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Orienting a Detergent Molecule in Water 4 questions
Orienting a Detergent Molecule in Water
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A class models a detergent molecule as a charged head attached to a long nonpolar tail. The class uses a simplified model in which the head interacts favorably with water and the tail can associate with a nonpolar oil droplet. Neither liquid reacts chemically in the model.
| Trial | Contents after mixing | Observation after a short wait |
|---|---|---|
| 1 | Water and oil | One oil layer forms again |
| 2 | Water, oil, and detergent | Many small oil droplets remain dispersed |
Question 1
Which orientation fits the model at the surface of a droplet in trial 2?
- The heads point into oil and tails point into water.
- The heads point into water and tails associate with oil.
- The heads and tails both point into the oil only.
- The heads and tails both avoid the oil.
Question 2
Use the model to explain why a molecule with both parts can stay at the boundary between the oil and water.
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A student says the oil must have reacted because trial 2 no longer shows a separate oil layer. The class takes a small sample and still sees tiny oil droplets.
Question 3
Use the droplet observation to explain why the missing layer is consistent with a physical change.
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Question 4
A model molecule keeps the same charged head but its tail is replaced by a part that strongly favors water over oil. Explain why it may be less useful at the droplet surface.
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