The following activities and demonstrations are meant to be incorporated into the unit, as appropriate, to provide students with hands-on experiences of the core science content in the unit (properties of matter, compounds, and mixtures). These activities should be adapted to fit either the “Explore” section or the “Elaborate” section of the 5-Es instructional model. Activities may be adapted into demonstrations for the “Engage” section as well.
Chemical and Physical Properties
Crime Scene Analysis – Pre-Assessment
Students will be able to use properties of matter to identify materials and substances found at a crime scene. In this pre-assessment activity students will receive a list of common materials and a list of their properties, e.g. density, solubility in water, magnetic attraction, color, melting point. Students will read about a crime that occurred and what was found at the scene, along with a report about the properties of the trace evidence. They will then work in groups to compile a list or graphic organizer of all of the ways in which they could identify the evidence.
Density of liquids/objects
Students will identify materials using their densities. Students will create a density column with different liquids (water, corn syrup, oil, rubbing alcohol). They will drop in different solid objects (beads, woods, glass, etc.) to estimate the densities of those objects by comparing them to a provided list of the liquid densities. Students will collect data and draw conclusions about the densities of the various objects.
Solubility
Students will determine which substances are soluble in water and which are not (practice measuring small volumes of liquid, using triple beam balance to measure mass of salt, sugar, baking soda, etc.) with the aim of identifying the white power found at the crime scene. Students can also explore the solubility of polymers by testing the solubility of polystyrene (Styrofoam) in acetone (nail polish remover) and comparing that to the solubility of polyethylene (common plastic).
Density of glass
Instruments for measuring tiny fragments of glass are expensive, but students can replicate glass analysis on a larger scale with different types of large glass pieces (e.g.. jars). Students can compare the densities of different types of glass.
Exploring refractive index
Students can explore how light is bent when passing from water to air by placing a pencil in a glass of water and observing it from the side. The pencil will appear to be broken at the point where it enters the water. This is due to the fact that the light reflected from the pencil that comes through the water is bent when it enters the air. As a result, the submerged portion of the pencil seems to be dislocated from the upper portion. (Crime Lab 101 Robert Gardner) Students can replicate the activity with a glass dropper – have them insert an empty glass dropper into the oil, and then have them compare that to when the dropper is full of oil.
Refractive Index
Students can dip transparent plastic tape into clear or light-colored cooking oil. Because the clear tape refracts light in almost the same exact way as the oil, it will disappear when submerged. (Crime Lab 101 Robert Gardner) Students will get a couple of different types of tape and/or transparent objects (e.g. marbles), and will identify the tape that has the same refractivity as the liquid. Students can also complete this activity using absorbing water beads and marbles.
Hair and fiber analysis
Students will analyze hairs and fibers under a microscope and compare them to a hair and fiber found at the scene of a crime. The teacher will have a group of volunteers provide one piece of hair. The teacher will choose one of those hairs and mark it as the crime scene evidence. The class will use microscopes to compare hairs from the volunteer group to the crime scene evidence to see if they can figure out to whom the hair belongs.
Boiling Point
Why do liquids have different boiling points? Boiling point depends on the intermolecular forces present between atoms or molecules in a liquid, and these forces must be interrupted when a liquid changes to a gas. Magnets are a great way for students to visualize and experience this concept. Get magnets of various sizes/strengths. Have students determine which magnets have a stronger force between them and which have a weaker force between them. As a class, compare the magnets to atoms/molecules in a liquid and your hands to the heat added to a liquid. Which liquid would have a higher boiling point and why? Which would have a lower boiling point and why? Students will see that the stronger the forces between the atoms/molecules, the more heat must be added in order to disrupt those forces. This demo/activity can be supplemented with the PHET interactive simulation on states of matter found online at https://phet.colorado.edu/en/simulation/states-of-matter.
Flammability
Dip a dollar bill into a 1:1 solution of isopropyl alcohol and water. Ignite the bill with a match. The dollar will catch on fire for roughly 10 seconds and then the flame will fade. Explain to students that although the two liquids look similar, they have different properties. One of those properties is flammability. Isopropyl alcohol is flammable, while water is not. The isopropyl alcohol burns, creating a flame. Water has a high boiling point (another physical property) due to its high specific heat; the heat from the burning isopropyl alcohol is not high enough to evaporate the water, so the water remains soaked into the bill keeping it protected from the flame.
Crime Scene Analysis –Post-Assessment
As a post-assessment for the unit, students will receive the same pile of “evidence” provided at the start of the unit, and they will come up with all of the ways they can use to describe it. Using their knowledge about properties of matter and how they are measured, they can create a list of all the ways they can describe the materials. As an alternative, they can create a graphic organizer of how they would identify the objects and substances.
Atoms, Elements, and Compounds
Elements
Students will research common materials or substances used in crimes, identify the elements that make up those materials, and use art supplies (modeling clay, pipe cleaners, wire, beads, etc.) to create physical models of their atoms. Students will create a “Wanted” poster for their atom to describe how it looks (protons, neutrons, electrons), where it is likely to be found in the Earth, as well as a list of properties that can be used to identify it. A great resource for this activity is an online interactive periodic table of elements found at http://www.ptable.com/.
Mixtures
Separating a mixture
Students will use properties of matter to separate a mixture of iron filings, sand, and salt. The mixture will be “crime scene evidence” and students must accurately report the mass of each component of the mixture for a trial. Students will work in groups to determine the best way to separate the mixture. To make it more engaging, the group that gets closest to the initial mass of each component is the group that was able to convict the suspect in the crime!
Paper Chromatography
Students will analyze the colors hidden in coloring markers using paper chromatography. Set up a scene for the students: “A note was found at a crime scene written in marker. It is your job to identify which marker was used to write the note, as that information will help to make a conviction.” Students will compare the patterns of different markers using paper chromatography to identify the marker used in the crime. Have the students draw a dot for each marker they are testing at the bottom of the paper. Hang the paper in water (making sure the marker ink does not touch the water) and give it enough time for the water to rise up the paper.
Blood as a mixture
Making blood – Students will create fake blood (water, oil, food coloring) and observe its properties as a mixture. Students can shake the fake blood in a bottle, and then let it sit. Ask them to explain what is causing the components of the blood to separate (density). Students can also complete a similar activity by using a centrifuge to separate fake blood into its individual components. This lab activity is available at: https://www.teachengineering.org/activities/view/gat_mixture_activity1
DNA
This activity fits nicely with the forensic theme. Provide students with an introduction to DNA and how it is used to solve crimes. Explain to students that they will be extracting the DNA from a strawberry, and that they will have to use properties of matter in order to separate the DNA from a mixture. In this activity, a coffee filter is used to separate DNA from the remaining parts of the cell based on its size. Cold isopropyl alcohol is used to cause the DNA to precipitate out of the solution. Review the concept of solubility with students during the activity. The procedure is outlined below:
-
Place one strawberry in a Ziploc bag.
-
Smash the strawberry using your fingers for 2 minutes to break open the cell walls. Careful not to break the bag.
-
Add 10 ml of DNA extraction solution ( to the bag to break up proteins and break through the cell membrane and the nucleus membrane. (Note: extraction solution consists of 90 ml water, 10 ml dish soap, and ¼ tsp salt)
-
Mush the strawberry in the bag again for 1 minute.
-
Place a coffee filter over the plastic cup. Pour the strawberry mixture into the filter and let it drip into the plastic cup to separate the DNA from the rest of the cell.
-
Slowly pour cold rubbing alcohol into the cup to take out the DNA from the solution. Observe!
-
Dip your pipette into the plastic cup to move around the DNA and make further observation.