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CHM 121 Week 6 Assignment
- Course
- Chemistry 121
- Institution
- Harper College
This is a comprehensive and detailed week 6 assignment on; Crystal Growth. *Essential!!
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Week 6 Assignment Crystal Growth Due October 4 @ 11:59 p.m.BACKGROUND
Crystals grow from melts (liquid rock) and from water-based solutions (aqueous solutions) in
the interior of and on the surface of Earth. They can even grow from other solid minerals in
something called the “solid state”. This latter process occurs in metamorphic rocks. In this
assignment, you will investigate the growth of some simple evaporite minerals from aqueous
solutions. These minerals, listed below, are also ore minerals for table salt (and sodium and
chlorine), boron, and copper. Evaporite minerals are “salts” that crystallize or precipitate as
water molecules move from the liquid phase to the gas phase by evaporation. Removal of
water increases the concentration of the cations and anions in the solution to levels that
exceed the amount required for the mineral to be in chemical equilibrium with the solution.
This triggers the nucleation and subsequent growth of the minerals.
The minerals you will grow today:
• Halite (NaCl)
• Borax (Na2B4O710H2O)
• Eriochalcite (CuCl22H2O)
(The formulas with the “2H2O” simply means that the minerals are “hydrous”, that is, they
have some water molecules in their crystal structures, like gypsum – CaSO42H2O.)
Saturated aqueous solutions of the minerals are ready for you to use in your experiments.
A saturated aqueous solution is a solution containing the maximum possible amount of
dissolved mineral in the form of its cations and anions when in contact with an excess of the
solid mineral at a fixed temperature. The saturated solutions you see here have been prepared
according to known solubilities for the phases at slightly higher temperatures (~40-50 degrees
C). All of these minerals are more soluble in water at higher temperature – this means that
more mass of the mineral will dissolve at a higher temperature.
Two processes will work together to start the growth of crystals: evaporation of the water and
cooling of the solution. Both processes cause the solution to become supersaturated with
respect to the minerals. Supersaturation means there are more dissolved components in the
solution than should exist when the solution is in equilibrium with the solid. As water
evaporates, the concentrations of the dissolved components in the water increase simply
because there are fewer liquid water molecules. As the water cools, fewer dissolved
components are required to be in equilibrium with the mineral, but the water cools faster than
the components can combine to form the solid. In both cases, we say that “the water becomes
supersaturated with respect to the mineral”.
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, Once a critical supersaturation is reached, crystals nucleate. As evaporation and cooling
continue, the crystal nuclei grow larger.
Where and how does water evaporate in nature?
a. Water evaporates around the border of a playa lake or in the vadose zone of a near-
surface environment (the zone of a soil that is unsaturated with water, i.e., above the
water table). Here the source of heat that causes water molecules to move from the
liquid to the gas phase is the sun. Evaporation would also occur anywhere hydrothermal
fluids are emitted at Earth’s surface. Here the source of heat is the hot water itself. As
the water molecules evaporate into the atmosphere, the solutes (the cations and
anions) stay in the solution and increase in concentration: the solutions become
supersaturated. At some point each solution will be sufficiently supersaturated that
crystals will nucleate. As evaporation continues, the crystals will grow larger.
b. Cooling has the same effect, even without evaporation. The water around the border of
a playa lake will cool down at night and hydrothermal fluids emitted at Earth’s surface
will cool down in contact with the colder air and land surface. Because these minerals
all dissolve less in water at lower temperatures, the simple effect of cooling causes
supersaturation. The solutions we have prepared are at a higher temperature than the
room air. The heat from the light bulb on the microscopes may contribute to
evaporating the water.
______________________________________________________________________________
INSTRUCTIONS
Use the glass watch glass or petri dish for the experiments with halite.
Use a separate plastic petri dish for each of the experiments with borax and eriochalcite.
IMPORTANT: Do not touch the borax or eriochalcite solutions with your hands.
The TA will place a drop or two of each heated solution into your petri dish.
• Place the container on the binocular microscope stage and turn the light on.
• Examine the droplets and look for evidence of crystallization as it occurs.
• Observe the crystals in the process of nucleating and growing as much as possible.
• You can start over if crystallization happens too quickly.
• Make drawings of the crystals that grew, or take photos with your phone cameras.
• Once crystallization has begun, continue to monitor the growth, and answer the
questions on the next two pages for each solution.
**** MAKE OBSERVATIONS FOR EACH CRYSTAL GROWTH EXPERIMENT AND WRITE A SHORT
DESCRIPTION OF YOUR OBSERVATIONS THAT REFERENCES YOUR FIGURES.****
______________________________________________________________________________
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