DENSITY

Unit Overview

This unit provides students with the opportunity to observe the property of density. Students will engage in inquiry about why and how some objects (solids and liquids) display the property of density. Students will measure the mass of solids and liquids. Students will experiment with solids and liquids of different densities. Students will also calculate the density of solids and liquids using hand-held calculators. The science skills emphasized in Density are collecting data, manipulating, observing, and predicting. Accuracy of measurement is critical to the success of this unit.

Scheduling

This unit may take from two to four weeks to complete depending upon the goals of the teacher and interests of the students. Use of the section included in this manual called More Ideas may extend the time span of this kit.

Materials to be obtained locally IN ADDITION TO MATERIALS REQUIRED BY SPECIFIC LEARNING EXPERIENCES:

Please make one student activity book for each student.

paper towels

water

chart paper

felt tip markers

notebooks

folders

calculator

metric ruler

Caution

Rubbing alcohol cannot be made safe to drink. Students should be instructed not to place any of the alcohol in their mouths and to avoid prolonged contact with the skin. Remind students to wash their hands after handling any of the materials. For eye or skin contact, flush with water. For ingestion, seek appropriate medical attention. Use caution if liquids are spilled near electrical outlets.

About the Format

Each learning experience is numbered and titled. Under each title is the objective for the learning experience.

Each learning experience page has two parts. The first part lists materials, preparations, basic skill processes, evaluation strategy, and vocabulary. The evaluation strategy is for the teacher to use when judging the student’s understanding of the learning experience.

The second part begins with a “Focus Question” which is typed in italicized print. The purpose of the “Focus Question” is to guide the teacher’s instruction toward the main idea of the learning experience. The “Focus Question” is not to be answered by the students. The learning experience includes direction for students, illustrations, and discussion questions. These discussion questions can be used as a basis for class interaction.

An optional Student Assessment has been included for Grade 6 only.

Background Information

The learning experiences in the density kit are designed to help students confront some of their “understandings” through scientific inquiry. During the inquiry, it is important to focus students, through discussions, on the question of “What is density?” and perhaps “How does the property of density interact in our environment?”

The focus of inquiry should be on student-generated questions or explanations in the student’s own language rather than defining them in specialized scientific terms. There is much agreement in science literature that an early focus on scientific terms often short circuits the development of student understanding. However. specialized language is very important. It facilitates deeper understanding after one has developed ideas in their own terms. Scientific terms should be presented for labeling, consolidating, and extending understanding.

The *Properties *of an object are determined by its material and its condition.

Everything you see and touch is matter. You are made of matter. The air you breathe is made of matter. So what is matter? Matter is anything that takes up space, and may have the observable properties that include color, texture, shape, volume and hardness. However, just because you might not be able to see all types of matter does not mean they are not there. Think of a balloon. If you pumped air into a balloon you would be able to see the volume of the air by the way it stretches and shapes the balloon. You cannot see air, but it is made of matter.

Mass is a measure of the amount of matter in an object, while the weight of an object is a measure of the gravitational pull on the object. Generally, a balance scale of some sort is used to measure mass, and a scale is used to measure weight. Students use a spring scale to measure the force of gravity on the object’s mass, in Newton’s, and that force is the weight of the object.

Weight is the result of the effect of gravity on the mass that stretches or compresses a spring. Mass is measured by comparing the gravitational pull to a standard mass. Therefore, some sort of a balance scale is needed to measure mass. Weight will vary with gravitational force, while the mass will remain constant. Therefore, if you measure a weight on earth and then measure the weight on the moon, you would find a difference because the force of gravity is different on the moon. However, if you find the mass on earth or the moon, it will be the same.

To Summarize:

Mass:

Amount of matter in the object

Measured with a balance

Does not change based on the gravitational pull

Units are in grams and kg

Weight:

Measure of the gravitational pull on the object

Measured with a scale

Depends on and changes with gravitational pull

Units are in Newton’s (N)

Density is the measure of the “compactness” of a material. It is the ratio of mass to volume for any material. It is usually measured in grams per cubic centimeter (g/cc, g/cm³) and tells how much matter is packed into a given space. Density is not a simple comparison of the “heaviness” or “lightness” of materials. It is instead, a comparison of the “heaviness” or “lightness” of the same volume (mass per unit volume). The density of a material is determined by the masses of the atoms in the material and the amount of space between the atoms. Gases have a low density not only because the atoms making up the gases have a small mass, but also because there is a large amount of space between the atoms. The heavy metals like gold, lead, and uranium are very dense because the atoms they are composed of are massive and spaced closely together.

Water has a density of one gram per cubic centimeter (1 g/cm³) at 3.98 degrees Celsius, and water is the standard for comparing the density of materials. Materials with a density greater than one gram per cubic centimeter are denser than water and will sink in water; materials with a density that is less will float in water. Lead has a density of 11.3 grams per cubic centimeter (11.3 g/cm³) that tells us it is more than eleven times as dense as water. It also means that a hundred grams of lead would have eleven times less volume than a hundred grams of water (atoms in lead are much more closely packed together so they take up less space.) The densities of ordinary substances here on earth vary from the least dense, hydrogen gas, with a density of 0.000089 grams per cubic centimeter to the element osmium which, at 22 grams per cubic centimeter (22 g/ cm³), is only slightly more dense than gold or platinum.

To calculate density, we divide the mass of the material in grams by its volume in cubic centimeters. We might just as well divide the mass in kilograms by the volume in liters, or the mass in metric tons by the volume in cubic meters, the answer in each case would be numerically the same. The density of an object equals its mass divided by its volume (D = M/V).

Some Effects of Density

• The block of wood will float if the wood is less dense than water. The mass of the volume of water displaced by the floating block of wood is equal to the total mass of the block of wood. Alternatively, the volume of water that has the same mass as the block would occupy less volume than the block of wood.

• The metal cube sinks because the metal in it is denser than water. For equal volumes of metal and water, the metal’s mass is more than the mass of the water. Alternatively, for an equal mass of metal and water, the water would have the greater volume.

• Salt water vs. fresh water – salt water is denser than the fresh water because of the mass of the salt that has been dissolved in it. The greater density of salt water increases the buoyancy, or the upward force of the salt water, of objects placed in it. Liquids with a greater density have a greater internal pressure and therefore push upon objects with a greater buoyant force.

Volume is the amount of space occupied by a three-dimensional object as measured in cubic units, i.e., cubic centimeters, cubic meters, milliliters or liters.

It is important to note that when measuring the volume of a liquid, the person reading the volume must be eye-level with the liquid, or a false value will be obtained.

Displacement: An object floats if it displaces an amount of liquid equal to its own weight (otherwise it sinks).

In order for an object to be submerged in a liquid, the object must push aside or displace some of the liquid in which it is placed. This is called displacement. Volume of an object can be determined by displacement. Let’s say that 10 mL of liquid is placed in a graduated cylinder. If an object is dropped into the cylinder, the water level appears to go up, even though no more water was added to the cylinder. This is due to the fact that the object has “displaced” or pushed aside some of the liquid when it was put into the container. If the object is submerged, it will displace the object’s volume. Therefore, if there was 10 mL of liquid before the object was placed in the fluid and the measurement of the liquid in the cylinder is 14 mL after the object is placed in the fluid then subtracting 10 mL from 14 mL would equal the space that the object took up in the cylinder. Since the definition of volume is “the amount of space taken up by an object” it can be concluded that the volume of the object placed in the fluid is 4 mL.

• The block of wood floats because the water displaced by the portion of the block in the water equals the total weight of the block.

• The metal cube sinks because the “cube” of water it displaced weighs less than the cube of metal.

The empty glass cup floats because its weight and the air it contains is less than the weight of the water it displaced, that is, the water that occupied the space which the cup presently takes up in the water.

Gravity affects everything on earth. This includes bodies of air and water and the things in them. Gravity pulls the air or water and the things in them toward the center of the earth because the earth has the most mass of anything on earth. Only the solid surface beneath the air or water prevents them from being drawn further toward the center of the earth.

Gravity interacts with the double-pan balance. If one side of the balance has more mass than the other, that side will be attracted more toward the center of the earth. Plasticine has been provided to equalize the mass on each side of the balance arm. (Adding Plasticine to one side will accomplish this.) This process is called calibration or zeroing the balance.

In some situations, a buoyant force is able to overcome the force of gravity. The result is that those objects float. We recognize objects such as styrofoam and wood as being able to overcome the force of gravity when placed in water. The buoyant force of the water on the materials that are less dense than water overcome the force of gravity and, therefore, the object can float.

Archimedes Principle

A buoyant force is an upward force exerted on an object by a fluid in which the object is immersed. When an object is placed into a liquid, the object displaces some of the liquid. The volume of the liquid that is displaced is equal to the volume of the object. The amount of buoyant force is equal to the weight of the displaced fluid. Archimedes concluded that an object in a fluid is acted on by a force equal to the weight of the fluid displacing the object. This statement describes Archimedes Principle.

Densities

The following densities have been provided as a general reference. The actual density of each material may vary with its purity and condition. Densities have been rounded to the nearest hundredth. The density chart serves to identify which material will float in which liquid. For example, all materials with a density less than one will float in water. In the chart below, all material from vegetable oil to hydrogen will float in water. All materials from seawater to osmium will sink in water. Mercury is a liquid metal with a density of 13.57 g/cc. All materials with a density above 13.57g/cc will float in Mercury. Lead will float in Mercury but Gold will not.

Densities sorted by Material and Density

Category Material Density (g/cc or g/cm³)

Gas Hydrogen 0.00009

Wood Red Oak 0.67

Wood Sugar Maple 0.69

Chemical Gasoline 0.72

Other Ice 0.90

Other Paper 0.93

Liquid Rubbing Alcohol 0.76

Liquid Vegetable Oil 0.93

Liquid Water, 4 °C 1

Plastic Polyurethane 1

Liquid Water, Seawater 1.03

Mineral Coal, Coke 1.20

Mineral Coal, Anthracite 1.55

Ceramic Glass 2.6

Metal Aluminum 2.7

Metal Titanium 4.5

Metal Steel, tool 7.72

Metal Brass 8.55

Metal Tool Steel 8.75

Metal Copper, 8.9

Metal Nickel, 8.9

Metal Silver, 10.5

Metal Lead 11.37

Metal Mercury 13.57

Metal Uranium 18.74

Metal Tungsten 18.82

Metal Gold, Pure 19.32

Metal Platinum 21.3

Metal Osmium 22.0

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