Exploring Rates of Crystallization

Background

Molten rock material, or magma, originates deep beneath the earthÕs crust and comprises the parent materials from which all rocks form. When magma cools, it solidifies through a process known as crystallization to form ingeous rocks. This crystallization process can take place slowly and at great depth, or more rapidly when magma rises and flows out at the earthÕs surface as lava.

When magma cools slowly beneath the earthÕs surface, ions present in the liquid melt are allowed to form large, well-developed crystalline grains. The rocks which form are said to have a coarse-grained texture and are classified as intrusive or plutonic rocks. In contrast, magma, which rises to reach the earthÕs surface produces volcanic eruptions and lava flows. The lava cools more rapidly at the earthÕs surface, not allowing the time are called extrusive or volcanic rocks and exhibit a fine-grained texture composed of individual mineral grains too small to be seen with the unaided eye. In all instances, the rate of cooling directly influences the crystallization process. The slower the cooling, the larger the individual mineral grains that will form. The more rapidly cooling occurs, the smaller the mineral grain size. 

Materials
Magnesium sulfate (Epsom salt)
3 test tubes
1 beaker
3 glass jars or beakers
1 thermometer
metal spatulas
water
ice cubes
pan or pyrex beaker
heat source

Procedure
In this investigation, you will explore how the rate of cooling effects the crystallization process in a common salt, magnesium sulfate. You will then compare your observations to the similar process which occurs when magma is cooled and crystallizes to form igneous rocks.

A. Heat water to a temperature of about 60 degrees Celsius. Do not let it boil.
B. While the water begins heating, measure out about 100 ml of magnesium sulfate. Slowly pour the magnesium sulfate into the beaker of warm water while stirring. Continue to stir and heat the solution in the pan until youÕve reached and maintained the desired temperature of about 60 degrees Celsius and nearly all of the  crystals have dissolved. 
Note: It is important that the solution reach supesaturation before crystallization can take place. If the magnesium sulfate crystals dissolve too readily in the water as it approaches 60 degrees, you may need to add more crystals to the solution before proceeding. Ideally, there should be a small residue of undissolved crystals that will not enter solution once this temperature is reached. 
You may also wish to add several drops of food coloring to the solution during this process to enhance visibility during crystallization. Be careful not to spill any of the food coloring as it may stain your skin or clothing. 
C. While proceeding with steps A & B, have a partner set up the three glass jars and label them. Fill each jar with water to about the 14 oz level as follows: Jar #1 Š water and ice cubes (about 1 degree C); Jar #2 Š tap water at room temperature (about 20 degree C to 25 degree C); Jar #3 Š hot tap water (about 50 degree C to 60 degree C).
D. After completing steps A, B, and C, scoop out the a small quantity of magnesium sulfate crystals and sprinkle a few grains into each of three large test tubes.
E. Next, carefully pour off some of the heated magnesium sulfate solution from the pan into the beaker. Use the beaker to transfer equal amounts of the heated solution to each of the three test tubes. Use a test tube or the tongs to hold and steady each test tube when pouring. Fill the test tubes about two thirds full. 
F. Cover the top of each test tube, one at a time with the cork provided and shake the solution. Remove the cork and then quickly place one of the test tubes in Jar #1, one in Jar #2, and one in Jar #3.
G. Observe what happens as the solutions cool at different rates over the next ten to fifteen minutes. Then set the jars aside overnight, and examine the solutions again the next day.

Questions
1. Examine the crystals formed in each of the three test tubes. Which jar contained the test tube with the largest crystals?
2. Which jar contained the test tube with the smallest crystals?
3. Which test tube cooled the fastest?
4. Which test tube cooled the slowest?
5. What can you concluded about the rate of cooling and its effect on crystal size? Explain your answer.
6. What do you think would happen to the crystal size if you placed a test tube of the Epsom salt solution in an insulated cup of hot water? Why?
7. Examine rock samples A, B, and C provided with this activity. Which of these samples exhibits coarse-grained texture? Which exhibits fine-grained texture?
8. List these three rock samples (A, B, & C) in order by their probable rate of cooling (slowest to fastest). How does your list correspond with the results of your crystallization experiment?
9. Examine your four rock samples (A, B, C, & D). Which rock samples represent intrusive igneous rocks? Explain your answer.
10. Which of the 4 rock samples represent extrusive igneous rocks? Explain your answer.
11. Which rock sample contains both coarse and fine-grained minerals? What is this texture called? How might you explain the cooling conditions present during this rockÕs formation.