How Does Soap Work?

“Like dissolves like”

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.

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.

“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)

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.

Long chain hydrocarbons • Long chain hydrocarbons such as

alkanes and alkenes are NON-POLAR Covalent molecules.

Alkanes – non-polar

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

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

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.

Soaps

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

Micelle

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!

Words you should understand

• Polar / polarity

• Non-polar

• Hydrophilic

• Hydrophobic

• Micelle