Mathematics/Physics

Compared to What? – Comparing the Density of Different Liquids

Objective(s):

Demonstrate that different liquids have different densities. Initial activity can be conducted with primary level children. Calculations for finding the densities are appropriate for sixth grade or above.

Materials Needed:

water, liquid dish washing detergent (Palmolive), cooking oil (Wesson), corn syrup, food coloring, small spatulas or scrapers, four tablespoons, digital scale, four graduated beakers, (each group will need 5 clear plastic cups)

Strategy:

Divide the class into four groups.

Teacher preparation: Place each of the four liquids in containers. Label the containers; A (cooking oil), B (water), C (dish washing liquid), D (corn syrup). Pour a few drops of red food coloring into the container holding the water. Place one tablespoon with each of the four liquids. Give each student group five clear plastic cups.

Student Activity: A representative from each group will put 3 tablespoons of cooking oil in a cup, 3 tablespoons of liquid detergent in another cup, 6 tablespoons of water in a third cup. Pour the contents of cup A (oil) into a clear empty cup. Pour ½ of the contents of cup B (water) into the cup with the oil. Observe and record the result. Pour the remaining contents of cup B (water) into an empty cup. Pour the contents of cup C (dish washing liquid) into the cup with the water. Observe and record the results. Make a prediction. If the contents of the first cup are poured into the second cup, how will the liquids layer themselves. Record that prediction. Now pour the contents of the first cup into the second cup. Check the accuracy of your prediction and record the results. Do not use the contents of cup D yet.

Teacher preparation: Distribute one liquid to each of the four groups. (The fourth group will get liquid D now.) Instruct the groups to follow the directions listed immediately below.

Student activity: 1^st student weighs the empty graduated beaker and records its weight. 2^nd student puts 50 ml. of the liquid into the beaker. 3^rd student weighs and records the beaker and its contents. 4^th student subtracts the weight of the empty container from the weight of the container holding the liquid to determine the weight (mass) of the liquid. 5^th student calculates the density by dividing the weight (mass) by the volume (50ml.)

Performance Assessment:

Primary Level:

Ask

• Which liquid is at the bottom? Why is that so?
• Which liquid is in the middle? Why is that so?
• Which liquid is at the top? Why is that so?

Intermediate/Upper:

After each group calculates the density of its liquid, those calculations should be arranged from lightest to heaviest on the chalkboard. If the students have performed the measurements and the calculations correctly the numerical arrangement of the densities should mirror the density layers they created in their cups. Ask the students to predict where liquid D should layer itself based on their calculations. Groups who have the corn oil, the water, and the dishwashing liquid should place three tablespoons of corn syrup into their density cups. Liquid D, the corn syrup, should sink to the bottom since its density is the heaviest of the four.

References:

A Quantitative Approach to Science. Chicago Board of Education, 1985.

Whelmers. Steven L. Jacobs. Science Demonstrations That Spark Your Imagination, 1994.