Crystal Formation

Year 9 Science

Initial Research

In physics, the term "crystal" describes a solid substance with internal symmetry and a related, regular surface pattern. Inside a crystal, all the atoms are arranged in the same pattern. This makes the crystal uniform and makes parts of it the same shape. It also allows light to travel through it in interesting ways. Some common examples of crystals are salt and sugar. In ancient times it was believed that crystals were ice that would never melt.

Crystals can be made from all sorts of things. The crystals we will be making will be large aluminium potassium sulphate dodecahydrate crystals. Crystals can easily be made from household items like salt and sugar.

Crystals can form in many different ways, for example diamonds are crystals where carbon atoms have been pushed together by the pressure underground. The crystals we will make are formed using water. The different chemical are dissolved in water. When the water evaporates, it allows the atoms in the water to fall back into a geometric pattern. As the water dissolves the atoms get closer and closer together until you have a crystal. Crystals can grow in different shapes and sizes depending on the amount and type of substances used. The crystals we make will be cubic, meaning the atoms form together in a cube shape. Salt and Cubic Zirconia are also cube shaped at an atomic level. Diamonds have a hexagonal atomic structure. Some other shapes are monoclinic, triclinic and rhombic. If you use more of the water and chemicals, your crystal will be bigger. Also, if you let it grow until all the water is evaporated it will be bigger. If you pull it out before all the solution has evaporated it will be smaller.

Crystals have interesting effects on light. Crystals generally have a high refractive index meaning that light inside them is bent and travels slower than in air. Because of their high refractive index, they often have total internal reflection which is demonstrated by the red line of light in the digram above.  Crystals in jewellery are often mounted in reflective substances to make them appear more brilliant. They are also cut at specific angle to make sure there is the maximum amount of total internal reflection. Sometimes the light exiting the crystal is refracted so much the dispersion happens. This is demonstrated by the blue line in the digram above. When you are looking at a crystal and you see a small spec of colour in it, this is dispersion. When talking about diamonds, brilliance describes the amount of total internal reflection and fire refers to the amount of dispersion. The diamonds pictured below have a high amount of 'fire.'

The ideal condition for growing crystals is warm. The best place for growing a crystal is generally a warm place, like a windowsill. However, not in direct sunlight. The temperature should stay as constant as possible. The reason this is the best condition is because when making crystals you want the solution to evaporate. Light and heat will do this much faster.

Method

Session 1 - Tuesday

Materials

  • 2 250ml beakers
  • Filter Funnel
  • Filter paper
  • Hot water
  • 25g potash alum
  • Stirring rod
  • Place 25g of potash alum in a beaker (250ml or bigger.)
  • Add 170ml of warm water.
  • Stir the mixture until all the crystals have dissolved.
  • Filter the solution through a filter funnel (with filter paper) into a clean beaker.
  • Cover the beaker with a watch glass or another piece of filter paper.
  • Leave in a cool sheltered place overnight.

Session 2 - Wednesday

Overnight the bottom of the beaker should have become covered with a small layer of crystals. If the are no crystals on the bottom, follow the steps below. If there are crystals, move onto session 3.

  • Add one potash alum crystal to the mixture.
  • Cover and leave overnight.

OR:

  • Scratch the bottom of the beaker with a glass stirring rod.
  • Cover and leave overnight.

Session 3 - Thursday

Materials

  • 2 250ml beakers
  • Tweezers
  • Ruler
  • Camera
  • Overnight, the bottom should have become covered with a layer of small crystals.
  • Carefully pour the liquid that remains in the beaker into a clean beaker.
  • From the crystals at the bottom of the beaker choose one good symmetrical crystal to be used as a 'seed' for a big crystal.
  • Remove this crystal from the beaker and put it into the other beaker. Try to put it into the middle.
  • Take a picture of the crystal next to a ruler and mark the height of the liquid on the beaker. Record the date.
  • Cover the beaker with paper and put it in a safe spot that isn't in direct sunlight or near a heater.

Materials Saftey Data Sheet

Observations - Day 1

When we poured the hot water over the potash alum, most of it dissovlved instantly. The rest dissolved easily afer we stirred it. After we filtered the it, the result was a clear solution.

Observations - Day 2

This morning when we checked the beaker the were lots of crystals! They were nicely shaped and bigger than I expected. They were clear.

Obsrevations - Day 3

Today we moved the crystals from the beaker into our own jars to take home. I took two crystals, on that stayed in the bottom of my jar and one that I suspended in the solution using string and a pencil. When I got home I put my jar in the laundry close to the window but not in direct sunlight.

Observations - Day 7

The crystals have grown so much! The one growing on the bottom is much bigger than the one hanging from the string. The solution has also evaporated a little bit.

Observations - Day 12

Again, the crystals have been growing, they are even bigger! There are a few tiny crystals starting to form in the jar as well.

Observations - Day 20

They have grown a little bit bigger today. Not much has changed though.

Observations - Day 27

6 days ago, I decided to shine a lamp on my crystals to speed up the evaporating a bit. It really did speed things up, but it changed some things too. The crystal that was hanging from a string went a bit weird. There was also some crystal formation on the bottom of the jar.

Levels of liquid in the beaker

Final Photos

Total Internal Reflection
Total Internal Reflection
Refraction

These photos demonstrate the crystals ability to refract and internally reflect light. It also shows that they are translucent, allowing some light to pass though.

My Photography set-up

To take my photos I used a Nikon D200 camera with a NIKKOR Micro 55mm lens. The shutter speed was really slow so I used a tripod and a remote shutter release to make my camera steady. I shone a torch through a slit in some black paper and put the crystals on black paper.

Activity Sheet

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