Crystals Teacher Guide

Unit

Crystals

Subject

Earth and Space Science

Grade Level

Activity Name(s)

Introduction to Crystals

Molecular Crystals

Being Prepared

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.

Getting Started

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.

In "Molecular Crystals" the model has two different controls. The control right under the model starts, stops, and resets the model. To start, use the right facing (>) arrow. To stop the model, use the vertical parallel lines (||), and to reset the model use the left facing arrow with on vertical line (|<). Remind students to take Snapshots before resetting the model.

Suggested Timeline

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.

Thinking about the Discovery Questions

In this unit students investigate the structure and properties of crystals. 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 first 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.

In the second activity "Molecular Crystals" students will again use a Molecular Workbench model. Using this model they will investigate the questions, "How does melting a molecular crystal affect its alignment?" and "How and why does order exist in a crystal?". In addition to exploring how crystals change when heated, they will attempt to reform the crystal to its original form by cooling it. The final data collection introduces the concept of polymorphs and how different crystal made of the same molecules react differently.

Misconceptions

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.

Learning Objectives

NGSS
- Performance Expectations
  - 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.
- Disciplinary Core Ideas
  - MS-PS1.A. Structure and Properties of Matter.
    - Substances are made from different types of atoms, which combine with one another in various ways. Atoms form molecules that range in size from two to thousands of atoms. (MS-PS1-1)
    - Each pure substance has characteristic physical and chemical properties (for any bulk quantity under given conditions) that can be used to identify it. (MS-PS1-2),(MS-PS1-3)
    - Gases and liquids are made of molecules or inert atoms that are moving about relative to each other. (MS-PS1-4)
    - In a liquid, the molecules are constantly in contact with others; in a gas, they are widely spaced except when they happen to collide. In a solid, atoms are closely spaced and may vibrate in position but do not change relative locations. (MS-PS1-4)
    - Solids may be formed from molecules, or they may be extended structures with repeating subunits (e.g., crystals). (MS-PS1-1)
    - The changes of state that occur with variations in temperature or pressure can be described and predicted using these models of matter. (MS-PS1-4)
- Cross Cutting Concepts
  - Patterns
  - Scale, proportion, and quantity
    - Students observe time, space, and energy phenomena at various scales using models to study systems that are too large or too small. They understand phenomena observed at one scale may not be observable at another scale, and the function of natural and designed systems may change with scale. They use proportional relationships (e.g., speed as the ratio of distance traveled to time taken) to gather information about the magnitude of properties and processes. They represent scientific relationships through the use of algebraic expressions and equations.
  - Systems and system models
    - Students can understand that systems may interact with other systems; they may have sub-systems and be a part of larger complex systems. They can use models to represent systems and their interactions — such as inputs, processes and outputs — and energy, matter, and information flows within systems. They can also learn that models are limited in that they only represent certain aspects of the system under study.
  - Energy and matter: Flows, cycles, and conservation
    - Students learn matter is conserved because atoms are conserved in physical and chemical processes. They also learn within a natural or designed system, the transfer of energy drives the motion and/or cycling of matter. Energy may take different forms (e.g. energy in fields, thermal energy, energy of motion). The transfer of energy can be tracked as energy flows through a designed or natural system.
  - Structure and function
    - Students model complex and microscopic structures and systems and visualize how their function depends on the shapes, composition, and relationships among its parts. They analyze many complex natural and designed structures and systems to determine how they function. They design structures to serve particular functions by taking into account properties of different materials, and how materials can be shaped and used.
- Practices
  - Developing and using models
    - Evaluate limitations of a model for a proposed object or tool.
    - Use and/or develop a model of simple systems with uncertain and less predictable factors.
    - Develop and/or use a model to predict and/or describe phenomena.
    - Develop and/or use a model togenerate data to test ideas about phenomena in natural or designed systems, including those representing inputs and outputs, and those at unobservable scales.
  - Constructing explanations and designing solutions
    - Construct an explanation that includes qualitative or quantitative relationships between variables that predict(s) and/or describe(s) phenomena.
    - Construct an explanation using models or representations.
    - Apply scientific ideas, principles, and/or evidence to construct, revise and/or use an explanation for real- world phenomena, examples, or events.
    - Apply scientific reasoning to show why the data or evidence is adequate for the explanation or conclusion.
  - Engaging in argument from evidence
    - Construct, use, and/or present an oral and written argument supported by empirical evidence and scientific reasoning to support or refute an explanation or a model for a phenomenon or a solution to a problem.
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.

Discussion: Setting the Stage

Have you ever seen or collected crystals? What can you tell us about the crystals you have seen?

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.
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?

The internal structure would still be the same, though what you would see on the outside would look different.
What are some other common crystals that you have seen?

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.
Name the phases of matter and how would you describe the properties of each phase?

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.

Discussion: Formative Questions

Introduction to Crystals

Would you able to make the atoms or molecules in a crystal stop vibrating?

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.
In the model there are small spaces between the atoms, are there really empty spaces in a matter?

Yes, even in solids there are spaces in the material that are empty.
In a real world situation will the crystal if heated and then cooled reform in exactly the same way?

They may or may not. This depends on temperature, pressure, and the type of atoms and molecules that make the crystal.

Moledcular Crystals

How are the structures in the 2 polymorphs different? How are they the same?

In polymorph 1 the internal structure more compact than polymorph 2. They appear to have the same configuration of atoms,
Why do the polymorphs behave differently?

Even though they have the same atoms in the same basic arrangement, the internal structure is different which will change their properties.

Discussion: Wrapping Up

Introduction to Crystals

What have you discovered about the way matter behaves when they are heated? When they are cooled?

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.

Molecular Crystals

If creating a polymorph crystal changes its reaction to temperature, how might this be important in designing a new drug using an already designed crystal?

By creating a polymorph that dissolves quicker you would be able get the chemical compounds into a body quicker.

Additional Background

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.

Analysis

Introduction to Crystals

What do you notice about how particles move in a crystal?

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.
What is the first sign of a crystal melting?

The particles around the edge begin to flow.
What can you conclude about the attractions among atoms and how they contribute to crystal growth?

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.

Molecular Crystals

In the first model, at what temperature does the molecular crystal appear to melt? What is the first sign of melting?

Answers may vary. The first sign of melting is the outside edges begin to move away from the crystal.
In the second model, were you able to return the liquid to a crystalline form? If not, why do you think it was not possible?

Answers may vary. They should be able to return it to a solid, but they should notice that the structure of the crystal will not be the same as the original.
In the third model, which polymorph structure did you think would lose its solidity most quickly? Were you correct? Why or why not?

Answers will vary. The polymorph loses its solidity quicker. The molecular structure is not as closely packed.
What is the relationship between a crystal's density and crystalline structure loss as the temperature increases?

The denser the crystal, the more heat is needed to cause structure loss.

Further Investigation

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.