We all expect that fluids will seek their own level. This counterintuitive event will leave students searching for a reason for why the two levels of water in a U-shaped tube are not the same. The explanation is that two liquids with different densities have been added to the two sides of the U-shaped tube.

Materials

2 U-shaped glass tubes (about 6-8 mm in diameter and 20 cm or more in height)

10 mL Saturated salt solution (add sodium chloride to 15 mL of water until no more salt dissolves after stirring) or ethanol or isopropyl alcohol 10 mL

Water

Food coloring

Safety and Disposal

No special precautions required for fresh and salt water. Eliminate spark sources and open flames if ethanol is utilized. Liquids may be disposed of in the sink. (Flush drain with plenty of water if alcohol is used.)

Procedure

1. Pour fresh water into one U-tube until it is about 2/3 full and note that the water level is the same on both sides. Placing food coloring in the water prior to the demonstration will help students view the liquid levels.



Remarks

Have the students write a short explanation of what they believe is causing the discrepancy in liquid levels in the second U-tube.

Reference

Taken from Doing Chemistry A02.

A Non-Newtonian Fluid: Cornstarch Putty

Cornstarch putty belongs to a class of materials which does not obey the usual laws of viscosity. Also included in this class of materials (known as non-Newtonian fluids) are the commercial products, SlimeÔ and Silly PuttyÔ. Under low stress, such as slow pulling or pouring, these materials flow and stretch much as normal fluids do. But when hit with a hammer, Silly PuttyÔ, for example, loses any resemblance to a fluid and actually shatters as if solid. Cornstarch putty exhibits similar behavior. If poured from container to container, cornstarch putty looks like pancake batter. If one quickly ÒslapsÓ the surface or ÒgrabsÓ a handful of this material, it becomes a rigid, solid-like material.

Materials

Plastic mixing bowl

500 mL Cornstarch

250 mL Water

Mixing spoon

Pie pan

Safety goggles

Laboratory apron

Paper towels

Safety and Disposal

No special precautions are required. Upon standing the putty will separate but can be reconstituted by kneading with your hands. The cornstarch putty may be disposed of in the laboratory sink with an adequate supply of water.

Procedure

1. Place the 500 mL cornstarch in the plastic mixing bowl.

2. Slowly add the 250 mL water while mixing the cornstarch with the mixing spoon or while kneading by hand. (Check the consistency prior to the demonstration of slapping the mixture in 4 below.)

3. In the presence of students, pour the cornstarch putty from the mixing bowl into the pie pan. It will pour like a normal fluid.

4. Place a laboratory apron and safety goggles on a willing class volunteer. Ask the volunteer to quickly slap the surface of the putty in the pie pan. (The volunteer and the class will expect to have a cornstarch shower. Instead the surface wonÕt even be penetrated by the slap.)

5. Instruct the student to reach into the pie pan and to ÒgrabÓ a quick handful of the putty. (It will come out as a ÒsolidÓ mass which will almost immediately liquefy and drip off of the studentÕs fingers like pancake batter.)

6. Supply the student volunteer paper towels to wipe off the cornstarch putty so that it doesnÕt drip all over the laboratory table.

Reference

Walker, J. (1978). The amateur scientist; serious fun with polyox, silly putty, slime, and other non-Newtonian fluids. Scientific American, 239(5), 142-149.

Continued

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