Saturated Solutions and Crystallization Lab
Rock Candy
PRE LAB DISCUSSION
This is an experiment in controlling crystal growth. Rock candy, like most candy, is made primarily from sugar. The candy can be anything from large single crystals to an amorphic solid. In this experiment the students will produce crystals of sucrose.
Making fudge is an excellent follow-up to this experiment. The key to making good fudge is controlling crystal growth in a supersaturated solution. Most fudge recipes use both chemical and mechanical means to reduce the size of the sugar crystals in order to have a creamy texture. It is difficult to make fudge in most classrooms, however. An analysis of fudge recipes will extend the appreciation of how chemical principles are used in candy making. Students may wish to make fudge at home and have the class evaluate their results.
This experiment will take several days to complete.
It's easy to grow your own sugar crystals! Sugar crystals are also known as rock candy since the crystallized sucrose (table sugar) resembles rock crystals and because you can eat your finished product. You can grow beautiful clear sugar crystals with sugar and water or you can add food coloring to get colored crystals. It's simple, safe, and fun. Boiling water is required to dissolve the sugar, so adult supervision is recommended for this project.
Difficulty: Easy
Time Required: a few days to a week
Remember: You should make your seed crystal first! Make a BRAND NEW sucrose solution to begin crystallization! You can also double the recipe to fill a larger glass or jar. Make sure you fully clean the container you are going to grow your crystals in!! Use a glass container and wash it a few times with very hot soapy water. To really insure a lack of contaminates rub the inside with some alcohol from your medicine cabinet and give it a last rinse with DISTILLED WATER. YOU MUST USE DISTILLED WATER FOR THE EXPERIMENT NOT FAUCET WATER!!!
Here's How:
1. Gather your materials.
2. You may wish to grow a seed crystal, a small crystal to weight your string and provide a surface for larger crystals to grow onto. A seed crystal is not necessary as long as you are using a rough string or yarn.
3. Tie the string to a pencil or butter knife. If you have made a seed crystal, tie it to the bottom of the string. Set the pencil or knife across the top of the glass jar and make sure that the string will hang into the jar without touching its sides or bottom. However, you want the string to hang nearly to the bottom. Adjust the length of the string, if necessary.
1. Begin adding sugar to room temperature water until there is sugar at the bottom of the pot that will not dissolve no matter how much you stir. You have officially reached the saturation point!
2. Now bring the water in the pot to a boil. Stir in the sugar, a teaspoonful at a time. Keep adding sugar until it starts to accumulate at the bottom of the container and won't dissolve even with more stirring. This means your sugar solution is supersaturated. If you don't use a supersaturated solution, then your crystals won't grow quickly. On the other hand, if you add too much sugar, new crystals will grow on the undissolved sugar and not on your string. DO NOT LEAVE UNDISSOLVED SUGAR CRYSTALS IN THE CONTAINER YOU ARE GOING TO GROW THE CRYSTALS IN!!!! REMOVE THEM!!!
3. If you want colored crystals, stir in a few drops of food coloring. Add flavor if you like.
4. Pour your solution into the clear glass jar. If you have undissolved sugar at the bottom of your container, avoid getting it in the jar.
5. Place the pencil over the jar and allow the string (with your seed crystal) to dangle into the liquid.
6. Set the jar somewhere where it can remain undisturbed. If you like, you can set a coffee filter or paper towel over the jar to prevent dust from falling into the jar.
7. Check on your crystals after a day. You should be able to see the beginnings of crystal growth on the string or seed crystal.
8. Let the crystals grow until they have reached the desired size or have stopped growing. At this point, you can pull out the string and allow the crystal to dry. You need to place them in a sealed ziplock bag with your name and period on it and bring to class.
Tips:
1. Crystals will form on a cotton or wool string or yarn, but not on a nylon line. If you use a nylon line, tie a seed crystal to it to stimulate crystal growth.
2. If you are making the crystals to eat, please don't use a fishing weight to hold your string down. The lead from the weight will end up in the water -- it's toxic. Paper clips are a better choice, but still not great.
What You Need:
· 1 cup water
· 3 cups table sugar (sucrose)
· clean glass jar
· pencil or butter knife
· string
· pan or bowl for boiling water and making solution
· spoon or stirring rod
OBSERVATIONS: make observations DAILY!!! Try and estimate accurate size, shape and please illustrate what you are looking at! (that means draw it)
PART I
Day one observations
Day two observation
Day three observations
Day four observation
Day five observations
PART II
Observations of the clarity or crystal structure of the candy
Calculations (you must show all work for credit!)
1. Sucrose is C12H22O11 calculate the molarity of a solution containing 54g of sucrose and 2.85 L of water.
2. What is the amount of moles of sucrose in a solution containing [3.2M] and 5.7L of water?
3. If your final saturate solution of sucrose contains 86g dissolved in .5L of water what is the molar concentration of the solution?
4. If you have an 8M stock solution of sucrose how much do you need to make a 2.5M solution at 3L? What amount of water do you need to add your stock solution to?
5. You are working for a candy company and you need to create a specific molar solution of sucrose to begin growing your crystals. You have a stock solution of 15M sucrose, you are going to make 500mL of a 6.5M solution to begin crystallization. How much stock solution is required and how much water do you need to add to it?
6. Explain the importance of the seed crystal.
7. Using what you have learned in class explain the process of sucrose crystallization starting with the unsaturated solution and ending with the full grown crystals.