Density of Liquids

University of Virginia
Physics Department

Density of Liquids

A Physical Science Activity

2003 Virginia SOLs

PS.1
PS.2

Objectives

Students will

define intrinsic and extrinsic properties;
determine if density is an intrinsic property or extrinsic property;
determine the density of various liquids;
record data in a table;
use the following skills: observing and recording data (observations), inferring from observations.

Motivation for Learning

Discrepant Event

Do two simple demonstrations to illustrate the key concepts studied in the lab exercise.

To differentiate extrinsic properties from intrinsic properties:

Materials

Two 250 mL beakers
300 mL water
Thermometer
Hot plate
100 mL graduated cylinder

Procedure

Mass both beakers separately and record the results on the chalkboard.
Place 100 mL of water in one of the 250 mL beakers. Mass and record the results on the chalkboard.
Place 200 mL of water in the other 250 mL beaker. Mass and record the results on the chalkboard.
Determine the mass of the water in each beaker by subtracting the mass of the beaker from the mass of the beaker and the water. Record the results on the chalkboard.
Place both beakers on the hot plate and bring them to the boiling point.
At this point define the terms extrinsic property and intrinsic property and ask the students which one mass is. (Mass is an extrinsic property.) Also ask them if mass would be useful in determining that both beakers actually contained water. Discuss some other examples of extrinsic properties and point out that extrinsic properties are not useful in determining the identity of a substance.
When the beaker containing 100 mL of water begins to boil, measure the temperature and record it on the chalkboard.
When the beaker containing 200 mL of water begins to boil, measure the temperature with the same thermometer and record it on the chalkboard.
Compare the boiling point temperatures. (They should be the same.)
At this point ask the students if boiling point is an extrinsic or intrinsic property. (Boiling point is an intrinsic property.) Also ask them if boiling point would be useful in determining that both beakers actually contained water. Boiling points are generally unique for any given substance. Discuss some other examples of intrinsic properties and point out that intrinsic properties are very useful in determining the identity of a substance.

To demonstrate that different liquids can have different densities:

Materials

75 mL cooking oil
225 mL water
Food coloring (any color)
Four 150 mL or 250 mL beakers
100 mL graduated cylinder

Procedure (Part 1)

Pour 75 mL of water into a beaker and then add one or two drops of food coloring to the water and mix.
Pour an equal amount (75 mL) of water into another beaker.
Explain to the students that you are going to pour the colored water into the clear water and have them hypothesize what will happen.
Gently pour the colored water into the beaker of clear water and observe.
Discuss the results. The contents of the beaker will mix and the food coloring will diffuse throughout the mixture. Eventually, the color will be uniform. You can stir the mixture to speed up the process of diffusion.

Procedure (Part 2)

Pour 75 mL of water into a beaker and then add one or two drops of food coloring to the water and mix.
Pour an equal amount (75 mL) of cooking oil into another beaker.
Explain to the students that you are going to pour the colored water into the cooking oil and have them hypothesize what will happen.
Gently pour the colored water into the beaker of cooking oil and observe.
Discuss the results. Why does the colored water sink to the bottom? Students will probably answer that the colored water weighs more. Remind the students that you used equal amounts of water and proceed with the experiment below. If you stirred the mixture in part 1, then you should stir the mixture in part 2. It will only take a minute for the mixture to separate again. In the assessment question section, students will return to the question - Why did the colored water sink to the bottom of the beaker of cooking oil? The answer is that the colored water has a greater density than the cooking oil. At this point in the activity, students should be able to draw this conclusion utilizing their experimental results. You can illustrate this concept in a post lab discussion by placing a piece of copper or similar metal into a beaker or glass of water. The copper will sink to the bottom because it has a density of 8.96 g/cm³, while water has a density of 1 g/cm³.

Background Information

Density is the ratio of mass of an object to its volume (D = mass/volume) and is generally reported in grams per cubic centimeter (g/cm³). You may recall that a milliliter (mL) is the same as a cubic centimeter (cm³). In this lab exercise, students will determine the density of various amounts of different liquids in order to determine if density is an extrinsic property (dependent on the amount of matter) or an intrinsic property (dependent on the type of matter). Examples of extrinsic properties include volume, mass, weight, and temperature. Examples of intrinsic properties include density, color, texture, freezing point, and boiling point. This activity allows students to perform several density calculations or to use a computer spreadsheet to generate the data. Then students interpret the data to answer the question - Is density an extrinsic or intrinsic property? This activity will probably require two 50-minute class periods.

Student Activity

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Materials

100 mL of 3 liquids (recommend cooking oil, water, and rubbing alcohol to maintain continuity with the discrepant events)
3 150 mL or 250 mL beakers, one for each liquid
Triple beam or electronic balance
100 mL graduated cylinder

Procedure

Determine the mass of a clean dry graduated cylinder - record in data table.
Measure out 50 mL of water- record amount (50 mL) in data table. (Remember - always measure from the bottom of the meniscus.)
Determine the mass of the graduated cylinder and the liquid together and record it in the data table.
Pour the liquid back into its container for reuse.
Subtract the mass of the cylinder from the mass found in step 3. This gives the mass of the liquid alone. Record it in the data table.
Calculate the density of the liquid by dividing the mass by the volume. Record it in the data table.
Repeat steps 1-6 using 75 mL of water.
Repeat steps 1-6 using 100 mL of water.
Repeat steps 1-8 for the other 2 liquids provided.

Data Sheet

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Water

Mass of Graduated Cylinder (g)

Volume (mL)

Mass GC+Liquid (g)

Mass Liquid (g)

Density of Liquid (g/mL)

Trial 1

Trial 2

Trial 3

Oil

Mass of Graduated Cylinder (g)

Volume (mL)

Mass GC+Liquid (g)

Mass Liquid (g)

Density of Liquid (g/mL)

Trial 1

Trial 2

Trial 3

Alcohol

Mass of Graduated Cylinder (g)

Volume (mL)

Mass GC+Liquid (g)

Mass Liquid (g)

Density of Liquid (g/mL)

Trial 1

Trial 2

Trial 3

Students with special needs

Students may have difficulty manipulating the liquids in the activity.

Click here for further information on laboratories with students with special needs.

Assessment

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

_____________________________________________________________________

2. As the volume of the liquid increases, what happens to its density?

_____________________________________________________________________

3. Based on your experimental results, is density an extrinsic or intrinsic property? Explain.

_____________________________________________________________________

4. From the colored water and cooking oil classroom demonstration, explain why the colored water sank to the bottom of the beaker. Use data from your experimental results to support your answer.

_____________________________________________________________________

Answers to Assessment

If the volume of a liquid is increasing then more of the liquid must be being added; therefore, the mass of the liquid is increasing.
The density of a liquid is constant despite mass and volume.
Density is an intrinsic property because it is based on the chemical nature of the pure liquid or the combined chemical natures of the liquids within a solution and not the physical properties of the specific matter (i.e. temperature, mass, and/or volume). These results show that the density does not change when the extrinsic properties change and therefore is not related to these.
The cooking oil is less dense than the water; thus, due to gravity, the water (with a higher density) will sink to the bottom. Because the cooking oil is made up of hydrophobic carbon compounds (which don't like the polar behavior of water), the two substances stay unmixed.