What Variables Affect Crystal Growth?
Almost every solid that occurs in nature is made up of crystals, though the crystals may vary greatly in shape, size, and color. Salt, for instance, looks like rough sand to the unaided eye. However, when looked at using a hand lens or a microscope, you can see that the salt is actually made up of small cube-shaped crystals. The crystals of salt and many other substances are very small, but others are large enough to be seen without a hand lens or a microscope. Also, many crystals are transparent, while others are colored.
The varied appearance of crystals stems, in part, from the different ways that crystals may form. Some crystals, for instance, are formed when molten rock cools and hardens. Others are formed when water containing dissolved minerals evaporates. In both cases, however, the atoms of the minerals form solids by coming together in regular repeating patterns, which makes crystals both strong and hard. Crystals of the same mineral always follow the same growth pattern.
What will the students do?
Students will be investigating variables that may influence the quality of crystals they will be making. Students were given instructions on how to make crystals in Activity 14 and now they will be expected to determine what variables affect the size of the crystals. There are two options for implementing this activity. Activity 15A is open-ended, students deciding what variables affect crystal growth and then deciding on the manipulating variables they would like to study. Activity 15B is prescriptive and explains to students how to test three variables in making crystals. Following are teacher instructions for both Activity 15A and Activity 15B.
Once you have discussed the concept of variables, encourage students to first discuss and list in their science notebooks things to consider when making crystals. Some variables that students may suggest are: temperature of the mixture, temperature in the room, type of container used, amount of water, amount of solid, placement of slides, how the mixture is heated, or atmospheric conditions. Students will then need to choose what they would like to change when making their crystals. This one thing is the manipulating variable.
Encourage students to design a way to find out how each variable affects the size, number, or formation of crystals. Students will write down in their science notebooks all of the steps involved in testing each variable. Students from another group should then read the instructions to see if they are easy to understand.
Students will predict the effect each variable has on the formation of the crystals they have chosen to make, then perform their experiment, and record all data in their science notebooks. Have students examine their crystals using hand lenses and field microscopes and compare their results with the predictions they made prior to making the crystals. Students should present their findings to the class.
Students will manipulate the temperature of the liquid used to make their crystals. Students should label their containers with their names and an H (heated) or a C (cold). The same amount of water and copper sulfate or other chemical for each container will be used. The only difference is the temperature of the water used to make each solution. Provide room temperature water for one container and heated water (not boiling) for the other.
Students should add 7 g (1 tsp.) of the chemical to 10 ml (2 tsp.) of water in each container and stir gently until the chemical dissolves. They will then place two drops of the warm solution on a slide, and two drops of the room temperature solution on another slide. These should be labeled and then placed where they will not be disturbed. Students will examine the crystals the next day with hand lenses and with the field microscopes, recording results in their science notebooks.
Students will investigate the effect of stirring or shaking crystal-making solution on crystal growth. Provide 2 different containers for students. Have them label their containers with their name and an S (stirred) or an NS (not stirred). Students will use the same amount of warm water and the same amount of copper sulfate or other chemical in each container. Everything but the variable should remain constant.
Have students add 10 ml (2 tsp.) of heated water and approximately 7 grams (1 tsp.) of the chemical to each container. They should then stir the chemical and warm water in container S. After 5 minutes, they will take 2 drops of the liquid from container S and place it on a slide, and 2 drops of the liquid from container NS on another slide. They should label these and place them where they will not be disturbed. Students will examine the slides the next day with hand lenses and with the field microscopes, recording results in their science notebooks. Have students repeat the process, shaking the mixture.
Students will investigate how crystal growth is affected by adding different chemical amounts to water. Provide 2 containers for students, and have them label each container with their names. Have students also include labels signifying the amount of chemical additive that will be used with each container (3.5g and 7g). Students will make solutions of two different concentrations. In the 3.5g container, they will add approximately 3.5 grams (½ tsp.) of copper sulfate or some other chemical to 10 ml (2 tsp.) of warm water. In the container marked 7g, they will add approximately 7 grams (1 tsp.) of the chemical to 10 ml (2 tsp.) of warm water.
Students should either stir or shake both containers until the chemical is dissolved. They will base this decision on results from investigating variable 2. Students will then take 2 drops of liquid from the 3.5g container and place them on a slide. They should also take 2 drops of liquid from the 7g container and place them on a different slide. Both slides should be labeled and set aside where they will not be disturbed. Students will observe the slides the next day with hand lenses and field microscopes, recording observations in their science notebooks.
Research/Art - There are six crystal systems: cubic, tetragonal, hexagonal, orthorhombic, monoclinic, and triclinic. Have your students research the different crystal shapes and design a model of one of them to be shared with the class.
Writing - Stalactites and stalagmites are structures often found in caves. Students should pretend that they are tour guides explaining how these structures form to people on a tour. Have them write a script explaining this process. Encourage students to use models or visual aids that explain these and other stone features created by groundwater deposits.
Historical Vignette: Dorothy Crowfoot Hodgkin
Dorothy Crowfoot Hodgkin (1910-1994), a Nobel Prize winning chemist, made important contributions to chemistry, biology, and physics during her lifetime. Though she was born in Cairo, Egypt in 1910, she spent most of her life in England. At an early age she developed an interest in chemistry and crystals, and on her sixteenth birthday received a book about using X-rays to study crystals from William Henry Bragg, a Nobel Prize winner in physics. The gift increased her interest in crystals and started her on a career path toward using X-rays to study the shape and structure of crystals found in living things.
Although she was sick with rheumatoid arthritis from the time she was 24 years old, Hodgkin became one of the most skilled crystallographers of her time. She always chose projects to work on that others thought were impossible or too difficult. Hodgkin's hard work and knowledge of math, chemistry, physics, and biology enabled her to determine the structure of penicillin, vitamin B-12, and insulin. The work she did using X-rays to discover the structure of these crystals helped other scientists learn how to determine the molecular structure of almost any crystalline material. Hodgkin was only the third woman to receive a Nobel Prize in Chemistry. The prize was awarded for her discovery of the structure of Vitamin B-12 and for other discoveries she made that changed the way scientists study chemistry.
Activity - Dorothy Hodgkin used X-rays to investigate crystals, but X-rays are utilized for many other purposes as well. Have your students research X-rays and each report on a different that way that X-rays are used, or may be used in the future.
Artist Vignette: Juan Gris
Juan Gris (1887-1927) was a Spanish painter who was important in developing the style of painting known as cubism. Two other important painters who used this style of painting were Pablo Picasso and Georges Braque. The cubist style of painting uses flat, two-dimensional surfaces with sharp lines and geometric shapes, instead of the more realistic forms and colors seen associated with many other styles.
The use of stronger colors and larger and more decorative shapes distinguished the work of Juan Gris from that of Picasso and Braque. Gris portrayed the synthetic cubism form that used decorative shapes, stenciling, collage and bright colors. Many of the cubists also combined smooth and rough surfaces, frequently using foreign materials, such as newspapers, that were pasted on the canvas in combination with the painted areas. This collage technique became another popular way of creating art. It was Gris who helped to popularize the cubist style by systemizing the ideas of Picasso and Braque.
Activity - The structure of ice crystals that produce a snowflake has been a source of fascination for young and old alike. It is said that no two snowflakes are the same, but is it true? Encourage students to devise a way to find out using a variety of print and electronic media. Have students draw or make a model of a snowflake and present findings to the class.
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