|
1. Fill the jar about half full of water and pour half that volume (1/4 of the jar volume) of oil over the water.
2. Ask the students if they think you can mix the liquids by stirring vigorously. Then stir the two liquids with a spoon or glass stirrer and leave it for a while, observing what is happening to the mixture.
3. Ask the students: "How can I make the tow liquids stay mixed?" Now add a few squirts of the liquid detergent and stir thoroughly.
4. After mixing thoroughly, leave the jar alone and observe (if the emulsion separates again, you may need more detergent or more vigorous stirring, or both). |
|
Predict, Observe, Explain (POE). Students will predict whether the liquids will mix alone and then with the addition of detergent. They will then observe that alone, the liquids do not mix, but form an emulsion with the addition of the detergent. Finally, they will have to explain why the liquids do not mix alone, but become soluble with the detergent. |
|
|
1. Do you think that they oil and water will mix? What about with vigorous stirring?
2. After stirring without detergent, what did you observe in the jar?
3. Why won't the oil and the water mix?
4. What causes polarity or nonploarity of molecules?
5. After stirring with detergent, what did you observe in the jar?
6. What made the two liquids stay mixed?
7. What is an emulsion?
8. How can this apply to real life? |
|
|
When mixing oil and water, the oil will break up into small droplets and be dispersed in the water temporarily. After leaving the jar with the mixture alone for a while, the two liquids will separate with the oil forming a layer on top of the water because of its smaller density. The insolubility of the liquids is due to the fact that the water is polar and hydrophilic, while the oil is nonpolar and hydrophobic, and only like substances are soluble in each other. Polarity is imparted by the unequal sharing of electrons in the bonding of hydrogen and oxygen to form water. This results in regions of partial positive and negative charge, which lead to hydrogen bonding, or "holding hands" of the water molecules. These bonds are weak, but reform quickly. The oil is a hydrocarbon that shares its electrons evenly, thus creating no partially charged regions or intermolecular bonding. Therefore, when only water and oil are mixed, the hydrogen bonds of the water reform, excluding the nonpolar, hydrophobic oil molecules, which separate out at the top due to their smaller density. Soap is a molecule with a polar, hydrophilic head region and a long, nonpolar, hydrophobic tail. Therefore, it can mix with with oil and water. the nonpolar tails of the soap associate with the drops of oil, leaving the hydrophilic heads interacting with the water (this is called a micelle). This way, the small drops of oil become interspersed in the water, since many of the H-bonds were broker, and the micelles slip into these newly formed spaces.
This relates to every day life in many ways.
1. This is how detergents clean the dirt and oil from your clothes. The hydrophobic tails of the soap interact with the oil droplet adhering to the cloth fibers. the droplet can now be washed away and interact with the water because it is surrounded by the hydrophilic heads of the soap molecule.
2. This is how salad dressings and mayonnaise are made. Proteins, such as in eggs, also have hydrophilic and hydrophobic regions. Therefore, they surround the oil and intersperse it in the polar vinegar to make mayonnaise. Commercial emulsifiers are used in many salad dressings.
3. This is how bile works in your digestive system to break down fats. It also has a hydrophilic and hydrophobic region. Therefore, it surrounds the ingested lipids in this same manner, so they can be distributed in the digestive system or the blood stream. The formation of the small, emulsified droplets also increases the surface area that digestive enzymes are able to attack. |
|
