Water Unit

The race is on!

Students will develop and produce a boat. Students will then compete in a race with their boat where they will later be able to grasp an understanding of “surface tension,” “molecules,” and “cohesion,” and how they relate to each other. Students will determine if soap will affect surface tension. Students will determine if water temperature affects surface tension.

Students will also be able to explain a water strider’s adaptations for walking on water.

Key Concepts:
Molecules, surface tension, and cohesion. The polarity of a water molecule. How polarity affects the properties of water. Hydrophilic and hydrophobic properties. What is a surfactant?

Question to be answered:
What concentration of a liquid soap solution will power a boat the fastest?

Materials List:
1.Boat Pattern
2. Index Cards
3. Clear 3” tape (to laminate)
4. Liquid Detergent 50/50 dishsoap and water and
100% dishsoap
5.Dishpan
6. Water
7. Inflatable kiddie pool


Getting Ready Notes to Teacher:

Students make a boat by tracing the pattern on an index card, then covering it with the clear tape, or you can laminate them to make them more durable in water, then students cut them out. The students then put the boat flat on the surface of water in a dishpan (low cost), with the notch near the edge. Students then carefully place a drop of dishwashing liquid in the water where the notch is located, and record the time and distance that their boat speeds across the surface of the water. The molecules of the dishwashing fluid are attracted to water, and the dishwashing liquid breaks the surface tension, causing a ripple effect that pushes the boat forward. Students also run a trial with hot water because hot water significantly lowers surface tension and makes detergents more effective.

Background Information

Water covers about three fourths of the surface of the earth. It is ubiquitous. It is also one of the simplest yet most important molecules in living systems.

The simplicity of the water molecule belies the complexity of its properties. Water boils and freezes at much too high, or low of a temperature for a molecule of its size. Many of these unexpected properties of water are due to the fact that water molecules are attracted to each other like small magnets. This property is called cohesion, which results from the structure of the water molecule and the characteristics of the atoms it contains. Each molecule of water is made up of two atoms of hydrogen and one atom of oxygen.

Hydrogen in water takes on a partial positive charge and oxygen takes on a partial negative charge. these partial charges cause water molecules to stick to each other like magnets. The stickiness is due to hydrogen bonding. In this case, hydrogen bonding involves the attraction between the positively charged hydrogen atom of one water molecule and the negatively charged oxygen atom of another water molecule.

Procedure:

1. Students work in groups of 4-6.

2. Prediction-Have each group discuss and make predictions as to what concentration of detergent will make the boats go fastest.

3. Variable-students will each decide on a different concentration of detergent, and perform time trials with it.

4. Constant-The same size and boat design will be used by all students. (Teacher will have varied design...more notches cut out...super-charged!)

5. Students will assemble their individual boats.

6. Students will team with a partner to race and measure time and distance and record.

7. Each group of partners will perform 2 trials and record their data. Water needs to be replaced after each trial or boats will not work. First trial the water is 25 degrees C. The second trial the water is 50 degrees C. approximately. The students measure temperature with a thermometer, and record on student worksheet.

8. When all students in each group have raced and recorded they will discuss the groups results and decide which boat and detergent concentration they want to choose to compete in the classroom finals.

9. Classroom finals will be performed in the kiddie pool...it is a winner takes all race!

10. After the race, students return to their groups to complete data sheets.

11. Students hypothesize changes that could be made to increase the speed of their boat.


Data Collection Sheet
The Race is On!
Name___________________________


Data Collection Table
Question and Answer
Name______________________
1. What is polarity?
2. How does polarity affect the properties of water?
3. Define the words hydrophilic and hydrophobic.
4. What is a surfactant and describe how a surfactant
can be used in our environment?
5. Explain cohesion.
6. How does a water strider use surface tension to walk on water?
7. Draw a model of a water molecule.


Take Home Activity
“Parent Partners in Science”
Name_________________________

Predict 1. Take a clean glass of water. Predict what will happen if you add one small drop of oil to the water.

To Do 2. Do this experiment. Can you see the oil? Was your predication correct? Describe what you see. Your description should focus on polar and non polar layers and why that occurs.

Predict 3. Predict what will happen if you add a drop of detergent to the glass.

4. Now add a drop of detergent to the glass of water with the oil on top. Record your results.

5. Compare the results with your predication, and explain how the detergent works in molecular terms. What does it do to the molecules? Your explanation should focus on the ways in which amphipathic (hydrophobic and hydrophilic) “water hating”and “water loving” molecules disrupt cohesion.

6. Explain two consequences of an oil spill in the sea. What effects do they have on sea life and bird life, and what methods are used to “clean-up” oil spills?

Signature of parent or guardian helping you with this
experiment_______________________________


Directions:Read the following article and complete the writing assignment.

How is it possible for insects and spiders to walk on water or walls?

Humans cannot, under normal circumstances, either walk on water or climb up smooth vertical surfaces. But many animals, such as small lizards, snails, slugs and arthropods, easily clamber up walls or hang from the undersides of smooth leaves. A few, including fishing spiders and water striders, habitually walk on the surface of water.

Small size unites these two capabilities. Small organisms have relatively large ratios of surface area to volume (S/V). The force that opposes both capabilities (gravity) acts on the mass of the animal, whereas the forces that support climbing (adhesion) and walking on water (surface tension and fluid drag) are related to the surface in contact with the substrate. A general consequence of the way the S/V ratio changes with size is that larger animals are more influenced by gravity and inertia, and smaller animals are more influenced by surface forces such as adhesion and fluid drag.

Apart from very small size being an advantage for both walking up walls and walking on water, the two phenomena are quite different. When a fly walks up a vertical piece of glass, adhesive forces between its tarsal pads and the glass are sufficient to resist both the tendency to slide downward and the tendency to fall away from the glass surface. If you were able to isometrically increase the size of the fly by a factor of 10, its volume and its weight would increase by a factor of about 103, whereas its surface in contact with the glass would increase by a factor of about 102. This creature would fall to the ground because the somewhat increased adhesive forces could no longer resist the greatly increased pull of gravity; moreover, its now too-small wings would not allow it to fly!

For the water strider or the fishing spider, adhesion is irrelevant; in fact, both animals' legs have a waxy, hydrophobic surface that repels water, so neither is wetted by the water it stands on. Because the legs are not wetted by the water, the animal does not become submerged until the downward pull of gravity (the animal's weight) exceeds the opposing vertical component of the water's surface tension. The opposing component of surface tension is proportional to the perimeter of the leg where it is in contact with the water.

Again, imagining an isometric increase in the animal's size is instructive; if you were to increase a fishing spider's linear dimensions by a factor of 10, its weight would increase by a factor of about 103, whereas its perimeter in contact with the water would increase by a factor of only 101. The spider might not sink to the bottom because it might not be denser than water, but it probably would become submerged.

Image: Robert B. Suter
UNSINKABLE. Surface tension allows water striders to stay high and dry. "Dimples" in the water created by the pressure of the insect's legs make it possible for it to move in a nearly frictionless environment.


R.A.F.T. Assignment
Name_____________________
Date______________________
Role:Water Strider
Audience: Other Insects
Format: Brochure
Topic: Vacation Destination:Water Surface