Rinse a buttery plate under the tap and the water runs straight off, leaving a greasy film behind. Grease and water keep apart because of the way their molecules share electrons.
Grease on plates is mostly fats and oils. The fat molecule you met earlier is glycerol joined to three long fatty acid chains. Those chains are hydrocarbons, made of carbon and hydrogen atoms only. Carbon and hydrogen share their electrons almost evenly, so the chains carry no charged ends. Chemists call molecules like these non-polar.
Water is polar. Its oxygen atom pulls harder on the shared electrons, so the oxygen carries a small negative partial charge, δ−, and each hydrogen a small positive partial charge, δ+. You have already seen what that does: the hydrogen of one water molecule clings to the oxygen of the next in a hydrogen bond, so water molecules hold on tight to each other.
Water molecules hold on to each other far more strongly than they hold on to grease. So the water gathers together and squeezes the grease into an oily blob, which stays stuck to the plate or floats to the top as a film. Hot water melts solid fats such as butter, so they run, and the melted fat still gathers into a film on top.
What mixes with what
The rule from lesson 12 is that polar mixes with polar, and non-polar with non-polar.
| Substance | Its molecules | In water |
|---|---|---|
| sugar | polar | dissolves |
| ethanol | polar | mixes in any amount |
| olive oil | non-polar | floats on top |
| candle wax | non-polar | floats on top |
To lift grease off a plate, you need a molecule that gets on with water and with grease at the same time. That molecule is soap.