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.
Tell students that the surface of water in a tube may not be completely flat. Instead, the surface may curve in a shallow U-shape called the meniscus. When measuring, read the line just at the bottom of the meniscus. Tilt the graduated cylinder and slowly slide the sample into the water. Hold the graduated cylinder upright. Record the level of the water. 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. Find the amount of water displaced by subtracting the initial level of the water from the final level. This volume equals the volume of the cylinder in cm 3. 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. 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 about 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. 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.
Project the image Polyvinyl Chloride medium-length rod. Polyvinyl chloride is made up of carbon, hydrogen, and chlorine atoms. If you compare polyvinyl chloride to polyethylene, you will notice that there are chlorine atoms in some places where there are hydrogen atoms in the polyethylene. In the chart, chlorine has a large mass for its size. This helps make polyvinyl chloride more dense than polyethylene.
The density of different plastics is usually caused by the different atoms that can be connected to the carbon—hydrogen chains. Density readings will be approximate--variations in temperature and atmospheric pressure will have slight effects on the density of the water used to compute the density for your object. An avid perennial gardener and old house owner, Laura Reynolds has had careers in teaching and juvenile justice. A retired municipal judgem Reynolds holds a degree in communications from Northern Illinois University.
Her six children and stepchildren served as subjects of editorials during her tenure as a local newspaper editor. Things You'll Need. How to Measure the Density of a Person. How to Calculate Weight of Plastic. How to Calculate the Volume of a Person. How to Find Density.
Fun Archimedes Principle Experiments. How to Calculate the Density of Plastic. What Is Low Density? How to Demonstrate Surface Tension with a Paperclip Why Rubber Floats in Water. If both hulls are square or rectangular, you can measure the length, width and height of each and multiply these dimensions together to get its total volume. If parts of the hull have an irregular shape, measure the volume piece-wise and then add these volumes together.
Use triangles to approximate any areas of the hull that are curved or angled. What is the volume of each hull? Write this down. To do this, carefully fill each hull with dry rice so that the rice is level with the top of the hull. Being careful not to damage the hull, transfer the dry rice into a measuring cup. W hat is the volume of each hull using the rice?
To prevent the hull from tipping, carefully balance the load as you add pennies left to right, front to back—or port to starboard, fore to aft, if you're feeling nautical. Dump any excess water back into the container. How many pennies could it support? Write this number down. Be sure to only add dry pennies. Why do you think using dry pennies instead of wet ones is important? Convert it if necessary and write it down. Cubic centimeters are the same as milliliters, or mL.
To do this, multiply the number of pennies by 2. How many grams could each hull support? Write this number down for each hull. This roughly gives you the hull's density.
What was the density of each hull right before sinking? How do you think this relates to the density of water? Do you get the same results if you use aluminum foil boat hulls that have a different shape? Do you see a pattern in your results? Thinking about what factors affected your calculation, develop a way to more accurately determine the density of water, such as by including the weight of the boat hulls, adding something smaller than pennies to measure how much weight the hulls can carry, and more accurately determining the volume of the hulls.
How close can you get your calculations to the actual density of water? Check with an adult to make sure it is alright if you use household liquids for your activity.
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