Science Activity
#4 - Growing Crystals
All experiments must be done in the presence of a parent or teacher.
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Ideas
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Key Words
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Materials
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Procedure
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Observations
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Summary
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Questions
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WWW Links

Ideas to be Developed

The study of crystals and their structure is a field called Crystallography. A crystal is a solid that consists of the various atoms, or molecules being arranged in a uniform repeating pattern based on its unique shape. This results in the material having a specific shape and colour, and having other characteristic properties. Crystals may be big or little, but they all have the same "shape". Take a look at the display of crystals in the lab. Salt and sugar are examples of crystals. Table salt is NaCl and has a cube-shaped structure. Snow crystals form a six-sided structure. Diamond (used in jewelery, and cutting tools) is also an example of a crystal; it is made of pure carbon. Graphite (used in pencils and lubricants) is also a crystal made from carbon.

How are crystals grown?

In a solution, a solvent (water) can only hold a certain amount of solute. This is called the solubility of a solution. When the temperature of the solution is increased, hot water can dissolve more solid substance than cold water. This is because heated water molecules move farther apart, making room for more solid substance to dissolve. When no more of the solid substance can be dissolved, the solution is said to be saturated. As this solution cools, the water molecules move closer together again and there's less room for the solution to hold onto as much of the dissolved solid.

Crystals begin to form and build on one another as the water lets go of the excess solute. This process is called recrystallization and, depending upon conditions, one may obtain a mass of many small crystals or one large crystal.

How do crystals form and how do we control its rate of growth?

In this experiment, you will be growing crystals from a saturated solution. During recrystallization, crystals start growing by a process called "nucleation". Particles of dust on the surface of the solution can initiate crystalization. However, this situation is somewhat uncontrolled. To get controlled growth, a "seed crystal" is tied on a piece of thread and is submerged in the solution. As the temperature of the solution continues to drop, more crystals will accumulate on the thread. The rate at which crystallization occurs will affect crystal quality. The best crystals are the ones that grow SLOWLY.


Key Words

Crystal - A crystal is a solid with a definite geometric shape. The shape consists of smooth, flat surfaces that meet in sharp edges or corners.

Crystallography - A branch of Chemistry that studies crystals and their structure.

Nucleation - When solute molecules in a saturated solution encounter a dust particle or a solid surface (like a string or a seed crystal), they will tend to adsorb and aggregate on the surface. The solid surface provides the nucleation site for the formation of crystals.

Recrystallization - Recrystallization is a process that has been used to purify solid material by dissolving the solid substance in an appropriate liquid and then having the material come out of solution in crystalline form.

Saturated solution - Solution where the maximum amount of solutes is dissolved in the solvent.

Seed crystal - A starting surface for a growing crystal.

Shape of Crystal - The atoms in a crystal occupy positions with definite geometrical relationships to each other. This structural arrangement of its atoms. is uniquely defined by the chemistry of the substance and determines the shape of the crystal. In crystallography, the shapes of crystals can be grouped into seven systems:
Cubic Tetragonal Hexagonal Trigonal Orthorhombic Monoclinic Triclinic
These are covered in more details in Activity #5 - Shapes and Polyhedra.

Solubility - The maximum amount of solute that can be dissolved in a certain volume of solvent at a given temperature is known as the solubility of the solute. The solubility of the solute usually increases with an increase of temperature.

Solute - Dissolved substance in a solution.

Solution - A uniform mixture of two or more substances. For example, sugar dissolved in water is a solution.

Solvent - The liquid into which the solute is dissolved. The solvent of choice in this lab is water.


Materials Required

The crystals that we will be growing in this lab are:

copper (II) sulfate pentahydrate
nickel sulfate hexahydrate
sodium thiosulfate
nickel ammonium sulfate
sodium nitrate
potassium sodium tartrate

You will be assigned one to two crystals to grow. Read the MSDS sheets of your crystals.

Procedure

1. Obtain a vial from your teacher. Neatly write your name and the name of the crystal near the bottom of the vial.

2. Obtain a seed crystal from your teacher. Do not touch the crystal with your finger. Record your observations of the seed crystal.

3. Cut a piece of thread of 20 cm long. Tie the seed crystal on the thread.

Obtain a seed crystal
Tie seed crystal on the thread

4. Tie the thread to the vial cap. Adjust the length of the thread making sure that the crystal does not touch the bottom of the vial and dangles inside the vial.

Tie the thread to the vial cap
Tie the thread to the vial cap

5. Your teacher will provide you with warm saturated solution of the crystal that you are assigned to grow. Your teacher will assist you in pouring the saturated solutions into your vial.

Wam solution on the hot plate
Pour solution into vial


4. When the solution has come to near room temperature, or when you see crystals start to form on the bottom of the vial, gently place the seed crystal in the solution.

Allow solution to cool to near room temperature
Do not disturb the crystal

5. Allow the vial to sit undisturbed.
6. Check back frequently and you'll see your seed crystal grow in size. Your teacher can assist you in removing the crystal from the solution.
7. Make observations and drawing on the growth of your crystals.


Observations

1. Sketch and describe the shape of the seed crystal before you add it to the saturated solution.

Chemical name of your crystal:
Chemical name of your crystal:

2. After your crystals grow, describe the crystals that forms on your string? Look at the shape again. How does their shape differ from the shape of sugar crystals that are on display?

Chemical name and formula of Crystal:
Chemical name and formula of Crystal:

Summary

 

Questions

1. Minerals are crystals. Choose a mineral that is displayed in the classroom and record the appearance of the mineral. Using the links below, research some facts about this mineral.


Mineral Name: _________________________________________________

Chemical Formula: ______________________________________________

General Appearance:


 





Some interesting facts that I found:


WWW Links
1. Alphabetical Listing of Mineral Species
2. Minerals and the 7 Crystal Systems

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Ideas
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Key Words
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Materials
bubble_t_pointer.gif (877 bytes)
Procedure
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Observations
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Summary
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Questions
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WWW Links

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