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Name: Gerri
Status: educator
Grade: 6-8
Location: TX
Country: USA
Date: N/A


Question:
I have a new lab kit and some of the answers are not explained in depth. Why do the larger intermolecular attractions make the drop size larger for water than ethanol? I know that water has a much greater surface tension. My students seemed to interpret that a larger surface tension makes drop size smaller with the data that they collected. Thanks for your help.


Replies:
Without having the "answers" at hand it is difficult to assess the accuracy of the explanation. A good way to think about surface tension is the following: How much energy it takes to increase the surface area of a fluid per unit area. If it takes a lot of energy to increase the surface area then that fluid has a large surface tension. This point of view eliminates the need to take into account the nature of the surrounding fluid, the temperature, and other variables that can cause confusion.

If a fluid has a strong attraction to itself, compared to its surroundings, it will have a large surface tension. Water is an example of such a fluid. Even more so are liquid metals, e.g. mercury.

If there is a component in the fluid that weakens the attraction of the fluid in the region near the surface -- for example a soap-like component -- the surface tension of the solution will decrease, because such components tend to accumulate at the surface, and they do not attract one another very strongly.

Your students are on the right track, associating higher surface tension (greater attraction for molecules for one another) with smaller drop size being more stable. It is perhaps more clearly seen by considering the inverse process. In order to make a film of a fluid easier to stretch, it is necessary to add a component to the solution that weakens the attraction of the fluid for its neighbors. That is what happens when you add soap to water. The surface film of soap weakens the attraction of water molecules for one another, allowing it to be easier to stretch, thus forming bubbles (or foam).

Vince Calder


Gerri,

If I understand you correctly, you are asking how to explain to your students why higher surface tension equals larger, not smaller, drop sizes, correct?

| Rubber balloons are often used as an example of surface tension, but I believe to explain your dilemma, plastic shopping bags might work better. The thicker and stronger the bags, (ie: higher surface tension) the more material they can hold, thus a larger drop size.

Ryan Belscamper

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