Surface Tension

Description

Powdered sulfur is dusted on the surface of some clean water; it floats. Upon the addition of a small amount of detergent, the sulfur wets and sinks. Dyes puddle when added to the surface of whole milk, but begin mixing in dramatic, colorful streaks when a drop of detergent is added to the container wall near the upper surface of the milk. Nearly 1 cm of liquid film may be drawn up from the surface of liquid water using a ring and a triple beam balance. These three phenomena illustrate some of the consequences of surface tension.

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1. Place 500 mL of tap water in a 1-L beaker. Sprinkle some powdered sulfur on the surface of the water. Dip your clean dry finger into the water, and note the effect on the sulfur.
2. Dip your finger into some liquid dish detergent, and then dip it into the beaker containing the sulfur. Note the effect on the sulfur.
3. Fill a Petri dish bottom half-full of whole milk. Drop some food coloring onto the surface. Fill a transfer pipet with some liquid dishwashing detergent. Place a few drops of the detergent on the inside wall of the Petri dish above the level of the milk. Note any evidence for reaction.
4. After a long period of time, the apparent changes cease.
5. A flat wire circle with a loop is constructed for this part of the experiment.
6. Hang the loop from the balance hook of a triple beam balance.
7. Fill a Petri dish bottom half-full of distilled water. Add a drop of food coloring to make the water more visible. Support the Petri dish on the ring of a support stand. Bend the wire loop so that the plane of the flat wire circle is precisely parallel to the plane of the surface of the liquid. Adjust the level of the dish so that the ring is just below the surface of the liquid in the dish.
8. Add weight by carefully sliding weights on the balance toward greater values.
9. Note the liquid surface.

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Name ___________________________ Class ________

Teacher__________________________

DoChem 068 Surface Tension

A detergent (surfactant) changes the surface characteristics of a suspension.

1. Why does the sulfur remain on the surface?

   Why does the sulfur sink when detergent is added?

2. What happens when detergent is added to the milk and food coloring?
3. Describe what happens when the ring is pulled by the balance. Describe the effect of a drop of detergent on this experiment.

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Purpose

To illustrate the change in water surface tension when a detergent is added.

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• 1-L beaker or shallow glass pie pan
• 1 g flowers of sulfur (fine sulfur powder)
• 1 mL dishwashing detergent in beaker
• tap water
• Petri dish
• 25 mL homogenized fresh whole milk
• food coloring
• thin-stem transfer pipet
• triple beam balance
• support stand with ring
• wire circle with wire loop (60 cm heavy gauge aluminum wire; solder; soldering iron; suitable soldering work surface)

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• The sulfur must be as a very fine powder.
• Manufacture the ring for the students. Bend a smooth, flat circle of heavy gauge aluminum wire into a circle of 8-10 cm diameter. Once the wire lays flat and the loop closes with a certain pressure, solder the circle closed. Fashion a loop of the same wire with a span equal to the diameter of the wire circle. Solder this loop perpendicular to the circle.

• Lifting the ring is the most difficult part of the experiment.

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Teacher preparation: 5 minutes

Presentation: 5-10 minutes

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There are no unusual hazards in this experiment.

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No special precautions are required in this experiment. Follow routine laboratory precautions.

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The materials used in this experiment may be disposed of safely at the sink.

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• Sulfur floats on water. The surface tension of the water is high, and the water is unable to wet the sulfur. The density of the sulfur is greater than that of the water. Just a small amount of detergent, enough to go unnoticed on fingertips, will lower the surface tension of the water enough to allow it to wet the sulfur which immediately sinks.
• A detergent (surfactant) changes the surface characteristics of a suspension. While the phenomenon involved is very complex, changes in surface tension play a major role in the mixing effect seen on changes in food coloring puddled on the surface of some milk once a detergent is added to the surface.
• Surface tension in water is sufficient that nearly 1 cm of film can be drawn from a water surface using a suitable device such as a flat wire ring and a gentle steady source of pull such as that from a triple beam balance.

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1. The sulfur is denser than water but the surface tension keeps the sulfur from wetting. It sinks because the surface tension is reduced by the detergent.
2. Currents are set up in the milk which causes the food coloring to mix.
3. A cylinder of water is lifted from the petri dish. A drop of detergent would dramatically reduce the surface tension so no visible amount of the solution could be lifted.

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• surface tension
• detergent
• surfactant

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