In this lesson, students will make measurements, calculations, and inferences to help solve a mystery about the identity of an unknown metal. After being presented with a scenario about a fire along with some “evidence” from the fire, students will be challenged to determine the temperature of the fire. They will calculate the density of various metals and then use the periodic table to identify them. They will then compare melting points of the metals and use this information to infer the temperature of the fire.
NGSS and Cross-Disciplinary Extensions addressed in this lesson.
By the end of this lesson, students should be able to:
- Measure the weight or mass of a substance.
- Measure the volume of a substance using the water displacement method.
- Calculate the density of a substance.
- Use the periodic table to identify a substance based on its density.
- Gather information from the periodic table.
- Infer the temperature of a fire based on knowledge of the melting point of metals.
This lesson supports students’ understanding of the following topics in chemistry:
- Melting point
Teacher Preparation: First use of curriculum, 1 hour; subsequent years, 10 minutes.
(Note: The amount of time required for each part of the lesson will depend on your students’ prior knowledge and on how they choose to present their results.)
Engage: 20 minutes
Explore 1: 10-45 minutes
Explore 2: 10-20 minutes
Explore 3: 30-45 minutes
Explore 4: 30 minutes
Explore 5: 20-30 minutes
Explain: 60 minutes
- Metal strips: iron, zinc, tin, aluminum, lead, nickel, and copper (can be ordered from a science supply company)
- Propane torch
- Firefighter (s) (or one or more people acting as firefighters)
- Metal strips, some melted, some not
- Melted: zinc, tin, aluminum, and lead
- Not melted: iron, nickel, copper
- Mystery metals Receipt
- Metal strips from Engage
- Periodic Table that contains information on density and melting point
- Optional: objects of various densities
- Scale or balance
- Graduated cylinder
- Pencil, colored pencils
- Graph paper or Science Journal
- Preparation: Use caution when using the propane torch to melt the metal samples. Metal can sometime “sputter” causing small amounts to jump from the vicinity of the torch that can burn skin or material surrounding the work area. Leather gloves, shirts with long sleeves, and safety goggles should be worn.
- Have students wash their hands after handling the metal strips.
- Although this activity is specifically designed with no safety issues, activities like this provide an opportunity to introduce lab safety to students. Forensic specialists make a practice of using safety goggles, rubber gloves, and rubber aprons.
- Physical change
- Melting Point
periodic table, element, metal, density, melting point, properties, forensics
- Order strips of iron, zinc, tin, aluminum, lead, nickel, and copper from a science supply company.
- Using a propane torch, melt the zinc, tin, aluminum, and lead strips (separately) in an iron bowl. Allow to cool. (After you have prepared the necessary samples, you can store them for follow on years.)
Logistics and Tips
- This activity is easily adaptable to fit the number of metal samples you have. You can conduct the activity with a single set of metals (dividing the single set among four groups—two samples each) or provide a full set of samples to each small group to analyze.
- This activity is best done after students have some familiarity with the concepts of mass, volume, density, and melting point. However, it can also be used as an introduction to these concepts. If the ideas and methods are new to your students, plan to spend more time guiding them through each step of the Explore activity.
- It is not necessary for students to understand the concept of an element yet. However, if they do, make it clear that the metals they are investigating are pure elements. This is why the periodic table of elements can be used to identify them based on density. (If the metals were compounds or mixtures, the periodic table would not help.)
- When measuring volume using water displacement, have students tie a piece of thread around the end of each metal sample and dip it into the water by the thread. The purpose of this is to allow the students to submerge the sample and then pull it out easily.
- The lesson can be simplified by
- bypassing exploration and use of the periodic table and simply giving students the simple table of properties provided below.
- reducing the number of types of metal strips examined. Using even just a single metal can help narrow down the temperature of the fire.
- telling students directly what to do, rather than guiding them to figure it out themselves
- Students who have experience calculating density should be encouraged to help plan the investigation.
In this activity, students identify metals based on density. Density is an intrinsic property of a material: it is the same regardless of the amount of material present. Density is a measure of the amount of matter in a given unit of volume. In general, density is measured in grams per cubic centimeter (g/cm3) and is calculated by dividing mass by volume. The densities of the metals examined in this lesson are given in the table below.
Measuring Volume: Water Displacement Method
The mass of a sample is easily measured using a scale or balance. Volume, however, can be trickier if the sample is not a regular geometric shape. In this activity, students use the water displacement method:
- a) Fill a graduated cylinder with water ½ to 2/3 full, to an even marking on the cylinder. Record the level of the water.
- b) Place the sample in the cylinder. Measure and record the new level of the water.
- c) Subtract the first reading from the second reading. This is the volume of the object.
Note that volume in a graduated cylinder is generally given in milliliters (mL). One milliliter is the same as one cubic centimeter (cm3).
It is important to make sure that students understand why this method works: the object is displacing a volume of water that is equal to its own volume. The total volume in the cylinder is the starting volume of the water plus the volume of the object.
|Sample||Volume of Water||Volume of Water + Sample||Volume of Sample|
|A||20 mL||24 mL||
24mL – 20 mL
= 4 mL
= 4 cm3