Introduction: (Initial Observation)
Crystals are beautiful and decorative objects. But that’s not why we make them. Crystallization is a very important process in many chemical and pharmaceutical and other manufacturing activities. Crystallization is the way that we purify many materials. For example rock salt (the salt we extract from the earth), caries some sand and mud and is not good for human consumption. Salt factories will purify the rock salt by crystallization and make pure salt, table salt and cooking salt.
Even sugar extracted from sugar cane is purified by the process of crystallization. During crystallization, pure material will form crystals and impurities will separate. In this project we discuss different methods and techniques of making crystals.
Ben had a different initial observation.
He made a cup of salt water for a buoyancy test and left it on his bookshelf and totally forgot about it. After a week, he accidentally noticed the cup with thousands of small crystals grown all over the cup and even on the shelf. It was a mess and it raised a lot of questions. How did the salt crystals come out of the cup and got to the shelf and outside wall of the cup?
Information Gathering:
Find out about crystallization methods. Read books, magazines or ask professionals who might know in order to learn about the growing crystals and the specific substance that you want to crystallize. Keep track of where you got your information from.
Following are samples of information that you may gather if you are planning to make salt crystals.
Salt Properties:
- Crystals or white crystalline powder
- Transparent and colorless in crystalline form, rather like ice Crystallizes in the isometric system, usually in the form of cubes Soluble in water (35.6g/100g at 0°C and 39.2g/100g at 100°) Slightly soluble in alcohol, but insoluble in concentrated hydrochloric acid
- Melts at 801°C and begins to vaporize at temperatures just slightly above this
- boiling point 1,413°C
- Hardness of 2.5 on the MHo scale of hardness
- Specific gravity of 2.165
- Non combustible – Low toxicity
- Hygroscopic – absorbs moisture from damp atmospheres above 75% relative humidity.
When we have a saturated solution of a substance, molecules of that substance tend to join each other in a structured form that we call it crystals. Certain conditions should be met for this process to happen.
Visit the following links for more details:
Growing one single large crystal is a challenge. In most crystallization methods you will get many small crystals. To grow one large crystal, you may carefully transfer the saturated solution to another container and then transfer only one of the small crystals to the new container. In this way, your single crystal will get chance to grow larger.
You may find smaller crystals are growing on your large crystal. Such small crystals may be separated with little force.
Salt crystals with high purity are mined in the form of huge, clear rocks. These rocks are very large crystals.
Many companies in the world cut and shape large rock salt crystals to make lamps.
Growing large salt crystals can be best performed using high quality – pure salt. You may purchase high quality pure salt from grocery stores as “Kosher Salt”. You may also buy high quality salt from hardware stores and water treatment companies. They sell it as “Solar Salt”. Each of the solar salt crystals may be up to one cubic inch in size.
Question/ Purpose:
What do you want to find out? Write a statement that describes what you want to do. Use your observations and questions to write the statement.
Crystallization is one of the important methods of purifying chemicals. The purpose of this project is getting familiar with crystallization process and find out about the conditions that may facilitate crystallization.
You may also pick any of these questions and seek for the answer while you work on this project.
Why do we make crystals?
How crystals are made?
Can we grow crystals at home?
Do all material have crystal?
What is the shape of a Salt Crystal?
How can we grow large crystals?
What businesses make crystals?
More Advanced question for students in grade 5 and up:
1. What is the effect of wind (air flow) on the rate of crystallization?
2. How does the cooling temperature affect the amount and the size of crystals?
Identify Variables:
(This section is not required for primary students, grades 1-4)
If you want to repeat the experiments of making crystal in order to find out the best conditions for making crystals, then you need to decide on your variables. Factors or variables that you control and set and may affect the production of crystals in your experiments are:
1. Temperature of solution.
2. Concentration of Solution (How many grams salt per Liter)
3. Heat exchange of your crystallization bath. (The speed that it cools off)
4. Method of Crystallization?
The above variables are called independent variables or manipulated variables. You must select only one of the above variables for your study.
The dependent variable (also known as responding variable) can be:
1. The speed of crystallization
2. Maximum size of crystal
3. Rate of crystal production
In other words you modify the value of independent variable during your experiment and record the value of your dependent variables.
For the advanced question #1 above, this is how you define variables:
Independent variable (also known as manipulated variable) is the wind condition.
Dependent variable (also known as responding variable) is the ratio of crystallization.
Constant is the crystallization time.
For the advanced question #2 above, this is how you define variables:
Independent variable (also known as manipulated variable) is the cooling temperature.
Dependent variable (also known as responding variable) is the amount of crystals and the size of the largest crystal.
Constant is the crystallization time.
Hypothesis:
Depending on the question that you have selected, make an educated guess about the answer. That will be your hypothesis. Do it after some study, but before your experiment. This is a sample hypothesis:
When a solution of salt water stands still for a long time, molecules of salt move and bind to each other to form crystals. In other words if we continuously vibrate or stir the solution, no crystals will form.
For the advanced question #1 above, this is a sample hypothesis:
I hypothesize that air flow will increase the rate of crystallization. My hypothesis is based on my observation that wind speeds up evaporation of water and drying of wet clothes.
For the advanced question #2 above, this is a sample hypothesis:
My hypothesis is that when the cooling temperature is lower, more crystals will form and crystals will be larger.
Please note that hypothesis may happen to be wrong. You will find out after doing your experiments.
Experiment Design:
Design display to show a habitat of any animal of your choice. As a part of your habitat display you must show the food in which your animal feeds. This can also lead to a food chain display.
Experiment 1: Effect of air flow or wind on the rate of crystallization of saltwater.
Introduction: Sea water and saltwater from salt lakes are among the sources of salt in many regions. Producers want to know the factors that can increase the rate of crystallization. In this way they can produce more salt and generate more income. If air flow can increase the rate of crystallization, then producers may use the natural air flow to produce more salt. All they need to do is to use natural barriers to direct the air flow to their crystallization containers.
Procedure:
1. Dissolve 340 grams of kosher cooking salt in one liter of hot water and stir it for 5 minutes. (this is about 11 ounces of salt in 1/2 gallon hot water). Let the mixture sit for 5 minutes until the top layer is completely clear and remaining salt precipitates at the bottom. (Do not use table salt in this experiment because table salts are often mixed with small amounts of flours, starch or other desiccants in order to keep them dry and flowing)
2. Use measuring cups or graduated cylinders to transfer equal amounts of the clear saltwater to 4 identical plates or bowls. Make a note of the amount of saltwater in each bowl. (For example you may have 250 mL of salt water in each bowl). Also weigh and record the weight of each bowl.
3. Place three bowls in a row in front of an electric fan. Position them so that the first one is one foot away from the fan, the second one is 2 feet from the fan and the third one is 3 feet away from the fan. Label the bowls as “High Wind”, “Medium Wind” and “low wind”. (High wind is the one that is closest to the fan).
Place the 4th bowl away from the electric fan. Label this bowl as “Control” or “No Wind”.
4. Start the electric fan and record the time. Crystals will start to form in about 5 minutes. Let the electric fan run for 12 to 24 hours (or until most of the saltwater in the first bowl are crystallized.
5. Stop the electric fan. Carefully dispose of the remaining saltwater in each bowel and only keep the crystals. Take pictures if you need. Place each bowl in a scale and record the weight of crystals and the bowl. By deducting the weight of the empty bowl from the total weight you can calculate the weight of crystals in each bowl.
Record your final results in a table like this:
| Air Flow | Saltwater | Crystallized | Ratio |
| High Wind | 250 mL | 210 grams | 0.84 |
| Medium Wind | |||
| Low Wind | |||
| No Wind (Control) |
In the above table the amount of Saltwater may be specified as volume in mL (milliliters) or may be specified as weight in grams. If you want to specify the weight of saltwater, then you must use a scale to measure it before turning on the fan.
Make a graph:
You may use a bar graph to visually present your results. Make four vertical bars and label them “High Wind”, “Medium Wind”, “Low Wind”, and “No Wind”. The height of each bar must show the ratio of crystallization. For example a bar that is 6.5″ tall may represent 65% crystallization. To make it more understandable, also write the crystallization ratio for each wind condition on the top of the bar it represents.
Experiment 2: How does the cooling temperature affect the amount and the size of crystals?
Introduction: In this experiment we will compare the amount of crystals formed in 3 different storage temperatures.
Procedure:
1. In a cooking pot make a saturated solution of hot salt water. (For example you may dissolve 340 grams of kosher cooking salt in one liter of hot water and stir it for 5 minutes. (this is about 11 ounces of salt in 1/2 gallon hot water). Let the mixture sit for 5 minutes until the top layer is completely clear and remaining salt precipitates at the bottom. (Do not use table salt in this experiment because table salts are often mixed with small amounts of flours, starch or other desiccants in order to keep them dry and flowing)
2. Transfer the salt water to 3 identical cups or bowls. Labels the cups as “Cold”, “Room”, and “Warm”.
3. Place the cup labeled “Cold” in the refrigerator. Keep the cup labeled “Room” at room temperature. Place the cup labeled “Warm” in a warm oven or any other warm space you may have at home.
4. After 24 hours empty excess water from all cups and compare the size and the amount of crystals in all cups. Possibly look at the crystals using a magnifying glass.
5. If you have a scale, weigh the cups to see how much crystal is formed in each cup.
6. If you have a ruler, measure the size of the largest crystal.
7. Record your results in a table like this:
| Storage Temperature | Amount of crystals formed after 24 hours | Size of the largest crystal |
| Cold | ||
| Room Temperature | ||
| Warm |
Which cup had the largest crystal?
Which cup had more crystals?
Make a graph:
You may make a bar graph to visually present your results. To make a bar graph for the amount of crystals, Make three vertical bars and name them “Cold”, “Room” and “Warm”. The height of each bar will represent the amount of crystals formed on that temperature. For example you may make a bar that is 5 centimeters tall to indicate that 5 grams of crystals was formed.
More experiment ideas: You may want to study the effect of temperature on the rate of crystallization. This is very similar to the experiment # 1. The only difference is that you will substitute the electric fan with a heat lamp. The closer a sample is to the heat lamp the more heat it will receive. You may also place one thermometer in each bowl so you know the exact temperature of each bowl. Everything else will be adapted from the experiment 1.
Other methods of making crystals:
There are several methods of making salt crystals. Some methods use molten salt (in about 900 degree centigrade) and some use a salt solution.
Here we only discuss the methods that use salt solution. These methods can be divided by three main group as follows:
1. Growing crystal using a seed crystal.
2. Growing Crystals using a wick.
3. Growing Crystals by condensation.
Wick Method
Procedure:
This is a simple procedure for growing salt (Sodium Chloride) crystals.
Add salt to a cup of very hot water until saturation is reached. The solution is saturated when no more salt can be dissolved and some salt remain at the bottom of the cup. Pour the solution off into a clean jar, leaving behind any un-dissolved salt.
Suspend a thin thread into the center of the jar. The thread can be tied to a pencil, Popsicle stick, or a hole made in the lid of the jar. Let sit, then after 15 minutes, swish the jar a bit. swish it again 15 minutes later, then one final time an hour later. Set the jar where it won’t be disturbed. The crystals should begin to grow in a few hours or so, and continue to grow for several days.
Old-fashioned rock candy is grown using basically the same setup as above. Sugar is used for the solution, and the crystals are typically grown on a wooden stick rather than a thread.
This is a step by step Procedure.
1. Get a cup or beaker.
2. Heat up some water in a kettle to boiling point.
3. Put some of the boiling water in a cup/beaker.
4. Add some salt.
5. Stir.
6. Add more salt.
7. Stir.
8. Keep adding more salt and stir until no more salt goes into solution, and there are a few Salt crystals left out of solution.
9. Take a pencil and tie a piece of string in the middle of it.
10. Make sure the string is about half the length of the cup/beaker.
11. Tie a big knot in the end of the string.
12. Put the knotted end of the string in the cup/beaker and let the pencil rest on the rim of the cup/beaker, so that the knot is dangling in the salt solution.
13. Leave the solution to cool SLOWLY!!
14. A crystal of salt should form on the knot.
Warning: When you put hot solution in a glass container, be very careful. Some glass jars and some cups are not made for hot liquids. If you are not sure, just use a good plastic container.
The same method can be used to grow a large crystal. To do that you attach a small crystal that you have already made to the Knot as a starter seed. You can repeat the above steps many times but continue to use the s
What is Saturation?
Saturation is when a solution can not accept any more solute.
To make a saturated salt solution, add enough salt so some salt will remain un dissolved.
why do we have to swish the jar of salt solution in a wick method?
Initially crystals have a tendency to form on the surface of water. Swishing will make them fall and possibly land on the knot as a seed crystal or starter crystal.
Salt-Water Ratio:
170 grams of salt can dissolve in 500 ml water to make a saturated salt solution. (That is about 5.5 ounces of salt in one quart water. Or 22 ounces of salt in one gallon water).
Seed Method
Growing a single large salt crystal
Procedure:
With a little more effort it is possible to produce a single large symmetrical crystal. First fill a jar with hot water, and stir in as much salt as will dissolve. This is a supersaturated solution. Allow the excess salt to settle out, and pour the mixture into a saucer, leaving all of the undissolved salt behind.
As the water cools, tiny crystals will begin to form on the bottom of the saucer. These are the “seed” crystals. Using a magnifying glass, pick out the largest perfectly formed crystal. Hold on to any well shaped seed crystals in case your first attempt does not go well.
Mix up another batch of hot supersaturated solution. Allow the undissolved salt to settle out as before, and pour off into a clean jar. You can use a coffee filter or clean napkin as a filter. Tie a thin thread to the seed crystal and hang it in the solution. Cover the jar with a piece of paper to slow evaporation.
As the water evaporates over the course of several weeks, the salt will attach to the growing crystal. Remove any other crystals as they form. From time to time, you should add more cooled, supersaturated solution. Don’t let the jar get too warm, as this may dissolve the crystal. Crystals formed this way tend to be very fragile, and of course will be dissolved by water.
This can be tried with other substances, such as bicarbonate of soda, alum, copper sulfate, photographer’s hypo, borax, laundry soda, etc.
Rock salt is used for melting ice and snow in winter. As you see in the picture, rock salt is not a pure salt. We dissolved some rock salt in the water and boiled it for a few seconds in a metal pot. After the salt solution gets cold, poured it in a glass tank with wicks hanged from a wooden dowel. After two weeks, mass of small crystals grow on the surface and the walls of the tank and just a little on the wick.
Some other interesting activities to do:
Growing one big alum crystal
Heat one cup (250 ml) of water to boiling. Stir in about 3 tablespoons (45 ml) of alum until it dissolves. Pour the solution into a clean jar. Cover with plastic wrap or wax paper and set in a place where it will not be disturbed for several days. If the solution was pure enough, you should end up with a single large crystal.
Epsom frosted glass
Dissolve Epsom salts (magnesium sulfate) in a pot of boiling water until no more will dissolve. Add a drop or two of white glue (not critical). Brush a small amount of the liquid onto a glass or bottle. The liquid will quickly evaporate, leaving behind a film of delicate crystals.
Stalactites
Stalactites are the long rock columns that grow from the roofs of many caves. Stalagmites are similar columns that grow from the cave’s floor. They are made from the minerals in water that drip down through the cave. It is possible to simulate the growth of these formations.
Fill two jars with very warm water. Add as much baking soda or Epsom salts to each jar as will dissolve. Place a saucer between the two jars, which are about a foot apart. Dip one end of a piece of absorbent (cotton, wool – some synthetics won’t draw the solution) yarn, thick thread or string into each jar. The ends should be weighted with washers, nails or what have you to keep them in the jars. Let it hang in the middle, over the saucer. Capillary action should draw the baking soda solution up through the yarn, where it will drip onto the saucer. Over the course of several days, the dripping water will deposit the baking soda, forming a tiny stalactite and stalagmite. Eventually these may join to create a single column, as in an actual cave. Epsom salts generally take longer, but yield more variety of shape.
Materials and Equipment:
Depending on the experiment you choose, you may need some of the following materials:
1.One bag or box of cooking/kosher salt (about 2 lbs).
2.Water or hot water.
3.Heat source such as an electric or gas stove.
4.Cooking pot used to heat up saltwater.
5.Electric Fan.
6.Heat lamp
7.Refrigerator
8.Bowls or deep plates (Such as Evaporating Basin or Petri-dishes)
Some salts such as Alum can form larger crystals or more crystals in less time. Most crystal growth kits are using Alum or other chemicals. This is a sample:
Results of Experiment (Observation):
Experiments are often done in series. A series of experiments can be done by changing one variable a different amount each time. A series of experiments is made up of separate experimental “runs.” During each run you make a measurement of how much the variable affected the system under study. For each run, a different amount of change in the variable is used. This produces a different amount of response in the system. You measure this response, or record data, in a table for this purpose. This is considered “raw data” since it has not been processed or interpreted yet. When raw data gets processed mathematically, for example, it becomes results.
Calculations:
If you do any calculation during your experiment, write your calculations here.
Summary of Results:
Summarize what happened. This can be in the form of a table of processed numerical data, or graphs. It could also be a written statement of what occurred during experiments.
It is from calculations using recorded data that tables and graphs are made. Studying tables and graphs, we can see trends that tell us how different variables cause our observations. Based on these trends, we can draw conclusions about the system under study. These conclusions help us confirm or deny our original hypothesis. Often, mathematical equations can be made from graphs. These equations allow us to predict how a change will affect the system without the need to do additional experiments. Advanced levels of experimental science rely heavily on graphical and mathematical analysis of data. At this level, science becomes even more interesting and powerful.
For this experiment I used a small (75mm) Evaporation Basin that is specially used for crystallization. I also dropped a small wood block on the surface of the saltwater so that the crystals will also form on the wood block. I placed my crystallization dish in front of an electric fan, so the crystallization completed in 24 hours.
Crystals grew to the edge of the evaporation basin as all over the wood piece.
I noticed three distinct crystal forms. the first group are small cubic shape crystals. The second group are thin flat crystals. The fourth group are the filament or fiber like crystals.
Most fiber like crystals are formed at the edges of the evaporating dish.
I did not get any large cubic crystals.
Crystallization basin is available at major scientific suppliers.
How can I make large cubic crystals?
There are two factors that affect the size of crystals. The first factor is the size of the evaporating basin. The largest cubic crystals are often 200 times smaller than the diameter of the evaporating basin you use. In other words to make crystals that are one cubic inch, you must have a very large evaporating basin that is about 200 inches (about 8 feet) in diameter. The second factor is the speed of crystallization. Large crystals need slow crystallization. To get large crystals you must not use excess wind and heat to speed up the crystallization.
Another type of crystallization container that I recommend are plastic petri-dishes.
In a crystallization experiment in petri dishers and using electric fan, crystals started to form in the first 5 minutes (before the liquid is cooled down).
In about one hour I was able to see crystals at the bottom and on the surface of the saltwater.
Some crystals also formed on the sides. These are the crystals that may grow large.
In about 12 hours exposure to the wind, almost 85% of the salt crystallized and I could see some relatively large crystals on the sides.
This picture shows some of the large crystals on the side. You can also see that some small crystals are formed over the larger crystals.
Petri dishes are available at MiniScience.com
Conclusion:
Using the trends in your experimental data and your experimental observations, try to answer your original questions. Is your hypothesis correct? Now is the time to pull together what happened, and assess the experiments you did.
Related Questions & Answers:
What you have learned may allow you to answer other questions. Many questions are related. Several new questions may have occurred to you while doing experiments. You may now be able to understand or verify things that you discovered when gathering information for the project. Questions lead to more questions, which lead to additional hypothesis that need to be tested.
Possible Errors:
If you did not observe anything different than what happened with your control, the variable you changed may not affect the system you are investigating. If you did not observe a consistent, reproducible trend in your series of experimental runs there may be experimental errors affecting your results. The first thing to check is how you are making your measurements. Is the measurement method questionable or unreliable? Maybe you are reading a scale incorrectly, or maybe the measuring instrument is working erratically.
If you determine that experimental errors are influencing your results, carefully rethink the design of your experiments. Review each step of the procedure to find sources of potential errors. If possible, have a scientist review the procedure with you. Sometimes the designer of an experiment can miss the obvious.
References:
Visit your local library and find books related to crystallization or general chemistry. List the books and the websites you use as your bibliography.
Following is a sample:
Price, Chris J., “Take Some Solid Steps to Improve Crystallization”, Chemical Engineering Progress, September 1997, p. 34.
Geankoplis, Christie J., Transport Processes and Unit Operations, 3rd Ed., Prentice Hall, New Jersey, 1993, ISBN: 0-13-930439-8
Brown, Theodore L., Chemistry: The Central Science, 5th Ed., Prentice Hall, New Jersey, 1991,
ISBN: 0-13-126202-5
Handbook of Separation Process Technology
Edited by: Rousseau, Ronald W. © 1987 John Wiley & Sons
After you complete your experiments about crystals, you may write a report and an abstract about your project and your results. Following is a sample abstract:
ABSTRACT
The effect of temperature on growing crystals
The goal of this project was to find out how does the temperature affect the growth of crystals. I made an over saturated solution of salt in hot water, transferred it to 3 identical plastic containers and placed them at 3 different temperatures of -10ºC, +5ºC and +22ºC for 2 days so the crystals will form. Observation showed that the most crystals are formed at -10ºC. As sources of temperature I used the freezer, refrigerator and room temperature.
