Every time you use your bottle of vinegar-and-oil salad dressing, you have to shake it up. Why?

Answer 1

vinegar is polar and oil is not, so they don't mix. You have to shake it up if you want to get some of each when you pour.

Commercial salad dressings add emulsifers to keep the layers together.

Answer 2

It is true that vinegar and oil have different densities, but this is not the reason why they don't mix. The basic rule is "like dissolves like." That is, a polar solution (like vinegar) will dissolve in another polar solution (like water). The interaction between two polar molecules (two vinegar molecules) is much stronger than the interaction between a polar molecule and a non polar molecule (a vinegar molecule and an oil molecule) . Therefore, these molecules will prefer to aggregate with each other. Molecules without any polar functional groups will in turn only dissolve in solutions that are also non polar for the same reason.
Density comes in after this. Once shaken the less dense molecules will sit on top of the more dense, but this has nothing to do with the reason the two don't mix.

Answer 3

Oil and vinegar don't mix. Oil is lighter than water (vinegar is mostly water), and oil molecules are more attracted to other oil molecules than they are to water, so the oil will float to the top and eventually you'll have two layers.

By shaking up the dressing, you mix the oil and water into a temporary emulsion where the oil is broken up into many tiny bubbles distributed throughout the vinegar.

Answer 4

Years ago the said that alcohol and gasoline do not mix. So why are we buying Gasohol? Oil and vinegar do not mix well because vinegar is mostly water and the oil does not mix well. So far I haven't heard of Oilagar on my salad

Answer 5

I think it is a heterogeneous mixture. You have to shake it in order to get both vinegar and oil. Otherwise you would just get what is on the top.