Crystals Facilitator Guide Introduction to Crystals

The activities in this unit all are model based, so the equipment needed will just be computers with internet access.  Using the activities will work best if students have their own computer to work individually and then after data collection is completed by all the students working as a group to debrief.  If students haven't used the modeling programs before taking sometime to project the models on the board so that the various button functions can be shared with the students.  This may prevent some frustration for some students.

The activities are model based. In "Introduction to Crystals" students should be sure to stop the model and take Snapshots to show what happened in each test they complete.

There are 2 activities in this unit.  To complete both will take 3-4 45-minute periods to complete both.  If you have block periods (90 minutes or more) students should be able to complete the activities in 2 days.

In this unit students investigate crystals--their structure and properties.  Crystals are a common form of a solid.  They are composed of repeated patterns of molecules held together by different kinds of bonds.  In some solids the arrangement of atoms and molecules can be random or can vary in the material.  In crystals the arrangement atoms and molecules is exactly repeated.  Students will understand crystal forms by using models to investigate their structure at the atomic and molecular level. They will experiment with Molecular Workbench models to learn how temperature affects crystals and the phenomenon of melting.

In the activity "Introduction to Crystals" students will use a Molecular Workbench model.  They will use this model to investigate the question, "What is a molecular crystal?".  In the activity they will test what happens when the atoms making up a crystal are heated or cooled and when atoms are added to a crystal.

At the middle school level, students may have a difficult time understanding the very small size of particles.  They may attribute macroscopic properties to microscopic particles.  The motion of atoms at in solids, liquids, and gases are difficult for students to appreciate.  Understanding that there is space between particles at the smallest level is important for students to understand as they do the activities in this unit.

NGSS
Structure and Properties of Mater
MS-PS1-1. Develop models to describe the atomic composition of simple molecules and extended structures.

MS-PS1-4. Develop a model that predicts and describes changes in particle motion, temperature, and state of a pure substance when thermal energy is added or removed.

NSES
Physical Science – Properties and Changes of Matter
A substance has characteristic properties, such as density, a boiling point, and solubility, all of which are independent of the amount of the sample. A mixture of substances often can be separated into the original substances using one or more of the characteristic properties.

Physical Science – Structure and properties of matter
A substance composed of a single kind of atom is called an element. The atoms may be bonded together into molecules or crystalline solids.

Physical Science – Structure and properties of matter
Bonds between atoms are created when electrons are paired up by being transferred or shared. A substance composed of a single kind of atom is called an element. The atoms may be bonded together into molecules or crystalline solids.

Q: Have you ever seen or collected crystals?  What can you tell us about the crystals you have seen?
A: Answers may vary.  If you have quartz crystals or if students bring them in, ask them to describe the structure.  Classic quartz crystals are 6-sided (hexagonal) prisms with a 6-sided prism at the ends.

Q: If we were to break the crystal into smaller parts would the same structure still occur? If you did this you would see the same structure through out its structure?
A: The internal structure would still be the same, though what you would see on the outside would look different.

Q: What are some other common crystals that you have seen?
A: Some that they should be familiar with are salt, sugar, and ice. Each of these its own unique crystal structure.  Salt crystals (sodium chloride) are cubes. Sugar crystals are more of a hexagonal prism.  Ice crystal (water) are hexagonal prisms.

Q: Name the phases of matter and how would you describe the properties of each phase?
A: The three phases they should be aware of at this point are solid, liquid, and gas.  A solid holds it's shape a given temperature range.  Water becomes a solid at  0° C (32° F).  A liquid doesn't have a definite shape but does have a specific volume. At sea level water will be in a liquid state from 0° C (32° F) to 100° C (212° F).  Over these temperatures water at sea level becomes gas.  A gas is formed when the movement of molecules is random and the substance has no fixed shape and or volume and will take up what ever space is available.

Introduction to Crystals:
Q:  Would you able to make the atoms or molecules in a crystal stop vibrating?
A: No. There is always movement of the atoms and molecules in matter including solid matter.  In theory, if you could reach absolute zero (-273.15° C or 0° Kelvin) all molecular motion would stop.

Q: In the model there are small spaces between the atoms, are there really empty spaces in a matter?
A: Yes, even in solids there are spaces in the material that are empty.

Q: In a real world situation will the crystal if heated and then cooled reform in exactly the same way?
A: They may or may not.  This depends on temperature, pressure, and the type of atoms and molecules that make the crystal.

Introduction to Crystals:
Q:  What have you discovered about the way matter behaves when they are heated?  When they are cooled?
A: When they are heated the atoms or molecules move at a more rapid pace.  When heated enough the atoms and molecules will start to move away from each other in a rapid and random way.  When cooled the atoms and molecules slow down and may reattach to form a solid.

Crystals are an important part of the world we live in.  A crystal is a group of atoms, molecules, or ions arranged in ordered pattern in all three dimensions.  Crystals can be both organic or inorganic.  Examples of organic crystals are foods like sugar, and the calcite crystals mollusks produce when forming their shells. Examples of inorganic crystals include quartz, sapphires, etc.

Some crystals form perfect patterns of molecules. For example, a diamond that is pure carbon will be clear without color.  Some crystals, however, may form with impurities or become flawed after formation.  Normal diamonds are pure carbon, but sometimes atoms of boron will mix with carbon which produces a diamond with a blue tint.  Scratches on the surface are another type of flaw.

Introduction to Crystals

Q: What do you notice about how particles move in a crystal?
A: The particles vibrate randomly within the crystal, "bumping" against each other.  When the crystals start to melt the movement speeds up and continues to be random.

Q: What is the first sign of a crystal melting?
A: The particles around the edge begin to flow.

Q: What can you conclude about the attractions among atoms and how they contribute to crystal growth?
A: Atoms appear to be attracted to each other.  The closer the atom starts  to the crystal as it is building the quicker it will attach to the crystal.  The new atom will be added to the outside structure of the crystal.

Students could grow salt, alum, or other water soluble crystals.  By growing them at different temperatures students could look for differences in the rate of growth, the size, and other characteristics.