Microscopic Observation of Crystal Growth
Science Process Skills:
Science as Inquiry
- position and motion of objects
- properties of objects and materials
Unifying Concepts and Processes
Change, Constancy, & Measurement
MATERIALS AND TOOLS
The mannite part of this activity should be done as a demonstration, using a microprojector or microscope with a television system. It is necessary to heat a small quantity of crystalline mannite on a glass slide to 168C and observe its recrystallization under magnification. The instructions call for melting the mannite twice and causing it to cool at different rates. It is better to prepare separate samples so they can be compared to each other. Th~ slide that is cooled slowly can easily be observed under magnification as crystallizes. You may not have time to observe the rapidly chilled sample properly before crystallization is complete. The end result, however, will be quite apparent under magnification. If students will be conducting the second part of theactivity, it is suggested that you prepare several sets of mannite slides so they may be distributed for individual observations. The salol observations are suitable for a demonstration, but because of the lower melting temperature (48C), it is much safer for students to work with that the mannite. A desktop coffee cup warmer is sufficient for melting the salol on a glass slide. Because of the recess of the warmer's plate, it is best to set several large metal washers on the plate to raise its surface. The washers will conduct the heat to the slide and make it easier to pick up the heated slide with forceps. Point out to the students that they should be careful when heating the salol because overheating will cause excessive evaporation and chemical odors, and will increase the time it takes for the material to cool enough for crystallization to occur. The slide should be removed from the hot plate just as it starts melting. The glass slide will retain enough heat to complete the melting process.
Only a very small amount of bismarck brown is needed for the last part of the activity with salol. Only a few dozen grains are needed. Usually just touching the spatula to the chemical causes enough particles to cling to it. Gently tap the spatula held over the melted salol to transfer the particles. It will be easier to do this if the salol slide is placed over a sheet of white paper. This will make it easier to see that the particles have landed in the salol.
If students are permitted to do individual studies, go over the procedures while demonstrating crystallization with the d-mannitol. Have students practice sketching the crystallized mannitol samples before they try sketching the salol.
Refer to the chemical notes below for safety precautions required for this activity.
Notes On Chemicals Used: Bismarok Brown Y
Bismarck brown is a stain used to dye bone specimens for microscope slides. Because bismarck brown is a stain, avoid getting it on your fingers. Bismarck brown is water soluble.
Mannite (d-mannitol) HOCH2(CHOH)4CH20H Mannite has a melting point of approximately 168C. It may be harmful if inhaled or swallowed. Wear eye protection and gloves when handling this chemical. Conduct the experiment in a well-ventilated area.
Salol (phenyl salicylate) C13H10O3
It has a melting point of 43 degrees C. It may irritate eyes. Wear eye protection.
Procedure: Observations of Mannite
Observations of Salol
Directional solidification refers to a process by which a liquid is transformed
(by freezing) into a solid through the application of a temperature gradient
(a temperature difference over a specified distance such as 10 degrees
C/cm) in which heat is removed in one direction. The heat travels down
the temperature gradient from hot to cold. A container of liquid will
turn to a solid in the direction the temperature is lowered. If this liquid
has a solute (something dissolved in the liquid) present, typically some
of the solute will be rejected into the liquid ahead of the liquid/solid
interface. However, not all of the solute can be contained in the solid
as it forms; the remaining solute is pushed back into the liquid near
the interface. This phenomenon has many important consequences for the
solid including how much of the solute eventually ends up in the solid.
The concentration of solute in the solid can control the electrical properties
of semiconductors and the mechanical and corrosion properties of metals.
As a result, solute rejection is studied extensively in solidification
The rejected material tends to build up at the interface (in the liquid) to form a layer rich in solute. This experiment demonstrates what happens when the growth rate is too fast and solute in the enriched layer is trapped.
Fluid flow in the melt can also affect the buildup of this enriched layer. On Earth, fluids that expand become less dense. This causes a vertical flow of liquid which will interfere with the enriched layer next to the growing solid. In space, by avoiding this fluid flow, a more uniform enriched layer will be achieved. This, in turn, can improve the uniformity with which the solute is incorporated into the growing crystal.