Why does salad dressing made from oil and vinegar separates upon standing?

Salad dressing made from oil and vinegar separates upon standing because oil and vinegar are immiscible. This means they don't mix evenly and form a homogeneous solution. Here's a breakdown:

* Oil and vinegar have different densities: Oil is less dense than vinegar, so it naturally floats on top.

* Oil and vinegar have different polarities: Oil is nonpolar, meaning it doesn't have a positive and negative end like water. Vinegar is polar, meaning it has a positive and negative end. This difference in polarity makes it difficult for them to mix.

* Surface tension: The surface tension of oil is higher than vinegar, contributing to the separation.

How to prevent separation:

* Emulsifiers: Adding an emulsifier like egg yolks, mustard, or honey can help to bind the oil and vinegar together, preventing separation. Emulsifiers create a layer around the oil droplets, allowing them to disperse evenly in the vinegar.

* Shaking: Vigorously shaking the dressing before use will temporarily combine the oil and vinegar, but it will eventually separate again.

In summary: Salad dressing made from oil and vinegar separates because of the inherent differences between the two liquids. Emulsifiers and shaking can temporarily prevent separation, but it's a natural process.