My freshman Physical Science classes just completed a lab on density. Density is the mass per unit volume of a substance. My students describe it as how closely the particles in a substance are packed together. If the particles are very close, the substance has a fairly high density. Density is calculated by using the following formula: Density = mass divided by volume. The correct unit for density is g/cm3.
Ms. Rose shows the formula for density.
My students used polystyrene cups in this experiment....WHY would we use polystyrene? I'll get to that in a moment. Most people call this type of insulated foam cup a "Styrofoam" cup. Styrofoam is a brand name...the Dow Chemical Company invented this type of foam cup and introduced it to the United States in 1954. Now, many companies make the product so it is no longer ONLY a "Styrofoam" product. It should be referred to as a polystyrene cup! Polystyrene is a strong plastic created from erethylene and benzine that can be injected, extruded, or blow molded. This makes it a very useful and versatile manufacturing material. As with all plastics, this should be recycled!
So, WHY did I choose polystyrene cups for my students to use in this experiment?? Polystyrene has many pockets of air that help this material to be a great insulator. Exposing polystyrene to depths in the ocean and pressure changes will cause a change in the material. My students collected initial data on their polystyrene cups. They measured the diameter of the top of their cup, the bottom of their cup and the height of their cup. They also calculated the density of their cup. These measurements will be repeated when I return with their polystyrene cup samples in January. My students will be able to determine if exposure to depths in the ocean and pressure changes will have an impact on their polystyrene cups.
Ryan and Devon measuring their polystyrene cup.
So...let's talk about density!! The first thing you need to obtain when calculating density is mass. The mass is the amount of matter in a substance. It is measured using a triple beam balance and the correct unit to measure with is grams.
Jordan using the triple beam balance to find the mass of her polystyrene cup.
Next, you need to obtain the volume. This was a little bit tricky to do using the materials and equipment in our science lab. When objects sink in water, they cause a rise in the water equal to the volume of the object. This is called displacement and it is the method we used to find the cup's volume.
We put 500 mL of water in a 1000 mL graduated cylinder. When we added the polystyrene cup to the graduated cylinder, it floated. Remember, it has all of those air pockets that make it a great insulator! My students had to add two 20 gram masses to sink the polystyrene cup. To obtain the volume of the cup, you have to take the Final water level in the graduated cylinder and SUBTRACT the water level that you started with. You also have to SUBTRACT the amount of water that the two 20 gram masses displaced. In our experiment, that was 6 ml.
Colin finding the volume of his polystyrene cup using the displacement of water in a 1000 mL graduated cylinder.
Once you have the mass and volume of the cup, you can calculate density by dividing the mass by volume. The average density of polystyrene cups as calculated by my students was .075 g/cm3. I am EXCITED to see what the density of these cups will be once I return with them in January!!
Jake adding the density of his polystyrene cup to the Class Data Table.
New Vocabulary
Density: the mass per unit volume of a substance.
Polystyrene: a strong plastic created from erethylene and benzine that can be injected, extruded, or blow molded.
Mass: the amount of matter in a substance.
Question of the Day
What changes do you think will occur to our polystyrene cups when they are submerged in the Bering, Beaufort and Chukchi Seas?
Stay tuned to this journal as prepare to embark on this very exciting expedition! Until then…
“Live as if you were to die tomorrow. Learn as if you were to live forever.” Mohandas Gandhi