6A: Polarity
Peter Yaukey
INTRODUCTION
Polarity refers to the existence of uneven charges on a molecule. Polar molecules have the ability to cling to each other, or to polar molecules of other kinds, by virtue of opposite charges on adjacent molecules attracting one another. Polarity is an especially important quality of water, and allows it to dissolve other polar compounds.
Nonpolar substances often remain segregated/clumped within water because of hydrophobic exclusion– the ability of water molecules to “squeeze” nonpolar substances out from among themselves (and into a clump) as the water molecules are drawn to each other by their positive and negative charges.
The purpose of this lab is to use oil and water to illustrate principles of polarity, and to examine how they affect the segregation of compounds.
SAFETY PRECAUTIONS
Alert the instructor to any broken glass.
Remember that an oil/water mixture is slick- clean up any spills promptly, and do not step in them.
Be careful with oil and food color, which stain clothing and skin.
MATERIALS
Vegetable oil
Tap water
Glass jar with lid
Red food coloring
PROCEDURE
Step 1: Pour a half inch of oil into your jar, and fill it all the rest of the way to the brim with water. Screw the lid on, and shake it. Then place it on a counter, lid-down, and let the oil and water separate for three minutes.
Step 2: In your lab notebook, answer: Which ended up on top, the oil or the water? So, which is less dense?
Step 3: Now, get your cell phone (or other camera) ready to take a picture. Then, rapidly invert the container on the counter, into the lid-up position. Snap a picture of the oil as it re-orients itself within the container, rising through the water (the best action may end in 1-2 seconds, so be poised to click!). Did the oil mix with the water as it rose, or remain in blobs? Does that suggest more that the oil is polar or nonpolar?
Step 4: After allowing the oil and water to separate again, remove its lid. Carefully, add several drops of food color to the top of the oil, making them reinforce each other (each drop combining with the previous ones). Ideally, you should place the drops in the same spot so they merge into a single larger drop that slowly sinks.
In your lab notebook, describe the appearance of the food color as it sank through the oil, and contrast this with its appearance after it entered the water. Did it appear to diffuse (spread) more within the oil or within the water? Based on this, would it be more reasonable to guess that the food color is polar or nonpolar?
Draw (using your photographs as reference) its appearance within the oil, and then within the water, in a single drawing
Step 6: Now shake the jar well, and allow it to separate again. Did the food color end up in the oil or the water? Does this support your earlier conclusion as to the food color’s polarity/nonpolarity? Why?
