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Whelmer #6: Falling Test Tubes?

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Illustration of how the inner test tube would stay inside the outer tube if inverted. Description:
Two glass test tubes and water are used to create a discrepant event demonstrating adhesion and cohesion.

Science process skills:
  1. observation
  2. inference
  3. prediction
Complex reasoning strategies:
  1. induction
Standards:
K-4:
  • Employ simple equipment and tools to gather data and extend the senses (Standard A.1.3).
  • The position of an object can be described by locating it relative to another object or the background (Standard B.2.1).
  • An object's motion can be described by tracing and measuring its position over time (Standard B.2.2).
5-8:
  • Unbalanced forces will cause changes in the speed or direction of an object's motion (Standard B.2.3).
9-12:
  • The physical properties of compounds reflect the nature of the interactions among its molecules (Standard B.2.4).
Above Standards from the National Science Education Standards.

Content topics:
  1. adhesion
  2. cohesion
  3. molecular attraction
  4. surface tension
You will need:
  1. a large glass test tube
  2. a smaller glass test tube
  3. water
  4. a pan or bucket
Instructions:
Glass test tubes work best for this activity. It does not matter what size test tubes are used. However, the smaller test must just fit into the larger tube. You may need to try different combinations of sizes to determine which pair works best.

Make certain the test tubes are clean, especially free of soap or detergent. Fill each test tube full with water. Holding both over the catch pan, lower and release the small test tube into the larger. Invert the larger tube. The smaller test tube does not fall out. Instead, water drips out of both tubes and the smaller tube rises up into the larger tube, seeming to defy gravity!

While rehearsing this activity, you may need to vary the amount of water in either tube to improve results. You may need to adjust the amount of water placed in the smaller tube if it sinks rather than floats on the water in the larger tube. Ideally, the smaller tube will float half way out of the larger tube.

To prevent breakage, you may want to place a sponge or some paper towels in the catch pan in case the small tube falls.

Presentation:
When presenting this activity, proceed at a moderate rate. Allow plenty of time at each step of the activity to elicit questions and model analytical thought.

Pour the established amount of water into each test tube. Before lowering the smaller into the larger, elicit predictions from your students. When they offer predictions, ascertain on what experiences they base their prediction (lowering themselves into a bathtub).

Inform students that you are going to invert the test tube system. Elicit predictions.

Before inverting the tubes, allow students to make careful observation of the two tubes. Some might notice a "bulge" of water around the lip of the larger tube. Ask students if that bulge of water has any significance. Might it give a clue to what is going to happen when the tubes are inverted? (It does.)

Slowly, but without hesitation invert the tubes. The smaller should rise up into the larger tube. Ask students to explain the discrepant event. Suggest that they use drawings to indicate all of the forces involved (gravity, adhesion, cohesion, air pressure, etc.).

Reinforce to your students that experience, very careful observations, and analytical thinking provide a basis for good predictions.

Content:
The polar nature of water molecules causes them to be attracted to each other. That is why water forms beads, drops, and the skin on the surface of a pool of water called surface tension. This type of intermolecular attraction is called cohesion.

An attraction between dissimilar types of molecules, such as glass and water, is called adhesion. Both forces are responsible for the bulge of water hanging out over the edge of the glass lip on the test tube.

The bulge is an indicator that there is an attraction between the water and the glass, and the water and itself. In this case, the attractive forces are stronger than the pull of gravity.

Assessment:
Type: small group or individual.
Content/Process: Surface tension (#1), adhesion, cohesion, and surface tension (#2).
Age/Level: all.

#1: Fill a small glass with water until it is full to the brim. Kneel and look at the top of the glass to assure that the water is filled to the top. Predict how many pennies you can drop into the glass before the water runs over. Record your prediction. Add one penny at a time to the glass. Carefully slide it in from the edge. Observe the glass from the side as you add more pennies. Draw the water in the glass at the beginning, after several pennies, and at the end. Record the number of pennies added until the water runs down the side. Describe what happened to the water as more pennies were added. What properties of the water allowed you to add so many pennies before the water spilled out? What gives water these properties?

Assess this concept based on the age of the child. The elementary student should discuss the rounded shape of the surface of the water and that water sticks together at the surface. Older students should discuss the polarity of the water molecule that gives water this characteristic.

#2: These three concepts are at play as water moves up the xylem of a plant. Following a study of plant physiology, students should use these terms along with transpiration to describe how water moves from the root of the plant out the leaf.

Notes:






  
     
 

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