A DIALOGUE WITH SARAH, AGED 3: IN WHICH IT IS SHOWN THAT IF YOUR DAD IS A CHEMISTRY PROFESSOR, ASKING “WHY” CAN BE DANGEROUS By Stephen McNeil

• SARAH: Daddy, were you in the shower?
• DAD: Yes, I was in the shower.
• SARAH: Why?
• DAD: I was dirty. The shower gets me clean.
• SARAH: Why?
• DAD: Why does the shower get me clean?
• SARAH: Yes.
• DAD: Because the water washes the dirt away when I use soap.
• SARAH: Why?
• DAD: Why do I use soap?
• SARAH: Yes.
• DAD: Because the soap grabs the dirt and lets the water wash it off.
• SARAH: Why?
• DAD: Why does the soap grab the dirt?
• SARAH: Yes.
• DAD: Because soap is a surfactant.
• SARAH: Why?
• DAD: Why is soap a surfactant?
• SARAH: Yes.
• DAD: That is an EXCELLENT question. Soap is a surfactant because it forms water-soluble micelles that trap the otherwise insoluble dirt and oil particles.
• SARAH: Why?
• DAD: Why does soap form micelles?
• SARAH: Yes.
• DAD: Soap molecules are long chains with a polar, hydrophilic head and a non-polar, hydrophobic tail. Can you say ‘hydrophilic’?
• SARAH: Aidrofawwic
• DAD: And can you say ‘hydrophobic’?
• SARAH: Aidrofawwic
• DAD: Excellent! The word ‘hydrophobic’ means that it avoids water.
• SARAH: Why?
• DAD: Why does it mean that?
• SARAH: Yes.
• DAD: It’s Greek! ‘Hydro’ means water and ‘phobic’ means ‘fear of’. ‘Phobos’ is fear. So ‘hydrophobic’ means ‘afraid of water’.
• SARAH: Like a monster?
• DAD: You mean, like being afraid of a monster?
• SARAH: Yes.
• DAD: A scary monster, sure. If you were afraid of a monster, a Greek person would say you were gorgophobic.
• (pause)
• SARAH: (rolls her eyes) I thought we were talking about soap.
• DAD: We are talking about soap.
• (longish pause)
• SARAH: Why?
• DAD: Why do the molecules have a hydrophilic head and a hydrophobic tail?
• SARAH: Yes.
• DAD: Because the C-O bonds in the head are highly polar, and the C-H bonds in the tail are effectively non-polar.
• SARAH: Why?
• DAD: Because while carbon and hydrogen have almost the same electronegativity, oxygen is far more electronegative, thereby polarizing the C-O bonds.
• SARAH: Why?
• DAD: Why is oxygen more electronegative than carbon and hydrogen?
• SARAH: Yes.
• DAD: That’s complicated. There are different answers to that question, depending on whether you’re talking about the Pauling or Mulliken electronegativity scales. The Pauling scale is based on homo- versus heteronuclear bond strength differences, while the Mulliken scale is based on the atomic properties of electron affinity and ionization energy. But it really all comes down to effective nuclear charge. The valence electrons in an oxygen atom have a lower energy than those of a carbon atom, and electrons shared between them are held more tightly to the oxygen, because electrons in an oxygen atom experience a greater nuclear charge and therefore a stronger attraction to the atomic nucleus! Cool, huh?
• (pause)
• SARAH: I don’t get it.
• DAD: That’s OK. Neither do most of my students.