A Sample Student Lab

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by  Burt Kessler,  Harold Meiselmen          Cindy Miyada,     Soo Boo Tan	Walking on Water:  Water Striders & Surface Tension

This sample activity investigates the specific question of the effect of a pollutant, sodium docedyl sulfate (SDS), on the mobility of water striders.  SDS is  found in most household detergents and is known to reduce the surface tension of water.  Water striders are dependent on surface tension of water to move around and capture food.  The hypothesis is that the water strider will break the surface tension and sink at some concentration of detergent.

Water striders (order Hemiptera, family Gerridae) are insects that live on the surface of the water in slow moving streams as well as in oceans .  There are about 75 to 85 species in North America and hundreds more in other parts of the world.  They are very sensitive to motion around them and can move at a rate of about a meter per second.  The legs of the water strider are divided into several segments: (from the body outward) the coxa, the trochanter, the femur, the tibia and the tarsus.   The water strider has three pairs of legs, each with a specific function.   The front legs are the shortest and are generally used to help support the insect.  The middle legs are primarily used to propel the insect using a rowing motion.  Though the rear legs mainly function to steer the insect as it glides on the surface,  they may also aid in propulsion.    Only the tarsus touches the water on the front and middle legs.  On the rear legs,  both the tibia and tarsus will touch.   They are also capable of jumping several centimeters on the water surface.  When stationary,  the water strider rests its weight on all six legs.

"Although molecules in a liquid are electrically neutral in nature, there are often small attractive forces between them. These attractive forces (called Van der Waals forces) are caused by the asymmetrical charge distribution inside the molecules. Within a body of a liquid, a molecule will not experience a net force because the forces by the neighboring molecules all cancel out (Figure 1). However for a molecule on the surface of the liquid, there will be a net inward force since there will be no attractive force acting from above the molecule (Figure 1). This inward net force causes the molecules on the surface to contract and to resist being stretched or broken. Thus the surface is under tension and has Surface tension.
 

Due to the surface tension, small objects will "float" on the surface of a fluid. This can be seen in Figure 2. When an object is on the surface of the fluid, the surface under tension will behave like an elastic membrane. There will be a small depression on the surface of the water. The vertical components of the forces by the molecules on the object will balance out the weight of the object."³
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   1.Sodium Dodecyl Sulfate:  This is the name of the active ingredient of nearly all modern detergents and "soaps", including dish soap, laundry detergent, hand soap (even Ivory), and shampoo.  It is often found in lists of ingredients as sodium lauryl sulfate or laureth sulfate.  Derivatives such as lauramide are also used.  It is an amphoteric as opposed to anionic or cationic surfactant.  It lowers surface tension making water wetter.  It does this by binding to the water molecules aligned at the surface of the water.  The polar sulfate head binds to water's positive pole.  The 12 carbon hydrocarbon tail can dissolve in nonpolar substances such as lipids, oil, or grease.  Thus it forms micelles in the same way traditional saponified oils do in old fashioned soaps. If SDS is not available, other detergents such as liquid soap, dish detergent, or laundry detergent can be substituted.  These will probably contain additional ingredients but will all lower surface tension.  Toxicity of SDS to the organisms is not an issue if organisms are removed from the solutions immediately after submersion and rinsed.
   2.Water Striders:  There are over 75 species and several genera of water striders, family Gerridae, found in the US.  Identification to the species level can be quite challenging.  They are caught fairly easily with a net or screen and can be kept in a bucket of water or an aquarium for weeks.  They eat almost any insect which falls onto the surface of the water.  Food must be alive and struggling.  Ants can usually be attracted to any sugary bait and will be consumed by the water striders if dropped onto the surface of their tank.  The water striders' needle-like mouthparts pierce their prey and the juices are sucked out.  Water striders that had been submerged in the higher concentrations of detergent in the experiment were at first attacked by the others as food.  This problem was circumvented by rinsing  the striders that had been even partially submerged.  The rinsed striders were then allowed to dry themselves on paper towel before being reintroduced to their fellows.  Water striders appear to be harmless to people.
   3.Serial Dilutions:  This lab utilizes serial dilutions of pollutants.  This is a valuable math-science link. Serial dilutions can be used to teach powers of ten, proportions, and concepts of scale.  The teacher may choose to introduce the technique of serial dilutions prior to this inquiry investigation.  Students would then be prepared to use the technique while investigating pollutants.  Alternatively, when students need to determine quantities of polluntants to test, they can then be taught about serial dilutions.  Below is the procedure for the serial dilutions used in this sample lab.

• SDS has a gram molecular weight of 288.
• 28.8 g were dissolved in 100 ml of water to make a 1 molar solution.
• This stock solution is sufficient for several classes.
• 1 ml of the 1M stock solution was added to 99 ml of water to make 100 ml of .01M (actually 500 ml are needed, therfore add 5 ml to 495 ml of water).
• 10 ml of the .01M was added to 90 ml of water to make 100ml of .001M.  This was a sufficient quantity to cover the bottom of the testing chamber.
• 20 ml of the .01M solution was added to 80 ml of water to produce 100 ml of .002M solution
• 30 ml of the .01M solution was added to 70 ml of water to produce 100ml of .003M solution , and so forth

6 water striders
Sodium Dodecyl Sulfate (SDS) - 1 Molar (may be substitued by Dish Detergent)
Triple Beam Balance
Distilled Water
Six micro-capillary tubes
Six short metric rulers
1-ml pipette
3-ml pipette
Five 1000-ml beakers
One 10-ml graduated cylinder
One 100-ml graduated cylinder
computer with video capture card
digital camera

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1. Obtain 6-10 water striders (catch from nearby streams or purchase from biological supply companies).
2. Prepare the following serial dilutions of  1-Molar SDS:  .001M, .002M, .003M, .004M, .005M and pure water (control) in 1000-mL beakers.
3. Measure the relative surface tension for each of the 6 solutions above by the elevation of liquid in a micro-transfer pipette (aka capillary tube).  See diagrams 1 & 2.
4. Carefully place a water strider into the beaker with the lowest SDS concentration.
5. Observe and record (with digital camera if available) whether water strider is capable of maintaining locomotion on the surface of the solution.
6. Repeat using the different SDS concentrations.
7. Multiple trials are recommended for each detergent concentration.
8. Other factors such as temperature and pressure that can influence surface tension should be kept constant during observations and collection of data.

                  

   
            0.003M                                    0.004M                                    0.005M

Results:

As detergent concentration increases, surface tension decreases.  The lower the surface tension, the deeper the dimple on the water surface in which the insect stands.  At an SDS concentration of 0.005M, the water strider is unable to stay above water.

Conclusion:

Detergent pollution at very low concentrations of 0.005M is sufficient to prevent water striders from standing on the water surface.
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Students will construct a graph of detergent concentration versus surface tension;
Students will present their results and conclusions in the form of a lab report;
Students will present the results of their investigation to the class.

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For further research, students may extend this activity to investigate:
a.  the effects of other chemical pollutants on water striders' ability to stand on water;
b.  the effects of thermal pollution on water striders' ability to stand on water;
c.  the effects of pollution (specific or general) on other stream organisms' ability to move in water;
d.  the effects of water pH on water striders' ability to stand on water;
e.  the effects on water movement on water striders' ability to stand on water.

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PETA ON-LINE: a link to ethical treatment of animals.  www.peta-online.org/index.html

Reducing Surface Tension www.ph.unimelb.edu.au/lecdem/fb6.htm

Surface Tension of Water. www.iit.edu/~smile/ch9510.htm

Walker, Jearl:   "The Amateur Scientist,"  Scientific American,1983, 249:  vol.5, pp 188-193

^{1, 3} Surface Tension: http://www.ferris.edu/htmls/academics/course.offerings/physbo/MultiM/surfacet/surfacet.htm

²Water Strider Rap Sheet http://www.pbrc.hawaii.edu/~kunkel/wanted/mugs/92296-rap.html

What is Surface Tension? www.epa.gov/owow/NPS/kids/TENSION.HTM

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