Crystals - a Handbook for School Teachers

CRYSTALS - A HANDBOOK FOR SCHOOL TEACHERS Elizabeth A. Wood, 1972 Written for the Commission on Crystallographic Teaching of the International Union of Crystallography Copyright c 1972, 2002 International Union of Crystallography This electronic edition may be freely copied and redistributed for educational or research purposes only. It may not be sold for profit nor incorporated in any product sold for profit without the express permission of The Executive Secretary, International Union of Crystallography, 2 Abbey Square, Chester CH1 2HU, UK Copyright in this electronic edition (c) 2002 International Union of Crystallography 2 CONTENTS Contents I Preface to the 2002 Edition 4 II Introduction 5 IIIEquipment and Materials 9 IV Crystals in Classroom and Home 10 A Growing Crystals from Solution ................. 10 1 Salt (table salt, sodium chloride, NaCl) ........ 10 2 Borax (Na2B4O7.10H2O) in water ........... 15 3 Sugar (sucrose or saccharose, C12H22O11) in water .. 17 4 Alum [ammonium alum, NH4Al(SO4)2.12H2O or potas- sium alum, KAl(SO4)2.12H2O] in water ........ 18 5 Copper sulfate (blue vitriol, CuSO4.5H2O) in water . 19 6 Epsom Salt (MgSO4.7H2O) in water ......... 20 7 What else? ........................ 22 B Crystals from the Melt ...................... 22 1 Ice (solid water, H2O) .................. 22 2 Salol (phenyl salicylate, HOC6H4COOC6H5) ..... 25 3 Bismuth (metallic element, Bi) ............. 27 C Crystals from the Vapor ..................... 28 1 Ice ............................. 28 2 Naphthalene (moth flakes, C10H8) ........... 29 D Experiments with Polarized Light ................ 30 1 The Nature of Polarized Light and the Means for Pro- ducing It ......................... 30 2 Crystals between Crossed Polarizers .......... 31 3 Ice and mica ....................... 33 4 Sugar solutions ...................... 33 V Crystals Outside of Home and Classroom 34 A In Museums ............................ 34 1 Displays of Rocks and Minerals ............. 34 2 The Museum Staff (suggestions for field trips) ..... 34 B Out of Doors ........................... 34 C At the Big Stores and Small Shops ............... 36 1 Decoration of buildings and counters .......... 36 2 Jewelry stores ....................... 37 CONTENTS 3 3 Drug stores (apothecaries, chemists’ shops) ...... 37 VI References for Further Reading 38 VII References 39 VIII Acknowledgement 39 4 I Preface to the 2002 Edition I Preface to the 2002 Edition The text of this edition is largely unchanged from that of the original 1972 edition; there has been a small amount of editing where materials are no longer considered safe for general use, e.g. the use of benzene as a solvent is no longer advocated due to its toxic and carcinogenic properties. Readers outside the USA and Canada should be aware that the mea- surements (e.g. cup) used in the practical sections are based on American cooking measures, which are not quite the same as the equivalents used in British or other Commonwealth kitchens! Users of “U.K.” or “British” English should be warned that spellings and usage in this document are “American” (e.g. “color” rather than “colour”, ‘Crystals Outside of Home” rather than “Crystals Outside the Home”. The original guide is available as an HTML document on the IUCr’s web- site (http://www.iucr.org/iucr-top/comm/cteach/pamphlets/20/index.html); there are translations (by native speakers) into several languages (currently (i.e. Autumn 2002) Arabic, Czech, Polish, Russian and Spanish). Harry Powell Cambridge, UK November 2002 5 II Introduction To the Teachers of Young Children Everywhere: This booklet was produced for the Commission on Teaching of the Inter- national Union of Crystallography, which is an organization for the benefit of the science of crystallography throughout the World. It is not a trade union, but a group of people of all nations interested in crystals. Many teachers have found that children are interested in crystals. There are good reasons for the teacher to encourage this interest. Children can perform simple experiments with crystals and so get the feel of doing science themselves, the experience of watching something happen in their own experiments. Crystals are of interest to chemists, physicists, geologists, biologists and mathematicians. To study crystals is to be part of all these fields and to become aware that Nature is not separated into chemistry, physics, geology, and biology. Most teachers at the present time (the 1970s) did not learn about crystals when they were in school and college. The purpose of this booklet is to give them some background of understanding of crystals so that they can enjoy working with children who are interested in crystals. It is not a systematic course in crystallography. This would not be suitable. Those students who want to know that much about crystallography will take courses in crystallography at the university. It is a handbook for your enjoyment. In this booklet, technical terms are avoided as much as possible, not to make it easier, but to avoid the pitfall of substituting learning names for thinking about what is going on. Children think they know why an apple falls because they have learned the word ‘gravity’, but our most competent scientists are puzzled by the way in which an apple and the Earth are drawn toward each other. Most books on crystallography, the subject that deals with the study of crystals, emphasize the importance of symmetry in the classification of crystals. However, many of the crystals that children can grow themselves, or find in nature, have shapes that do not exhibit perfect symmetry, because the growing conditions were not the same all around the crystal. It takes a mature imagination and experience with some perfectly symmetrical crystals to imagine what such a crystal would have looked like if the growing conditions had been uniform. Unless students can be convinced from their own observations that symmetry is really useful in classifying crystals, there is no merit in having them memorize the symmetry terms, since they have no meaning for them. The 6 II Introduction essence of science is observation and wonder, curiosity and the effort to sat- isfy that curiosity. Learning what others have found out is part of learning about science, but first we must see how scientists learn what they know about nature so that we may be convinced that their results are based on repeatable experiments. For these reasons this booklet does not deal with systematic crystallography, the classification of crystals according to their symmetry. It seeks to lay a firm foundation for later study of crystallography by encouraging observation and experimentation. Over a period of time, the students’ observations will probably lead them to conclusions such as the following: 1. Under suitable conditions some kinds of solid matter form in shapes called crystals. 2. Crystals grow bigger by adding more layers of solid matter around their outsides. 3. Crystals form from solution when the solvent evaporates. Crystals form from the molten state when the liquid cools. Crystals form from warm invisible vapor when that vapor meets a cooler surface. 4. Crystals of different substances have different forms. 5. Crystals of different substances have different properties; that is, some are colored and some are not; some grow nicely and some do not; some have cleavage (to be discussed later) and some do not; some look bright between crossed polarizers (see section III-D) and some do not. 6. (for older students) There must be something orderly about the way a crystal forms that is responsible for its flat faces, its characteristic shape, and the way it affects light. This orderliness must be different for different substances. If your students have some conviction about such conclusions from their own observations, they will have a good foundation in the science of crystallography. *** A crystal of a given substance or material shows plane faces always at the same angles to each other and has its other orderly properties because 7 it is made up of atoms, ions, or molecules arranged in a very orderly way. This orderliness of structure is found in almost all solid matter, though some substances have a more orderly arrangement than others. Even in wood the molecules are arranged in good order along the fibers, though there is not much orderliness from one fiber to the next. Is wood, then, a crystal? It doesn’t show shiny faces. Some crystallographers (people who study crystals) would say its fibers are crystals; some would not. A substance that is made up of crystals is called a crystalline substance. Sometimes the word polycrystalline is used to indicate a substance made up of many crystals. In a single crystal, the orderliness of rows of atoms is not interrupted and does not change direction. When two crystals grow against each other, the boundary between them marks the place where the orderly array of one makes an angle with the orderly array of the other. A slice through four crystals with such boundaries is crudely suggested by this sketch. The solid lines represent boundaries between crystals (sometimes called grain boundaries). The dotted lines represent layers of atoms, ions, or molecules. Many substances that we are familiar with are made of very orderly crystals that do not show their bright faces because neigh- boring crystals have grown against each other with irregular boundaries. Nearly all rocks are made up of crystals and the different kinds of crystals in a rock can often be dis- tinguished from each other. Metal objects are made up of interlocking crystals. Some- times their boundaries can be seen, as in the zinc coating on galvanized iron often used for pails (buckets) and garbage cans. Sometimes a brass door handle shows the boundaries between the crystals of which it is composed. A substance in which the atoms or ions or molecules are not arranged in orderly rows is called a glass. Window glass is a familiar example.

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