Note: This video is designed to help the teacher better understand the lesson and is NOT intended to be shown to students. It includes observations and conclusions that students are meant to make on their own.
Students use the water displacement method to find the volume of different rods that all have the same mass. They calculate the density of each rod and use the characteristic density of each material to identify all five rods. Then students consider the relationship between the mass, size, and arrangement of atoms to explain why different rods have different densities. Students will be briefly introduced to the periodic table.
Students will be able to explain that materials have characteristic densities because of the different mass, size, and arrangement of their atoms. Students will be able to use the volume displacement method to find the volume of an object.
Be sure you and the students wear properly fitting goggles.
For this lesson you will need a set of five solid rods, each with the same mass, same diameter, but a different volume. Each rod is made of a different material. There are several versions of these rods available from different suppliers. This activity uses the Equal Mass Kit from Flinn Scientific but can be adapted to any set of equal mass rods. Since there are only five samples in the Equal Mass kit, you may need two kits so that each group can work with a sample.
This chart will help you identify each rod. Do not reveal this information to the students. They will discover the identity of each rod and the inverse relationship between the density and the length of each rod later in this lesson.
Approximate density (g/cm 3 )
Relative length
Shiny gray metal
Get the entire lesson plan and Student Activity Sheet for "Lesson 3.2 - Finding Volume: The Water Displacement Method."
Supplement in-class learning with interactive, multimedia-rich Google Forms lesson modules, perfect for reinforcing key chemistry concepts and scientific investigation skills.
Show students the five rods and explain that they all have the same mass. Then hold up the longest, middle-sized, and shortest rods and remind students that they have the same mass.
Ask students to make a prediction:
Students may reason that since the mass of each rod is the same, the volume of each rod must have something to do with its density. Some may go so far as to say that the rod with the smallest volume must have the highest density, because the same mass is packed into the smallest volume. Or that the rod with the largest volume must have the lowest density, because the same mass is spread out over the largest volume.
Tell students that like the cubes in the previous activity, they will need to know the volume and mass of each of the samples. They will also calculate the density of each sample and use this value to figure out which material each rod is made from which material.
Project the animation Water Displacement.
Play the animation as you demonstrate the water displacement method using a cup of water, a graduated cylinder, and a rod, the way students will do in the activity. Use the dark gray plastic sample so that students can see it better.
Students may be confused that the unit for volume in the graduated cylinder is milliliters (mL), when in the previous lesson students calculated volume in cubic centimeters (cm 3 ). Explain to students that 1 ml is the same as 1 cm 3 . Click on the oval-shaped button on the first screen of the animation marked “1 mL = 1 cm 3 .”
Student groups will not need to measure the mass of the rods. The mass of each rod is the same, 15 grams, and is given in their chart on the activity sheet. They will need to measure the volume of each of the five different rods and calculate their densities. Students will use their values for density to identify each rod.
Demonstrate how to calculate density (D = m/v) by dividing the mass by the volume. Point out that that the answer will be in grams per cubic centimeter (g/cm 3 ).
Give each student an activity sheet.
Download the student activity sheet, and distribute one per student.
The activity sheet will serve as the “Evaluate” component of each 5-E lesson plan. The activity sheets are formative assessments of student progress and understanding. A more formal summative assessment is included at the end of each chapter.
Students will record their observations and answer questions about the activity on the activity sheet. The Explain It with Atoms and Molecules and Take It Further sections of the activity sheet will either be completed as a class, in groups, or individually, depending on your instructions. Look at the teacher version of the activity sheet to find the questions and answers.
Give students time to answer questions 1–5 on the activity sheet before starting the activity.
Note: The densities for the three plastics are similar, so students need to be very
careful when measuring their volume using the water displacement method. Also, it is difficult to measure the volume of the smallest rod. Give students a hint that it is between 1.5 and 2.0 mL.
Question to investigate
Can you use density to identify all five rods?
Materials for each group
Teacher preparation
Volume
Density
Initial water level (mL)
Final water level (mL)
Volume of the rods (cm 3 )
Density (g/cm 3 )
Identify the samples
Note: The densities students calculate may not be exactly the same as the given densities in the chart. As students are working, check their values for volume to be sure that they are using the difference between the final and initial water levels, not just the final level.
Approximate density (g/cm 3 )
Sample (Letters A–E)
Discuss student values for density for each of the samples. Point out that different groups may have different values for density, but that most of the values are close to the values in the chart.
Remind students that in the beginning of the lesson they made a prediction about the density of the small, medium, and long sample. Students should have predicted that the longest cylinder has the lowest density, the shortest cylinder has the highest density, and the middle is somewhere in between.
Project the image Atomic Size and Mass.
Tell students that this chart is based on the periodic table of the elements but that it only includes the first 20 elements out of 118. A representation of an atom for each element is shown. For each element, the atomic number is above the atom and the atomic mass is below. This chart is special because it shows both the size and mass of atoms compared to other atoms.
Note: Students may want to know more about why atoms have different atomic numbers and different sizes. These questions will be covered in later chapters but you can tell them that the atomic number is the number of protons in the center or nucleus of the atom. Each element has a certain number of protons in its atoms, so each element has a different atomic number. The difference in size is a little harder to explain. Atoms have positively charged protons in the nucleus and negatively charged electrons moving around the nucleus. It’s really the space the electrons occupy that makes up most of the size of the atom. As the number of protons in the atom increases, both its mass and the strength of its positive charge increases. This extra positive charge pulls electrons closer to the nucleus, making the atom smaller. The atoms get bigger again in the next row because more electrons are added in a space (energy level) further from the nucleus.
Let students know that they will learn more about the periodic table and atoms in Chapter 4. For now, all students need to focus on is the size and mass of the atoms.
Tell students that the difference in density between the small, medium, and large samples that they measured can be explained based on the atoms and molecules they are made from.
Project the image Polyethylene (longest rod).
Polyethylene is made of long molecules of only carbon and hydrogen atoms. In the Atomic Size and Mass chart, the mass of carbon is pretty low, and the mass of hydrogen is the lowest of all the atoms. These low masses help explain why polyethylene has a low density. Another reason is that these long, skinny molecules are loosely packed together.
