BioEd Online

Teacher Background

Water molecules are attracted to each other because, in many ways, they act like tiny magnets. Each molecule in liquid water has a positive end and a negative end. The forces of attraction between these opposite charges bring the molecules together very tightly. Attraction among molecules of the same kind is called cohesion.

The forces of attraction among the molecules in most liquids are not as strong as those that occur among water molecules. The “stickiness” of water accounts for much of its behavior, including the formation of rounded droplets and its ability to creep upward inside a narrow tube (capillary action).

In this activity, students discover some of the unique qualities of water and compare and contrast water with another liquid (mineral or salad oil) that behaves differently.

Objectives and Standards

Concepts

• Polarity of the water molecule is responsible for the unique properties of water.

Science, Health and Math Skills

• Predicting

• Making and recording observations

• Drawing conclusions

Materials and Setup

Teacher Materials (see Setup)

• Cup containing a small amount of water

• Cup containing a small amount of clear cooking oil (or mineral or baby oil)

• 2 pipets (or droppers)

• 4 toothpicks

• Food coloring (a few drops)

• Paper towel

• Overhead or document projector

Materials per Student

• Sheet of cm graph paper (approx. 8 1/2 in. x 5 1/2 in.)

• Sheet of wax paper to cover graph paper

• Hand lens (or magnifier)

• Crayon, colored pencil or marker to match the food coloring used

• Copy of “Do Your Liquids Behave?” page

Setup

1. This activity can be done in one or two class periods. Have students work in teams of two to share materials.

2. Colored wooden toothpicks work best for this activity. If you prefer plain, wooden toothpicks, soak them in a glass of water for an hour or so before using them. (Dry, unvarnished toothpicks will absorb the water droplets.)

3. Do not substitute plastic wrap for wax paper in this activity, as static charge on the sheets of plastic wrap may affect the behavior of the water drops. Resealable plastic bags may be substituted for the wax paper.

4. Cut the cm graph paper sheets in half.

5. Pour a small amount of water (Liquid 1) into 12 cups. Pour a small amount of clear cooking oil (or mineral or baby oil) into 12 cups (Liquid 2).

6. Each team will use one color of food coloring. Each team also will need a crayon, colored pencil or colored marker that matches the food coloring used.

Procedure and Extensions

Session 1: Examining Liquid 1

1. Demonstrate the use of a pipet (or dropper) by placing several drops of Liquid 1 (water) on an overhead projector or under a document projector.

2. Ask students to describe the drops being projected. Explain that they will be examining drops of two different liquids at their own working areas.

3. Have the Materials Managers collect the supplies from a central location. Each student should prepare a working surface by placing the wax paper over the graph paper.

4. Have the students practice making equal-sized drops of Liquid 1, sharing the dropper and using the graph paper as an approximate guide to size. Students should examine the drops with their hand lenses.

5. Ask the students to draw a drop from the side and top on their student sheets, and to describe the drop using at least three descriptive words.

6. Next, ask the students to try to split one drop into smaller drops using a toothpick. They should draw the results on their sheets.

7. Have the students push two drops together and discover what happens. Have them draw the new drop that forms when the two smaller drops come in contact.

8. After forming the new larger drop, students should dip their toothpicks into a drop of food coloring and mix it into the new drop. Have students draw the drop again and color it appropriately.

Session 2: Examining Liquid 2

1. Have students repeat the preceding exploration using Liquid 2 (oil) and record their results in the second column on their worksheets.

2. Afterwards, have the students answer the comparison questions at the bottom of the student page. Discuss student observations as a class. Ask, Did the two liquids behave in the same way? OR ask, Which liquid made round drops? How were the drops of each liquid alike? How were they different?

Extensions

• Challenge students to use their toothpicks to push water drops (size of their choice) as quickly as possible from the top of the wax paper to the bottom. Ask, What size drop moves fastest? Is there anything besides size that affects how fast a drop can be pushed?

• Encourage students to consider other variables. For instance, what happens when they mix Liquid 1 and Liquid 2 together? What happens if food coloring is added to the mixture?

• Have students add a drop of liquid soap or detergent to a drop of water and observe what happens. (The soap decreases the attraction among water molecules, thereby causing the drop to spread out.)

• Make paper boats (see Mystery of the Muddled Marsh, pages 32–33). Use the activity as part of a mathematics lesson or afurther exploration of the properties of waters.

Handouts and Downloads