This is the second Investigation of the LabLearner CELL Density. In this Investigation, students determine the mass and volume of various samples of water and calculate water’s density.

______________________________________________

A. Begin this part of the investigation by encouraging students to think about what they discovered about density in Investigation One.  Pose the following questions to prompt student discussion.

1. Ask students: How does the density of a liquid affect its interaction with other liquids?  How did you demonstrate this in the experiments in Investigation One? The density of a liquid determines whether it will float on the surface of other liquids or sink to the bottom of the container.  Students demonstrated this by combining water with rubbing alcohol and vegetable oil and rubbing alcohol with vegetable oil.  Students concluded that water had the greatest density because it sank below both vegetable oil and rubbing alcohol, and rubbing alcohol had the lowest density because it floated on both water and vegetable oil.

2. Ask students: How does the density of a solid affect its interaction with liquids?  How did you demonstrate this in the experiments in Investigation One? The density of a solid helps determine whether it will sink or float in a liquid.  Students demonstrated this by placing a metal cube and ice cube in each of the three liquids. They discovered that the metal cube had a greater density than all three liquids. Students concluded that the ice cube was more dense than alcohol but less dense than both water and vegetable oil.

B. Continue this part of the Investigation by informing students that in Investigation Two they will explore density another way: through calculation.  Explain to students that density can be calculated because it is a quantitative property.  Encourage students to recall what they know about quantitative properties.  The following questions may be useful in guiding student recall.

1. Ask students: What is a quantitative property?  What are some examples of quantitative properties?

Students should indicate that a quantitative property is a characteristic of matter that can be measured.  Examples of quantitative properties include length, temperature, mass, and volume.  Encourage students to realize that quantitative properties can be thought of as properties that differ in their amount.

2. Ask students: What instruments have you used to measure quantitative properties? Student answers will vary.  Students have used meter sticks and metric rulers to measure length, thermometers to measure temperature, triple beam balances to measure mass, and graduated cylinders to measure volume.

______________________________________________

C. Remind students that they have been discussing types of quantitative properties that can be measured using appropriate scientific equipment.

1. Ask students:  Do you know of any piece of scientific equipment that might be used to measure density? Student answers will vary.  Students most likely will indicate they do not have experience with any piece of scientific equipment that might measure density.

2. Explain to students no single piece of scientific equipment can measure density.  Tell students that a property that cannot be measured directly often can be determined or calculated from other quantitative properties that are measurable.  These properties are termed derived properties.

3. Encourage students to locate the term derived property in their Scientist’s Glossary and review the meaning of the term.

Derived property: To calculate the numerical value of a property using two or more measured amounts or properties.

4. To help students better understand the term, think aloud about other properties that are derived.  Some of these properties students may have encountered in their previous science experiments and some of these properties that students may have measured directly.

5. Examples of derived properties may include:

a. Calculation of how much work is done when lifting a load

• Work = force x distance

• Work is not measured directly.

• The force needed to lift an object is measured using a spring scale and the distance it is lifted is measured using a metric ruler.  Work is calculated by multiplying the force by the distance.

b. Surface area of a rectangular object

• Surface area = length x width of an object

• Surface area is not measured directly.

• The length and width of an object are measured.  The surface area is calculated by multiplying the length times the width of the rectangular object.

6. Examples of properties that can be measured directly may include:

a. Length of an object

• Length is measured directly.

• Length is measured using a metric ruler or meter stick.

b. Temperature of an object

• Temperature is measured directly.

• Temperature is measured using a thermometer.

______________________________________________

7. Explain that density is a derived property.  Even though density cannot be measured directly, density can be calculated from the measurable quantitative properties of mass and volume.

8. Tell students that density can be calculated from mass and volume because density is the property that indicates how much mass of a type of matter is packed into a certain volume.  Reinforce this concept by drawing students’ attention to the three boxes on the slide.  Remind students that each box represents a liquid, the area within each box represents the volume of each liquid, and the dots represent the molecules of matter within the liquid.

a. Ask students: How would you measure the volume of a liquid? Students should indicate that the volume of a liquid can be measured using a graduated cylinder.  Volume is measured in units of milliliters.

b. Ask students:  How would you measure the mass of a liquid? Students should indicate that the mass of a liquid can be measured using a triple beam balance.  Mass is measured in grams.

c. Ask students: How would you measure the amount of molecules in a liquid? Student answers may vary.  Encourage students to understand that measuring the mass of a substance is a way in which the number of molecules in a substance can be estimated. A greater mass suggests a larger number of molecules.

d. Ask students: How do you think you might be able to use the mass and the volume of a liquid to calculate density?  What type of mathematical operation would you use with these two properties? Student answers will vary.

______________________________________________

D. Introduce students to the formula for density and provide them with an opportunity to practice calculating density.

1. Explain to students that thus far in this CELL, they have learned that density is a way of explaining how much matter is found in a certain volume.  Tell students that another way to explain density is to say that it is the amount of mass in a certain volume or the ratio of mass to volume.  This concept can be represented by a mathematical formula.  Density = mass ÷ volume 

2. Explain that this formula can also be written as:

Density = mass / volume

3. Ask students: What metric units do you think are used to the describe density of a liquid?  Can you answer the question by thinking of the units used to measure mass and volume? Student answers may vary.  Students may indicate that one possible answer would be that density is reported as g/ml because mass can be measured in units of grams and volume in units of milliliters.  Encourage students to realize that scientists generally report the density of a liquid in the units of grams/milliliter (g/ml).  This can be stated as “grams per milliliter.”

4. Remind students that formulas are tools, and that like other tools they use in their CELLs, this tool can be found in their Procedural Toolbox.

______________________________________________

5. Walk students through the process of calculating density using the measurements of mass and volume. Encourage students to follow along with Problem 1 of their Scientist Data Record. 

6. Provide a second problem that the class can solve together: A piece of cheese has a mass of 200 g and a volume of 100 ml. What is its density?

Density = mass / volume 

Density = 200 g / 100 ml 

Density = 2 g/ml 

7. Direct students’ attention to Problems 2a and b of their Student Data Record.  Provide students with sufficient time to practice calculating density.

E. Once students have finished working through the practice opportunities for density in their Student Data Record, explain that they will measure mass and volume of samples of water in the lab and use the formula for density to answer the following question:

What is the density of water?