Cool Crystals: How does the effect of cooling impact crystal growth?
(From Experiment Central, published by U·X·L and available online in Science Resource Center)
Temperature is one of the key environmental factors that affect crystal growth. This experiment examines the outcome of the same crystal-growing solution cooling at three different temperatures. You will place one jar in a cold environment while the crystals grow, the other jar will cool under room temperature conditions, and you will enclose the third jar and store it in a warm area so that it cools the slowest of the three. If the cooling is faster, the particles do not have time to form a large-scale orderly arrangement and a mass of little crystals will form instead. The size of each crystal will demonstrate how temperature impacts the growth of a crystal.
Having a control or standard crystal will help you measure the changes in the dependent variable. Only one variable will change between the control and the experimental crystals, and that is the size of the crystal. For the standard crystal, you will soak a seed crystal in plain water, which will not react with the seed crystal. At the end of the experiment you will compare the size and shape of the seed crystal with each of the other crystals.
What Are the Variables?
Variables are anything that might affect the results of an experiment. Here are the main variables in this experiment:
- The solution's rate of cooling
- The crystal-growing substance
- The surrounding air temperature
- The container the crystals are grown in
- The string the crystals are grown on.
Level of Difficulty
Moderate.
Materials Needed
- Epsom salt
- dental floss
- glass saucepan
- hot plate or stove
- saucer
- measuring cup
- measuring spoons
- four small glass jars
- small piece of cloth to cover glass container
- warm towel
- cold-water bath (pan with ice in cold water)
- stirring spoon
- four pencils (long enough to lay across the tops of the four small glass containers)
- marking pen
Approximate Budget
$2 (most materials are common household items).
Timetable
20 minutes initial time; 30 minutes after several days; 20 minutes over the next 2 weeks.
How to Experiment Safely
You are using very hot water in this experiment. Ask an adult to help you when using the stove or hot plate.
Step-by-Step Instructions
- To grow a seed crystal, heat a half a cup of water until it is almost at the boiling point and carefully pour it into a glass. Add 5 tablespoons of Epsom salt and stir mixture until all the salt dissolves. Continue adding Epsom salt, stirring after each addition, until the solution is completely saturated. You will know you are at the saturation point when a small amount of Epsom salt sinks to the bottom no matter how hard you stir.
- Pour the solution into a saucer and wait at least 24 hours until small crystals have grown in the saucer. This could take 2 or 3 days. Pour out any remaining liquid and choose the four largest crystals that are roughly the same size. These are your seed crystals.
- Cut four pieces of dental floss about 6 inches long. Take each piece and tie one end around a pencil. Cut the piece of dental floss so the other end hangs slightly above the bottom of each jar.
- Carefully tie a seed crystal to the loose end of each piece of dental floss.
- Heat 2 cups of water in the saucepan until it is almost boiling. Remove from heat and add 3/4 cup of Epsom salt and stir. Continue to stir while you add as much Epsom salt as you can--until no more will dissolve. When the solution is saturated, set the saucepan aside to cool for two minutes. Pour equal amounts of the solution into three glass jars.
- In the fourth glass jar pour a roughly equal amount of plain warm water. Hang a seed crystal in each solution by laying the pencil across the jars.
- Let Jar 1 completely cool and then place it in a cold-water bath. Leave Jar 2 at room temperature. Warm a towel in a clothes dryer, wrap it around Jar 3, and drape the piece of cloth over the top of the jar before placing the jar in a warm area, like a cupboard near the stove. Leave Jar 4 at room temperature.
- Every day place fresh ice in the cold-water bath for Jar 1, and reheat the towel for Jar 3. After about a week, compare the crystals.
Summary of Results
Compare the rate of crystal growth, using the control crystal in Jar 4 as your standard. Examine if there are small crystals on the side or the bottom of the jars. Estimate the size of each crystal on the string compared to the standard, or control crystal, that was sitting in the water. Graph your results, using the percentage of growth on the y-axis and the rate of cooling on the x-axis.
Troubleshooter's Guide
Below are some problems that may arise, some possible causes, and some ways to remedy the problems.
Problem: No crystals grew in one or more of the solutions.
Possible Cause: The solution may not have been saturated when the water was hot. You may not have stirred enough to dissolve the Epsom salt. Take out the seed crystal and pour the solution back into the saucepan. Reheat the solution, adding more of the Epsom salt and stirring well after each addition until you see bits of the Epsom salt fall to the bottom.
Possible Cause: The water may not have been hot enough to become completely saturated. It should not be at the boiling point, but it does need to be hot. Take out the seed crystal and pour the solution back into the saucepan. Reheat the solution, adding more of the Epsom salt and stirring well after each addition until it is saturated.
Problem: The crystals are cloudy.
Possible Cause: There may be impurities in the water or the jar. Examine the jar and, if it is dirty, try the experiment again with a clean jar. If the glass is clean, try repeating the experiment using distilled or purified water.