|MadSci Network: Chemistry|
The simple answer has to do with how things are dissolved in general. When salt (NaCl) is placed in water, it "dissolves" because all the sodium ions and chloride ions separate from each other and "hide" (become dispersed) within the water molecules. This is an example of an ionic system: Na+ and Cl- interact with the water molecules in a specific way...the positively charged Na+ is attracted to the part of water that is negatively charged and the negatively charged Cl- is attracted to the positive part of the water molecules. Its the same general principle when talking about salt dissolving in vegetable oil. Vegetable oil is made of lipids: chains of carbon and hydrogen, which are not ionic at all. Therefore there are no places for the ionic sodium and chloride to hide within the lipids. The Na+ and Cl- would prefer to stay with each other where they have nice ionic interactions..the result is that the salt does not dissolve. Sugar, on the other hand, can have interactions with the lipids. Sugar is composed of carbon, hydrogen and oxygen in a ring structure. It has chemical groups that can interact with the CH's of the lipids AND it also has chemical groups (OH) that can interact with water molecules. It makes contacts with the lipids and slowly starts to hide within the lipid molecules until its so dispersed that you cannot see it in granular form anymore. Due to these properties, sugar can dissolve in both water and in the vegetable oil. Things become more complex when you move to other types of oils because they may or may not have chemical groups that can interact with the sugar groups. It depends a lot on the hydrogenated nature of the oils. Try the experiment with liquid butter and see what happens. When butter is a solid, the tails of the lipids do not line up and therefore it is a solid. When you heat it, the tails all line up and the molecules pack together much better allowing it to liquify. Yeast is even more complex because it is not a molecule but an actual organism in the yeast kingdom. Yeast is made of actual yeast organisms, which can be observed under the microscope. Dry yeast is kind of living in suspended animation: when you put dry yeast in water and give it some sugar, it comes out of its suspended state and goes back to living in a normal way, converting sugar into carbon dioxide for example which makes the bread rise.
Try the links in the MadSci Library for more information on Chemistry.