University of Virginia Physics Department Investigating Density A Physical Science Activity

2003 Virginia SOLs

• PS.1
• PS.2

Objectives

Students will

• use the following skills: measuring, recording data, interpreting data, graphing, and interpreting graphs;
• understand the relationship between mass and volume for a substance;
• determine the relative densities of liquids;
• calculate densities of various materials.

Motivation for Learning

Discrepant Event

Materials

• Two 500 mL beakers
• Two ice cubes (you will need extras)
• 200 mL of alcohol, isopropyl
• 200 mL of water
• Plastic wrap
• 1000 mL beaker

Procedure

1. Show the students a beaker of water and a beaker of alcohol, both filled to the same level. Ask them to make detailed observations of the two beakers and solicit their input on similarities and differences between them. Tell them that they are not allowed to smell or touch the liquids, only look.
2. After observations have been made, show them two ice cubes. Ask for predictions on what will happen if one ice cube is dropped into each beaker.
3. Drop the ice cubes into each beaker, so that the students can observe one float and the other sink. Ask the students if they can explain why the ice floats in one beaker and not the other.

   Remind the students that they have been actively engaged in the first step of the scientific method, observation, and that they are now ready to hypothesize. If someone argues that the ice cubes are different, you can take each cube out and put it in the opposite beaker. Someone, sooner or later, will guess that the beakers contain different liquids.

4. Now you can allow them to smell the two beakers. But the question still remains, why did the ice sink in one beaker and not the other?

This would be a good time to point out that in the scientific process answering one question often leads to another. Once the discussion has come around to a density or weight difference in the liquid, you can extend your demonstration:

1. Add food coloring to the water and pour it into the larger beaker.
2. Drape a piece of plastic wrap on top of the water inside the beaker so that it looks like a loose fitting cover.
3. Add a different food coloring to the alcohol in the other beaker and gently pour it onto the plastic wrap in the large beaker.
4. Very gently remove the plastic wrap and the alcohol will sit on top of the water.
5. You can drop an ice cube into the mixture again and watch where it floats.

 

Background Information

Density is a measure of how much mass per unit volume a certain substance or object has (d = m/V). This property is an intrinsic property to each particular substance. This means that it is a property of the substance and depends on the type of material as opposed to the amount. Extrinsic properties depend on the amount of the material. Some examples of other intrinsic properties include color, melting point, freezing point, and boiling point. Extrinsic properties include volume, mass, and temperature. This activity will help students differentiate between extrinsic properties such as volume or mass alone and intrinsic ones such as density. Because density is intrinsic, as you add more and more of a substance to the sample, the density does not change. The volume and the mass both change as you add more to a sample. This is the relation that will be explored fully in the following activity. The density scale is historically related to the density of water, which is 1.0 g/cm³ (or 1000 kg/m³) at 15°C. In this lab, students will measure volume in mL. 1mL = 1 cm³ .

Student Activity

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Density of Liquids

Materials:

• 50 mL of isopropyl alcohol (colored red)
• 50 mL of water (colored blue)
• 50 mL of salt water (colored green) (preparation of salt water: 4 parts water to 1 part salt by volume)
• Graduated cylinder
• Calculator
• Electronic balance or triple beam balance

Pre-Lab Introduction

Show the students the three liquids and tell them what they are. Ask them to devise a way to compare the masses of the three liquids. They should eventually come to the conclusion that they must mass the same volume of each liquid in order to compare the masses fairly. Tell them that scientists and engineers use density to compare masses of different materials. Density (in units of g/cm³) is the mass of 1 cubic centimeter (cm³) of the substance. For liquids, 1 cm³ in volume is equal to 1 mL. The formula for density is:

Procedure

1. On the electronic balance, mass the graduated cylinder and press "tare" to subtract the mass. If you are using a triple beam balance, mass the graduated cylinder and record this mass.
2. Pour 10 mL of the alcohol (red liquid) into the graduated cylinder (To get an accurate measurement place the cylinder on a flat surface and read the bottom of the meniscus).
   

   Find the mass of 10 mL of the alcohol and record in the table. If using a triple beam balance, mass the cylinder with the 10 mL of alcohol in it and record this value. Subtract the mass of the cylinder to obtain the mass of the alcohol. Find the density by dividing the mass of just the alcohol by 10 mL. Record the density in the table.

3. Add 10 more mL of alcohol to the cylinder (for a total of 20 mL) and record the mass. Find the density again by dividing the mass by 20 mL and record it in the table.
4. Keep adding 10 mL of alcohol, recording the mass and calculating the density by dividing the mass by the amount of alcohol in the cylinder until all 50 mL of the alcohol has been used.
5. Repeat the procedure for each liquid, finding mass and density.
6. Graph mass (y-axis) vs. volume(x-axis) for each liquid on the graph paper provided.

Data Sheet

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Red Liquid Volume (mL) Mass (g) Density (g/mL)  10 mL          20 mL          30 mL          40 mL          50 mL             Blue Liquid Volume (mL) Mass (g) Density (g/mL)  10 mL          20 mL          30 mL          40 mL          50 mL         Green Liquid Volume (mL) Mass (g) Density (g/mL) 10 mL          20 mL          30 mL          40 mL          50 mL

Extensions

Students can determine the relative density of several different liquids by placing a hydrometer in each. Hydrometers can be made by putting a penny or other small weight in the bottom of a 75 mL test tube. The liquids being tested can be placed in a 100 mL graduated cylinder. Place the hydrometer in the liquid. It will float, and the sides of the graduated cylinder will keep it from leaning over. Measure the length of the hydrometer that is sticking out of the liquid. Repeat the process with the other liquids. The lengths of the test tube (hydrometer) sticking out of the surface of the liquids can determine their relative densities.

Students can extend this activity by using excel to make this graph of mass vs. volume. The students will make a spreadsheet of the data and then make a graph using excel to show the relationship between mass and volume. For instructions on how to use excel, click here.

Students with Special Needs

Some students may have difficulty manipulating the different liquids in the activity. Click here for further information on laboratories with students with special needs.

 

Assessment

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1. As the mass increases, what happens to the volume?

2. As the volume increases, what happens to the mass?

3. How many grams of alcohol (red liquid) would occupy 25 mL?

4. What volume would 65 grams of water (blue liquid) occupy?

5. How many grams of salt water (green liquid) would occupy 80 mL?

Answers to Assessment

1. Volume must increase when mass increases so that density remains constant. This is a linear representation.
2. Mass must increase as volume increases so that density remains constant. This is a linear representation.
3. (.785 g / ml) x 25 ml = 19.625 g or approximately 20 grams
4. 65 ml
5. Must get answer based on data. Do not know off hand the density of this particular solution.