Summary

In this lab, students cast tin into molds and observe physical changes of metals while creating an ornament. They will apply their knowledge of metallic bonding to analyze and explain their observations.

Grade Level

High School

NGSS Alignment

This lab will help prepare your students to meet the performance expectations in the following standards:

• HS-PS1-1: Use the periodic table as a model to predict the relative properties of elements based on the patterns of electrons in the outermost energy level of atoms.
• HS-PS1-3: Plan and conduct an investigation to gather evidence to compare the structure of substances at the bulk scale to infer the strength of electrical forces between particles.
• HS-PS2-6: Communicate scientific and technical information about why the molecular-level structure is important in the functioning of designed materials.
• Scientific and Engineering Practices:
  • Asking Questions and Defining Problems
  • Developing and Using Models

Objectives

By the end of this lab, students should be able to:

• Describe metallic bonding and properties of metals.
• Describe casting of metals.
• Compare the strength of metallic bonding of different metals.

Chemistry Topics

This lab supports students’ understanding of:

• Molecules and Bonding
• Metallic bonding
• Physical Change
• Physical Properties

Time

Teacher Preparation: 15 minutes (note that the first time using the lab requires about an hour to prepare the casts, and must be completed 1–2 days in advance)
Lesson: 60–120 minutes (depending on number of students and supplies)

Materials

• Lab Preparation
  • Plaster of Paris
  • Plastic weigh boats, ∼3-inch square (medium size)
  • Disposable stirring stick (wooden splint or similar)
  • Disposable cup (sturdy plastic type is best, 8 oz or larger)
  • Water
  • Silicon candy molds of desired shapes (holiday or other, about 1 inch tall)
• Lab Day Supplies
  • Tin (bulk metal) and any other desired metals depending on direction (Tin obtained online from MS Metal Shipper)
  • Crucible tongs
  • Glass stirring rod or small metal spatula
  • Hot pot (Brand: Do-It Molds Hot Pot-2)
  • Optional: ornament hooks (paper clips or wire)

Safety

• Students should always wear safety goggles when handling chemicals in the lab.
• Students should wear hot gloves and a lab coat or apron during this activity.
• Long hair should be tied back. No loose or baggy clothing or jewelry is permitted.
• Exercise caution when using a heat source. Hot plates/pots should be turned off and unplugged as soon as they are no longer needed.
• Materials will be hot!  Take caution before handling equipment or materials.
• Students should wash their hands thoroughly before leaving the lab.
• Provide instructions for cleanup of materials and disposal of chemicals.

Teacher Notes

• I use this lab as a ‘holiday fun’ lab for my students. It gives them the opportunity to see molten metal and investigate a topic that we wouldn’t do in our typical chemistry curriculum. They also get to make a neat little ornament.
• Casting is the process in which a metal is melted into a liquid and poured into a mold to create a desired shape. It is an ancient process used for many applications of metal-forming.
• Tin is used for this experiment because it melts at well above room temperature but within the reach of easily available equipment (about 232 °C) and is relatively unreactive. 
• Metallic bonding is the attraction of metal cations to the sea of shared electrons in the sample.  These forces are weakened upon heating and allow a liquid to form, and upon cooling, a ‘skin’ can be seen forming on the surface of the metal as energy is lost and the forces pull the metal ions back into structure. Tin forms a diamond cubic structure as a solid, however, is a ‘softer’ metal, and if you have a pencil-size rod of tin, it can be bent by hand.  It will also “cry” and make audible sounds as the lattice rearranges itself. 
• There are many ways to incorporate this lab, depending on your curriculum and students. Several suggestions for content extensions include:
  • Compare the melting point of tin to other metals: It has a higher melting point due to stronger metallic bonds, more electrons in the valence level and a higher cation charge.  This idea could be predicted, researched, or modeled. 
  • Test other metallic properties of tin, such as conductivity, solubility, or malleability.
  • Discuss or Investigate oxides: When tin is heated, an oxide can form. The properties of tin versus the oxide could be discussed or investigated.
  • A comparison of properties between metallic, ionic and covalent substances could be performed.

First-Time Lab Set Up:

• First, the molds to be used for the casting part of the lab need to be made in advance. But once they are made, they can be reused for years to come. 
• If you are ambitious, you can let students create the molds in advance of the lab day, but if not, just make them yourself. They are quick to make but need to dry for a day or two before using in this lab.
• Directions for creating the molds:
  • In a plastic cup or another disposable container, combine about one-half to one cup of Plaster of Paris with a small amount of water.  
  • Mix with a disposable stirring stick until a thick consistency is achieved, like toothpaste.
  • Add more water or plaster as needed. You are aiming to create enough to fill one weigh boat, so the amounts can be scaled as necessary.
  • Transfer the mixture into a weigh boat and fill close to the top (see photo).
  • Take a silicone candy mold and press it into the plaster, without getting plaster on top of the silicon. (If your candy molds came in a “sheet”, cut one out of the sheet to use.)
  • I use molds that include a few holiday shapes like a Christmas tree, candy cane, or a star, but many students like a smiley face, music notes, heart, or other fun shapes!
  • Try to level off the plaster in the weighing boat so the surface is flat.
  • Allow to dry for one or two days, and then remove silicon mold (refer to the photo for a before and after view).

Equipment, Materials, and Logistics:

• The tin can be purchased online in bulk. In my class, a pound of tin can be used for about 10–15 students, depending on the size of the molds and how full they are filled. More is better!
• During this activity, I typically invite half of the students into the lab space at a time, while having the other students work on an activity or problem set elsewhere. I use one lab table and set up two hot pots. I allow four students at the table at a time. Two students melt their sample in the hot pot, while the other two watch. I rotate students through this process, and this lets me closely supervise. While more stations could be faster, more safety equipment and supervision would be needed.
• While we are getting set up, I discuss metallic bonding with the students at the station and point out my observations of the metal as it changes. Students don’t have time to take observations on their own while doing the procedure but could write a reflection or draw a model afterward.
• I have a set of ∼10 molds available for students to choose from (they can be reused again as soon as the metal is removed). For popular shapes, I have two of each of those molds available.
• The hot pots take a little while to get hot, plugging them in a little early while you give instructions helps move things along.
• Casting the metal makes a metal form, like a small paperweight. While the metal is molten, an ornament hook (or another hook-type metal form) can be sunk into the shape to create an ornament that can be hung. This works best when the hook is bent twice (two right-angle bends) so the hook can be lowered into the metal flat and then will hang vertically (see photo below).
• It’s helpful to have an extra mold nearby for any extra molten metal. This helps save any extra from becoming oxidized and can be reused.
• Small traces of metal left in the pot will oxidize and form a ‘slag’. This can be scraped out with a glass rod or spatula and discarded between students.
• When pouring the hot tin into the mold make sure to tell students to keep the hot pot above the mold so that it doesn’t touch the weigh boat plastic. Teachers can demonstrate this to students (see photo).

• The casted metal solidifies pretty quickly, within ∼5 minutes it can be picked up by the hook (that’s a conservative estimate). However, it will be hot! Having a hot pad available for cooling or a ring stand with clamp to hang them on can be helpful.
• If the casted metals do not have hooks, the molds can be flipped over, and the metal shape will usually fall out pretty easily. A few gentle knocks of the mold against the table can be used for tricky ones.