How Does Soap Work?

1. How Does Soap Work? “Like dissolves like” ¿What do love and hate dissolve then?

2. Polar Molecules • Some Molecules, such a H2O, are POLAR -meaning that they have a (slightly) positive end and a (slightly) negative end like a battery. • Polarity is the result of unequal sharing of electrons in a bond causing one atom in the bond to be more negative than the other.

3. Non-Polar Molecules • Other Molecules, such as hydrocarbons, or symmetrical molecules (O2, N2, CO2) are non -polar,meaning that their electrons are shared more or less equally and there is no positive end or negative end.

4. “Like dissolves Like” • Polar things interact with Polar things, and repel non polar things • Ex: Water (polar) will dissolve salt (ionic), but will not mix with oil (non polar). • Non Polar things interact with non Polar things and repel Polar things • Ex: Kerosene (non polar) will mix with oil (non polar) but not water (polar)

5. Dirty Laundry Lab • What did the kerosene clean well? • What did water clean well? • What can you infer about those stains? • Soap can clean both polar and non-polar stains – what makes it so special?

6. Hydrophilic and Hydrophobic • “Hydro” – water • “phobic” – doesn’t like! (non-polar) • “philic” – does like (polar) • Things that interact with polar water molecules are water-loving or hydrophilic. Hydrophilic substances include Salts, alcohols, and anything with polarity. • Things that don’t like water are water-hating or hydrophobic. Hydrophobic substances are oils, grease, and anything Non polar.

7. Long chain hydrocarbons • Long chain hydrocarbons such as alkanes and alkenes are NON-POLAR Covalent molecules.

8. Alkanes – non-polar

9. Alkanes with stuff added on… • Adding functional groups such as alcohol (-OH) or Carboxyl (COOH), causes one part of the molecule to become Polar • The rest of the molecule is still non-polar, especially if it’s a really long chain

11. Saponification reaction • Soap is made by reacting a fatty acid with a base • This breaks the Carboxyl bonds to form a soap (with a polar end) and Glycerol

12. Soaps • A soap molecule has a Polar hydrophilic ‘head’ and a Non polar hydrophobic ‘tail’ • The head will interact with the polar water molecules and the tail will interact with the grease molecules. • This means a soap can interact with both oil and water.

13. Soaps • In this way, the soap molecules can cause grease molecules to dissolve into the water, by formingmicelleswhich allow small particles of grease to be lifted up into the water.

14. Micelle

17. Soap dissolves grease • The non-polar part of soap dissolves the grease, • The other end dissolves salts, polar stuff and associates with water • Voila! Clean dishes!

18. Water is Special • The Polarity of water molecules, causes them to ‘cling together’ much more than we may expect. This accounts for several unique properties including • High Melting and Boiling Points (large IMAF) • High Heat Capacity (remember that? Its 1cal per gram, or 4.18J) • Surface tension • Ability to dissolve certain substances

19. Bubbles • Soap Molecules come between water molecules but don’t stick to them as strongly as they stick to each other. • Therefore adding soap will decrease the surface tension of water in a cup.

20. Bubbles • When you blow a bubble the hydrophyllic part of the soap molecules turn inside and the hydrophobic part turns outside. • The water molecules cant pull together in a tight drop like they want to. This allows a bubble to form.

21. Words you should understand • Polar / polarity • Non-polar • Hydrophilic • Hydrophobic • Hydrocarbon – alkane (alkene?) • Functional group