by Nicole Chin
What is a crystal?
A crystal is a piece of a homogeneous solid substance having a naturally geometrically regular form with symmetrically arranged plane faces.
What substances can be used to make crystals?
Some substances that can be used to make crystals are alum, sugar, Rochelle salt, table salts, silver, tin, nickel sulphate.
What are some examples of crystals in nature? What household items can you also grow crystals from?
Crystals are found in almost every rock and many other solid type substances found in nature. Some crystals are so small they cannot be seen by the naked eye. You can use sugar and water or salt and water to grow crystals.
Explain the process of crystal growth.
Crystal growth is a major stage in the crystallisation process. You dissolve the solute into the solvent which creates a solution. After a few
Explain how crystals can grow in different shapes and sizes.
Crystals grow into certain shapes because the atoms or molecules join together in a pattern that repeats itself over and over to create a certain shape.
A crystal grows by adding atoms or molecules to all its sides in the exact same pattern as the atoms and molecules that were added before.
Because each different crystal is made up of a different building block (atom or molecule) they each have a different structure or shape.
Outline a few different types of crystals.
- Triclinic - usually not symmetrical from one side to the other
- Monoclinic - like skewed tetragonal crystals, often forming prisms and double pyramids
- Orthorhombic - like tetragonal crystals except not square in cross section (when viewing the crystal on end), forming rhombic prisms or dipyramids (two pyramids stuck together).
- •Tetragonal - similar to cubic crystals, but longer along one axis than the other, forming double pyramids and prisms
- Trigonal - possess a single 3-fold axis of rotation instead of the 6-fold axis of the hexagonal division
- Hexagonal - six-sided prisms.
- Cubic - not always cube shaped. They may be octahedrons (eight faces) and dodecahedrons (10 faces).
What effect do crystals have on light travelling through them? (relate to reflection, refraction, dispersion and diffraction)
Crystals have a high refractive index. This means that light inside them is bent and travels slower than in air. Because of their high refractive index, they often have total internal reflection.
What are the optimum conditions for crystal growth?
Warmth is key to forming crystals, the jar's surroundings should be warm also for optimum crystal growth. Warm air temperature aids water evaporation, causing the crystals to grow more quickly. Crystals will still grow in cooler temperatures, but it will take much longer for the water to evaporate.
Crystal growth also requires light. Again, the crystals will eventually grow in the dark, but it will take a very long time. Light evaporates water as heat does; combine them by placing your jar on a warm, sunny windowsill and you should have crystals in a few days.
· 25 g of potash alum
· 2 beakers (250 ml or bigger)
· 170 ml of hot water
· Filter Funnel
· Filter Paper or Watch Glass
Make sure all apparatus is clean and dry.
1. Place approximately 25 g of potash alum in a
beaker (250 ml or bigger) and add approximately 170 ml of hot water.
2. Stir the mixture until all the crystals have
dissolved. The solution may have a slightly cloudy or milky appearance due to
impurities in the technical grade of alum used.
(NOTE: don’t use all the alum you have been given – keep a few crystals back in
reserve in case you need to “seed” the solution later.)
3. Filter the warm solution through a filter funnel
(in which filter paper has been inserted) into another clean beaker.
4. Cover the beaker of solution with the watch
glass or fresh filter paper and set aside in a cool sheltered place and allow
it to stand undisturbed overnight. The first beaker and funnel can be washed
and dried and the residue and filter paper used for filtration discarded.
· Stirring Rod
1. The next day, observe the beaker of solution.
The bottom should have become covered with a layer of smallish crystals which
formed spontaneously as the solution cooled.
Pour off the clear solution above the crystals
into a clean beaker and set it aside for later, leaving the crystals behind in
the other beaker. If for any reason, no crystals have formed after leaving the
solution to stand overnight, the solution can be “seeded” by adding a crystal
from the original alum.
Let it stand overnight again and observe the
formation of crystals. Alternatively, if nothing has happened, crystal growth
may also be induced by scratching the bottom of the glass beaker with a glass
· Plastic Spoon/Spatula/tongs/tweezers
· Nylon thread
· Stirring rod
1. From the bed of crystals, one good symmetrical crystal or group of crystals needs to be selected to act as “seed” for your big crystal.
2. Using a plastic spoon, spatula, tongs or tweezers, transfer the selected crystal to the beaker containing the decanted solution, trying to place it in the centre of the beaker. A good technique to promote uniform growth is to suspend the crystal with a nylon thread tied round a stirring rod or pencil resting on the rim of the beaker. This step is not essential, however, and good a crystal can usually be obtained just by leaving the crystal on the bottom of the beaker.
In conclusion, I found this task very enjoyable and interesting, watching crystals grow. However, during the holidays, I noticed the crystals' sizes hardly changed compared to the second day of growing the crystals. The liquid levels in the jar hardly changed.
The picture below demonstrates the properties of the crystals well for it shows light bouncing off it, creating a rainbow effect on the table.
Above is a picture of my longest crystal. It is approximately 9mm long.