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Making and Observing Crystals

DESCRIPTION OF ACTIVITY

Cinnabar, Dolomite & Quartz, Tongren Mine, Wanshan District, Tongren Prefecture, Guizhou Province, China (Image by Rob Lavinsky, Creative Commons)

Young children often have a fascination with jewels and crystals, not only as objects of value in their imaginations and play, but also as objects of beauty and wonderment. In this activity, students will learn about crystal structure, compare the crystalline structure of real and artificial sugar, and grow and observe their own crystals under the microscope.


GRADE LEVELS

2-8

TEACHER BACKGROUND

Halite crystal
NaCl crystalline structure. Purple Na+ ions and green Cl- ions

A crystal (or crystalline solid) is a material that has its atoms, molecules or ions arranged in a highly organized three-dimensional pattern. The word comes from the Greek krustallos (ice or rock crystal). When ice forms, it starts as tiny crystals which ultimately fuse together to form a polycrystalline structure. An ice cube is not a true crystal, since the periodic pattern of water molecules is broken at the interface between each component crystal. However, a single snow flake is a true crystal. Salts, like NaCl (sodium chloride, or table salt) also form true crystals, as does table sugar (sucrose) and many sugar substitutes. Ionic substances, like salts, form crystals made of alternating positive and negative ions, which form bonds based on strong electrostatic attractions. Their crystals have high melting points and are good conductors of electricity when dissolved or in their molten state. Polar covalent substances, like sucrose, form crystals based on weaker London or dipole-dipole interactions. Their crystals tend to have lower melting points and are not particularly good conductors of electricity.

Crystallization is the process of forming crystals from a fluid. Many students may have had the experience of making or eating rock candy (crystallized sucrose), which is prepared by evaporating the water from a sugar-water solution.

Sources “[1],” Wikipedia, (last modified on 3 July 2013 at 16:12).


LEARNING GOALS

(Students Will Be Able to)

  • Conduct experiments that describe and classify various materials based on their physical properties
  • Explain that some solids may be formed from extended structures with repeating subunits (e.g., crystals)
  • Develop a model to describe the atomic composition of simple molecules and extended structures.
  • 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


STUDENT PREREQUISITE KNOWLEDGE

Students should already be familiar with states of mater (e.g., solids, liquids, gases); physical vs. chemical properties; and observation vs. inference. They should also be familiar with basic microscopy and the microscopes that will be used in these activities.


SEQUENCING OF LESSONS

Before beginning these activities, students should already have had basic microscopy lessons and should already be familiar with the composition and states of matter. The activities in this lesson can be done in any order, but should be preceded by the anticipatory set.


EXPECTED TIME FOR COMPLETION

The anticipatory set and Activity 1 can be completed in 30-50 minutes. Activity 2 can be completed in 15-30 minutes, depending on your equipment and the weather (or it can be set up in 15 minutes and allowed to dry overnight). Activity 3 can be completed in 15-30 minutes.


MATERIALS

(per each student team)

  • Anticipatory Set (optional): examples of crystals (e.g., quartz, salt or sugar crystals, geodes, amethyst)
  • Activity 1: small tube of sugar, small tube of artificial sugar, digital microscopes, microscope slides
  • Activity 2: sugar solution, black construction paper, small paint brush, digital microscopes
  • Activity 3: Each student team needs either 1 beaker, cup or container of Epson salt (magnesium sulfate) solution prepared in advance by the teacher or the materials to prepare their own (e.g., 1 beaker, 1 hot plate, Epson salts); 1 digital microscope; 1 small, clean paintbrush or toothpick; 1 microscope slide


PREPARATION

  • Activity 1: label enough small tubes so that each student team can have 2 tubes; fill half the tubes with real sugar and the other half with artificial sugar; set up lab stations with 1 tube of each substance, 1 microscope, 1 slide
  • Activity 2: Prepare sugar solution by mixing 2 cups of sugar in 1 cup of water and boiling until fully dissolved; set up lab stations with sufficient materials so that each student has 1 sheet of black construction paper and 1 paint brush, and each student team has 1 digital microscope and 1 small beaker or cup of sugar solution
  • Activity 3: Either prepare the Epson salt solution in advance (mix 1 cup Epson salts + 1 cup water and boil until completely dissolved) or provide a beaker, hotplate and Epson salts to each student lab team; set up student lab stations with 1 microscope, 1 slide, 1 clean paintbrush or toothpick
Amethyst crystal from Madagascar (Image by Didier Descouens, Creative Commons)

ANTICIPATORY SET

  1. Review matter with students (e.g., things are made of matter, which is composed of atoms and molecules).
  2. Review states of matter with students (i.e., solids, liquids, gases)
  3. Show images of crystals and/or pass around crystals for students to examine
  4. Ask students what they already know about crystals. Ask them to provide examples (e.g., salt, sugar, snowflakes)
  5. Explain that crystals are solids with regularly shaped, flat sides. They are made of atoms or molecules that are arranged in symmetrical, three-dimensional patterns
  6. Review microscope operation and safety


THE LESSON PLAN

Sugar (real & fake) viewed through the QX5 Digital Blue microscope (Image by M4k)

Activity 1: Comparing Different Types of Crystals

  1. Students pour or spoon a small sample of real sugar onto a microscope slide and examine under the microscope and a small sample of artificial sugar onto another microscope slide.
  2. Have them make macroscopic observations of the 2 samples. Can they tell the difference between the 2 samples?
  3. Next, have them examine the samples under the microscope and shoot a digital photograph
  4. Have students describe and record the physical properties of the 2 samples
  5. Have them make a data table that indicates the similarities and differences between the two types of crystals. (They can include this and their digital images of the crystals in their lab reports).

Extension: You can have students examine various kinds of sugars such as fructose, maltose, lactose, glucose—all available from science supply companies like Ward’s Science, Sargent-Welch, Carolina Biologicals). You could also have them compare a variety of synthetic sugars such as Sweet One (Acesulfame potassium), NutraSweet or Equal (Aspartame), Truvia or Stevia (Rebaudioside A), Sweet’N Low (saccharin), Splenda (Sucralose)—mostly available from grocery stores. Furthermore, you could have them research the chemistry of these substances (e.g., How are the true sugars similar to each other in chemical structure? How are they different chemically from the synthetic sugars?)


Activity 2: Making Sugar “Snow” Crystals

  1. Tell the class that they will be making pictures with a “paint” made from sugar water. Explain that at first the paint will look like water, but that their pictures will develop as the water evaporates, leaving behind sugar crystals
  2. Have students paint their pictures onto a black sheet of construction paper using the sugar solution as their “paint.” Remind them that their pictures will be invisible at first, so they will have to imagine what they will look like when developed.
  3. Have them hold their painted paper over a heater or place in a bright window to evaporate the water and allow the crystals to grow. (If you have no heater or the day is cool and overcast, you can leave the pictures overnight to develop and finish the following day)
  4. Students can look at the individual crystals under the digital microscopes and take pictures of the crystals under low or medium magnification.

Adapted from Crystal Activities,” Science Kids, (7/13/13).

{mediaplayer fullscreen="true" src="tiki-download_file.php?fileId=104" type="mov" style="maxi" align="center"}

Activity 3: Making Salt Crystals

  1. Set the microscopes so they are ready to take time lapse movies
  2. Add equal amounts of Epson salt and water in a beaker
  3. Heat on a hot plate, stove top, or microwave until the salt dissolves upon stirring

Note: Steps 2-3 can be done in advance by the teacher and provided for the students, or they can be done by the students during class.

  1. Using a clean paintbrush or toothpick, spread a thin film of the solution on a glass microscope slide
  2. As it cools, the magnesium sulfate will crystallize on the slide within a few minutes
  3. Have students make a time lapse movie of the crystals as they form on the slide

Extension (Making Bath Salts): As long as you have kids working with Epson salts anyway, why not also make some inexpensive artisanal gifts? The recipe is simple: In a bowl or beaker mix 1 cup of Epson salts, 5-6 drops of the essential oil of your choice (e.g., lavender, patchouli, rosemary, rose), a few drops of food coloring. Transfer to a plastic baggy or jar.

{mediaplayer fullscreen="true" src="tiki-download_file.php?fileId=105" type="mov" style="maxi" align="center"} Extension (Polarized video of Epson salt crystal formation): The second movie with the dark background (polarization microscopy, 5.3 MB) is made with two cheap sheet polarizers- one placed on the stage (and taped) and on the other taped or held between the sample and the microscope lens. The angle needs to be adjusted so that the background image appears dark and the crystal appears bright (this is a special technique called polarization microscopy).

Polarization microscopy is a complicated subject, too advanced for kids. But if you want to know how this works, click [2].


Adapted from Microscopy4Kids and “Crystal Activities,” Science Kids, (7/13/13).


QUESTIONS FOR DISCUSSION

Activity 1

  1. Describe the macroscopic differences between your two samples? Were you able to tell the difference without using the microscope? If so, how? (Answers will vary, particularly depending on which type of artificial sugar is used and the punctiliousness of the students. They may be able to see differences without the microscope, though these differences will seem subtle compared with those visible under magnification)
  2. Describe the microscopic differences between the various substances you examined? How are they similar? How are they different? (Answers will vary depending on which substances were examined, but could include differences in size, shape, color, clarity of the crystals)
  3. Why were they different? ( {k-8}: The crystals were made out of different substances, with different chemical compositions. {Ans 6-8}: Crystals form as a result of intermolecular forces between adjacent molecules. The intra-molecular structure of a substance determines the type of crystal that can form).

Activity 2

Grades K-8
  1. Why did images appear on your paper after heating it up? (Ans: The “paint” was a solution of sugar and water. The heat caused the water to evaporate, causing the sugar molecules to form crystals).
  2. Do you think it is possible to “erase” your painting? If so, how might it be done? (Ans: rinse with water. This would cause some of the sugar to go back into solution, while the force of the water hitting the paper would push some of the crystals off).
Grades 6-8
  1. What happened to the motion of the sugar molecules as the water evaporated and the crystals formed? (: they slowed down)
  2. Is it possible to convert the solid sugar into a liquid? (Ans: adding water would dissolve the sugar, converting it from precipitated crystals to an aqueous solution of sugar. However, solid pure sugar can be converted to liquid pure sugar by providing sufficient heat).


Activity 3

Grades 2-8
  1. The crystals are made out of Epson salt (Magnesium sulfate) which had been dissolved in water. What caused the salt to form crystals rather than remain dissolved in water? (Ans: the water evaporated)
  2. What type of energy caused the water to evaporate? (Ans: thermal energy or heat)
  3. Is this a reversible change? Can the crystals become liquefied again? (Ans: If water and heat are added, the crystals can be re-dissolved in water. However, the crystals themselves can be converted from solid to liquid if sufficient heat is added. This is an important distinction that is worth discussing with students. The starting material in this activity was an aqueous solution of Magnesium sulfate dissolved in water, MgSO4(aq). The liquid was water, not MgSO4(l). However, pure MgSO4 crystals are a solid MgSO4(s) that can be melted to form MgSO4(l).
Grades 6-8
  1. What happens to the movement of MgSO4 particles as they go from the solid to liquid state? (Ans: they move faster)
  2. Would it be possible to convert MgSO4(s) to MgSO4(g)? If so, how? (Ans: Yes, by adding even more thermal energy or heat than was added to convert it from solid to liquid).


STANDARDS

Next Generation Science Standards

2-PS1-1 Plan and conduct an investigation to describe and classify different kinds of materials by their observable properties.

2-PS1-4 Construct an argument with evidence that some changes caused by heating or cooling can be reversed and some cannot.

4-PS3-2 Make observations to provide evidence that energy can be transferred from place to place by sound, light, heat, and electric currents.

5-PS1-3 Make observations and measurements to identify materials based on their properties

MS-PS1-1 Develop models to describe the atomic composition of simple molecules and extended structures. Solids may be formed from molecules, or they may be extended structures with repeating subunits (e.g., crystals).

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

Common Core State Standards (CCSS)

CCSS.ELA-Literacy.RI.2.3 Describe the connection between a series of historical events, scientific ideas or concepts, or steps in technical procedures in a text.

CCSS.ELA-Literacy.RI.3.3 Describe the relationship between a series of historical events, scientific ideas or concepts, or steps in technical procedures in a text, using language that pertains to time, sequence, and cause/effect.

CCSS.ELA-Literacy.RI.4.3 Explain events, procedures, ideas, or concepts in a historical, scientific, or technical text, including what happened and why, based on specific information in the text.

CCSS.ELA-Literacy.RI.5.3 Explain the relationships or interactions between two or more individuals, events, ideas, or concepts in a historical, scientific, or technical text based on specific information in the text.

CCSS.ELA-Literacy.RST.6-8.3 Follow precisely a multistep procedure when carrying out experiments, taking measurements, or performing technical tasks. (Grades 6-8)

CCSS.ELA-Literacy.RST.6-8.9 Compare and contrast the information gained from experiments, simulations, video, or multimedia sources with that gained from reading a text on the same topic. (Grades 6-8)

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