Lesson

Background

If you’ve ever tried to make salad dressing from scratch, you know that one of the biggest challenges is getting the oil and the vinegar to mix. No matter how hard you try to shake, stir, or whisk oil and vinegar together, they eventually separate. This happens because oil and vinegar are made of very different types of molecules that are attracted to their own kind.

Most vinegars are solutions of acetic acid and water (plus some other acids and alcohols, depending on the type of vinegar you are using). Water, acetic acid, and alcohol are all examples of polar molecules—molecules that have a slightly negative charge at one end, or pole, and a slightly positive charge at another end. These slightly charged poles arise because one or more atoms in the molecule are electronegative, meaning that they tug electrons—which are negatively charged—towards them, creating an uneven distribution of charge within the molecule. Polar molecules are generally attracted to other polar molecules because their slightly negative poles have an affinity for their slightly positive poles. Polar molecules are attracted to water molecules—which are also polar—and are called hydrophilic, which means “water loving.”

Oils are a different story. Oils are a type of fat (like butter, shortening, and lard) and are considered nonpolar. Fats and oils are composed primarily of long molecules called fatty acids (usually bound together by glycerol molecules into groups of three called triglycerides). Most of the atoms in a fatty acid molecule share electrons evenly and are neither negatively nor positively charged (although fatty acids do contain small regions of polarity—just not enough to make the whole molecule polar.) Nonpolar molecules love other nonpolar molecules and will glom together when mixed with water. You can observe this phenomenon by placing a few drops of oil on the surface of a bowl of water—eventually the drops will form a single large oil slick. Oils repel polar molecules such as those found in vinegar. Because oils also repel water, they are called hydrophobic, which means “water-fearing.”

How can you bring together polar and nonpolar molecules to make something delicious like mayonnaise (which is essentially a combination of water and oil) or salad dressing? You need an emulsifier. Emulsifiers are the hand-holders of the molecule world. They contain both hydrophobic and hydrophilic regions and are able to attract and “hold hands” with polar and nonpolar molecules simultaneously, pulling them together to form a special type of mixture called an emulsion. For instance, after adding an effective emulsifier to oil and vinegar and mixing thoroughly, separation of the oil from the vinegar will take much longer or won’t happen at all.

In this experiment, you will test a few common household ingredients to see what the most effective emulsifiers for making salad dressing are.

Procedure

1. Set out four clean 20-mL test tubes in a test tube rack.
2. Use a scale to measure 2 g of each of the emulsifiers you will test.
3. To each of three test tubes, add 2 g of an emulsifier to be tested, putting a different emulsifier in each test tube. Label each test tube with the emulsifier that was added, and label the empty one “control.” Label the data sheet with the emulsifiers you will test.
4. Using a pipet, add 8 mL of vinegar to each test tube and swirl to fully mix in the emulsifier.
5. Using a clean pipet, add 8 mL of oil to each test tube. Take a moment to observe the two layers of oil and vinegar as they avoid mixing with one another. This is what separation looks like, a process you’ll need to be familiar with to collect data in the next step.
6. Using your thumb or a stopper, cover the opening to the control test tube, and shake it up and down for 30 seconds (time it with a clock or stopwatch). At the end of 30 seconds, place it back in the test tube rack and start the stopwatch, watching the sides of the glass for 1-5 minutes for signs of separation. When you see that most of the oil has separated from the vinegar, stop the stopwatch and record how long the process took in your data table in the column marked “separation time.”
7. Repeat step six for each of the test tubes containing an emulsifier, making sure not to contaminate one test tube with the emulsifier from another. If an emulsion has not separated after 5 minutes, write “> 5 mins” and the time of day in your data table.
8. After you have mixed and observed all of the emulsifiers, go back and check to see if any of the emulsions that didn’t separate earlier have now separated. Record your observations in your data table.

Analysis

1. Did the mixtures with the emulsifiers take more or less time to separate than your control? Is this what you expected?
2. How would you expect the separation time to change if you added more emulsifier? Why? What about if you added more oil than water?
3. Lemon juice is mostly citric acid and water. Would you expect it to mix better with olive oil or vinegar? Why or why not?
4. Look for recipes for other salad dressings or vinaigrettes online. For each, identify which ingredient is the polar molecule (hydrophilic), which ingredient is the nonpolar molecule (hydrophobic), and which ingredient is the emulsifier.

Conclusion

Which emulsifier would you recommend using for making salad dressing, based on your data?

Connections to Standards

Next Generation Science Standards

This lesson supports the following:

NGSS DCI PS1-A: Each pure substance has characteristic physical and chemical properties (for any bulk quantity under given conditions) that can be used to identify it.

NGSS MS-PS1-1: Develop models to describe the atomic composition of simple molecules and extended structures

NGSS HS-PS1-3: Plan and conduct an investigation to gather evidence to compare the structure of substances at the bulk scale to infer the strength of electrical forces between particles