:41 Satwik Pasani 4,14643465 Basically, yes, by common terms, soft soap has K+$$\ce{K+}$$ and hard soap has Na+$$\ce{Na+}$$. They are both alkali metals, but K+$$\ce{K+}$$ is below Na+$$\ce{Na+}$$ on the periodic table so maybe we should consider periodicity trends. As you go down a table the atoms become more reactive, so K+$$\ce{K+}$$ is more likely to react with polar parts of water than Na+$$\ce{Na+}$$. Here are solubilities for both (though there are other values that would tell this story, perhaps better than solubility of the product): Solubility for KOH [Solubility for $$\ce{KOH}$$] in H2O$$\ce{H2O}$$ at 25C: 121 g / 100 mL Solubility for NaOH [Solubility for $$\ce{NaOH}$$] in H2O$$\ce{H2O}$$ at 25C: 100 g / 100 mL Regarding your question of going straight into hard soap, based on this link it seems doable; he just used NaOH $$\ce{NaOH}$$ (lye) and oils with no mention of KOH $$\ce{KOH}$$ (potash?) to speak of. And yes, the positive ions connect to the O-$$\ce{O-}$$ of the fatty acid as seen in this diagram. Basically, yes, by common terms, soft soap has K+ and hard soap has Na+. They are both alkali metals, but K+ is below Na+ on the periodic table so maybe we should consider periodicity trends. As you go down a table the atoms become more reactive, so K+ is more likely to react with polar parts of water than Na+. Here are solubilities for both (though there are other values that would tell this story, perhaps better than solubility of the product): Solubility for KOH in H2O at 25C: 121 g / 100 mL Solubility for NaOH in H2O at 25C: 100 g / 100 mL Regarding your question of going straight into hard soap, based on this link it seems doable; he just used NaOH (lye) and oils with no mention of KOH (potash?) to speak of. And yes, the positive ions connect to the O- of the fatty acid as seen in this diagram. Basically, yes, by common terms, soft soap has $$\ce{K+}$$ and hard soap has $$\ce{Na+}$$. They are both alkali metals, but $$\ce{K+}$$ is below $$\ce{Na+}$$ on the periodic table so maybe we should consider periodicity trends. As you go down a table the atoms become more reactive, so $$\ce{K+}$$ is more likely to react with polar parts of water than $$\ce{Na+}$$. Here are solubilities for both (though there are other values that would tell this story, perhaps better than solubility of the product): [Solubility for $$\ce{KOH}$$] in $$\ce{H2O}$$ at 25C: 121 g / 100 mL [Solubility for $$\ce{NaOH}$$] in $$\ce{H2O}$$ at 25C: 100 g / 100 mL Regarding your question of going straight into hard soap, based on this link it seems doable; he just used $$\ce{NaOH}$$ (lye) and oils with no mention of $$\ce{KOH}$$ (potash?) to speak of. And yes, the positive ions connect to the $$\ce{O-}$$ of the fatty acid as seen in this diagram. 1 answered Mar 14 '14 at 21:29 rch 1,3561816 Basically, yes, by common terms, soft soap has K+ and hard soap has Na+. They are both alkali metals, but K+ is below Na+ on the periodic table so maybe we should consider periodicity trends. As you go down a table the atoms become more reactive, so K+ is more likely to react with polar parts of water than Na+. Here are solubilities for both (though there are other values that would tell this story, perhaps better than solubility of the product): Solubility for KOH in H2O at 25C: 121 g / 100 mL Solubility for NaOH in H2O at 25C: 100 g / 100 mL Regarding your question of going straight into hard soap, based on this link it seems doable; he just used NaOH (lye) and oils with no mention of KOH (potash?) to speak of. And yes, the positive ions connect to the O- of the fatty acid as seen in this diagram.