Crystal growth experiments involve precipitating substances dissolved in solution as crystals and observing their size, shape, color, purity, yield, and other properties.
In basic chemistry, inorganic chemistry, and organic chemistry experiments at universities, they may be conducted as recrystallization experiments, preparation of coordination-compound crystals, crystallization of salts, or introductory single-crystal growth experiments.
In a crystal growth report, it is not sufficient simply to write that “crystals formed” or “large crystals were obtained.”
It is necessary to discuss how crystal size and shape are related to cooling rate, degree of supersaturation, nucleation, crystal growth, type of solvent, impurities, stirring, temperature changes, and other factors.
In addition, when the crystals are fine, unevenly colored, low in yield, or low in purity, the causes of each result can also be explained.
This article clearly explains how to interpret the results of crystal growth experiments, factors affecting crystal size, shape, and purity, discussion of unsuccessful results, points for improvement, and discussion examples that can be used in reports.
Note:
This article is a reference intended to assist with discussions of results obtained in chemistry experiments at universities and similar institutions.
For the actual heating and cooling conditions, handling of solvents, filtration and drying methods, and safety precautions, always follow the instructions in your university’s laboratory manual and those given by your instructor or TA.
- What Is a Crystal Growth Experiment?
- Results to Examine in a Crystal Growth Experiment
- How Crystals Form
- Degree of Supersaturation and Crystal Size
- Effect of Cooling Rate on Crystals
- Discussion When Cooling Slowly
- Discussion When Cooling Rapidly
- Effect of Solvent Type on Crystals
- When Too Much Solvent Is Used
- When Too Little Solvent Is Used
- Effects of Impurities on Crystals
- Discussion of Crystals With Uneven Color
- Causes of Distorted Crystal Shape
- Causes of Fine Crystals
- Conditions for Producing Large Crystals
- Discussion When Seed Crystals Are Used
- Crystal Loss During Filtration
- Crystal Loss Caused by Washing
- Reduced Purity Caused by Insufficient Washing
- Errors Caused by Insufficient Drying
- Discussion of Crystal Water and Solvates
- Evaluating Purity by Melting-Point Measurement
- Relationship Between Yield and Purity
- Discussion When No Crystals Form
- Discussion When Recrystallization Is Performed
- When Crystals Can Be Considered Good
- Example Discussion When Crystal Growth Did Not Go Well
- How to Write Points for Improvement
- Difference Between a Superficial Discussion and a Good Discussion
- Examples of Expressions That Can Be Used in Reports
- Points to Check When Discussing a Crystal Growth Experiment
- Summary
What Is a Crystal Growth Experiment?
A crystal growth experiment is an experiment in which a solute in solution is precipitated as an orderly solid.
Crystals are produced by lowering the solubility of the solute through cooling the solution, evaporating the solvent, or adding another solvent.
Crystals are solids formed by the orderly arrangement of solute molecules or ions.
Therefore, the size and shape of crystals reflect how nuclei formed in the solution and how the crystals subsequently grew.
To obtain clean, large crystals, it may be important to allow crystal growth to proceed moderately and slowly rather than causing rapid precipitation.
In a report, record the appearance of the obtained crystals and discuss why they formed in that manner in terms of supersaturation, nucleation, crystal growth, and the effects of impurities.
Results to Examine in a Crystal Growth Experiment
In a crystal growth experiment, observe not only whether crystals were obtained but also their size, shape, color, transparency, luster, amount, and drying condition.
If yield, melting point, spectra, or similar measurements were obtained, they can also be used to evaluate purity.
Main Items to Include in the Results
- Substance and solvent used
- Appearance during heating and dissolution
- Cooling or standing conditions
- Timing at which crystals began to precipitate
- Crystal size
- Crystal shape
- Crystal color
- Transparency and luster
- Amount of crystals
- Condition after filtration, washing, and drying
- Yield
- Purity-confirmation results such as melting point and spectra
Example of How to Write the Results:
The sample dissolved completely in the solvent upon heating, forming a clear solution.
When the solution was then allowed to stand at room temperature, crystals began to precipitate, and white needle-like crystals were obtained after cooling.
After the crystals were filtered, washed, and dried, the dry mass was 0.84 g.
The obtained crystals were relatively fine, and some powdery portions were observed.
How Crystals Form
Crystals form when a solution becomes supersaturated.
Supersaturation is an unstable state in which more solute is dissolved than would normally be soluble at that temperature.
When small crystal nuclei form in this state, surrounding solute particles arrange themselves regularly on the nuclei, and the crystals grow.
In crystal growth, the balance between nucleation and crystal growth is important.
If many nuclei form, many small crystals are produced.
If only a small number of nuclei form and each grows slowly, larger crystals are more likely to be obtained.
Supersaturation → Nucleation → Crystal Growth → Crystal Precipitation
Example Discussion:
The crystals are considered to have formed because the solution became supersaturated.
In a supersaturated state, the solute in solution becomes unstable and nucleation occurs.
The crystals then grow as solute particles are regularly incorporated onto the surfaces of the nuclei.
Therefore, the size and number of crystals are affected by the number of nuclei formed and the rate of crystal growth.
Degree of Supersaturation and Crystal Size
The higher the degree of supersaturation, the more readily nucleation occurs in the solution.
When many nuclei form, the solute is distributed among many nuclei as they grow, so many small crystals are likely to form.
In contrast, when the degree of supersaturation is low and nucleation proceeds slowly, a smaller number of crystals can grow larger.
Therefore, when large crystals are desired, methods such as slow cooling or slow evaporation of the solvent may be effective instead of rapidly producing supersaturation.
Example Discussion:
One possible reason the obtained crystals were fine is that the degree of supersaturation increased rapidly and many nuclei formed at the same time.
When many nuclei form, the solute is distributed among the individual nuclei as they grow, making it difficult for each crystal to become large.
Therefore, rapid cooling or rapid concentration may cause the formation of small or powdery crystals.
Effect of Cooling Rate on Crystals
Cooling rate greatly affects crystal size and shape.
Rapid cooling causes solubility to decrease quickly and the degree of supersaturation to become large, making many nuclei more likely to form simultaneously.
As a result, many small, fine crystals are likely to form.
In contrast, when cooling proceeds slowly, nucleation is less likely to occur rapidly and a small number of nuclei can grow over time.
Therefore, relatively large and well-shaped crystals may be obtained more easily.
| Cooling Condition | Likely Result | Perspective for Discussion |
|---|---|---|
| Rapid cooling | Many small crystals form | Extensive nucleation and insufficient crystal growth |
| Slow cooling | Larger crystals are more likely to form | A small number of nuclei can grow |
| Insufficient cooling | Few crystals form | Degree of supersaturation is insufficient |
Example Discussion:
One possible reason the crystals became fine is that the solution was cooled rapidly.
Rapid cooling caused the solubility to decrease sharply and the degree of supersaturation to become large, so many nuclei formed simultaneously.
As a result, the amount of solute incorporated into each crystal decreased, leading to the formation of many small crystals.
Discussion When Cooling Slowly
When cooling proceeds slowly, the degree of supersaturation increases gradually and rapid nucleation is less likely to occur.
Therefore, a small number of nuclei form and can grow over time.
Slow cooling may be effective when large and well-shaped crystals are desired.
Example Discussion:
One possible reason relatively large crystals were obtained is that cooling proceeded slowly and the number of nuclei formed was limited.
When only a small number of nuclei are present, the solute in the solution is concentrated into those limited nuclei, allowing each crystal to grow larger.
From this, slow cooling is considered to have been favorable for crystal growth.
Discussion When Cooling Rapidly
When rapid cooling is performed, the degree of supersaturation becomes high in a short period of time and nucleation proceeds all at once.
As a result, fine crystals or solids resembling powdery precipitates may be obtained.
Rapid cooling may sometimes increase yield, but it can be disadvantageous for crystal size and purity.
Example Discussion:
Because many nuclei formed simultaneously as a result of rapid cooling, the obtained crystals are considered to have become fine.
When many nuclei are present, the solute is distributed among many crystals, making it difficult for each individual crystal to grow sufficiently.
In addition, rapid precipitation may increase the incorporation of impurities, so the purity of the crystals may also decrease.
Effect of Solvent Type on Crystals
Solvent selection is very important in crystal growth.
If a solvent is selected in which the solute dissolves well at high temperature but poorly at low temperature, crystals can be readily precipitated by cooling.
Conversely, if the solute remains highly soluble even at low temperature, crystals are less likely to form upon cooling.
The extent to which the solvent dissolves impurities also affects purity.
If conditions are such that only the target compound crystallizes while impurities remain in the mother liquor, crystals of higher purity are more likely to be obtained.
Example Discussion:
One possible reason sufficient crystals did not precipitate is that the target compound remained relatively soluble in the solvent even at low temperature.
If solubility remains high, the solution is less likely to become supersaturated even after cooling, and crystallization does not proceed readily.
Therefore, in crystal growth, it is important to select a solvent in which the target compound dissolves at high temperature and precipitates readily at low temperature.
When Too Much Solvent Is Used
If too much solvent is used, the target compound tends to remain dissolved in the solution even after cooling.
As a result, less material precipitates as crystals and the yield decreases.
In recrystallization and crystal growth, an amount of solvent close to the minimum necessary to dissolve the target compound is often used.
Example Discussion:
One possible reason the amount of crystals obtained was small is that too much solvent was used.
When a large amount of solvent is used, the target compound tends to remain dissolved in the solution even after cooling.
Therefore, the solution is less likely to become supersaturated, the amount precipitating as crystals decreases, and the yield may have been reduced.
When Too Little Solvent Is Used
If too little solvent is used, the sample may not dissolve completely even when heated.
If undissolved solids remain during cooling, they may become mixed in as impurities or act as nuclei and trigger rapid crystallization.
In addition, if the solution is too concentrated, crystals may precipitate rapidly and become fine.
Example Discussion:
If too little solvent was used, the sample may not have dissolved completely and undissolved material may have remained.
If cooling is performed while undissolved material remains, it may become incorporated as an impurity or act as a crystal nucleus and trigger rapid precipitation.
As a result, the crystals may have become finer or their purity may have decreased.
Effects of Impurities on Crystals
Impurities greatly affect crystal growth, shape, and purity.
If impurities adsorb onto the crystal surface, growth of particular faces may be inhibited and the crystal shape may become distorted.
If impurities are incorporated inside the crystal, uneven coloration, reduced transparency, and a broader melting-point range may result.
In crystal growth experiments, it is important to discuss not only whether clean crystals were obtained but also whether impurities remained in the mother liquor or were incorporated into the crystals.
Example Discussion:
One possible reason uneven coloration or cloudiness was observed in the obtained crystals is that impurities were incorporated into the crystals.
If impurities adsorb onto the surface during crystal growth, the growth of crystal faces may be disturbed and the shape may become uneven.
In addition, if impurities are incorporated inside the crystals, transparency decreases and purity is also considered to decrease.
Discussion of Crystals With Uneven Color
If crystals show uneven color, possible causes include incorporation of impurities inside the crystals, variation in the amount of crystal water or solvent from place to place, partial changes in oxidation state, or residual mother liquor.
Especially in metal complexes and inorganic salts, even small differences in composition may cause changes in color.
Example Discussion:
One possible reason uneven coloration was observed in the crystals is that impurities or components of the mother liquor were partially incorporated during crystal growth.
In addition, in metal complexes and hydrates, the color may change depending on the state of the ligands or crystal water.
Therefore, uneven coloration provides a clue that the uniformity and purity of the crystals may have been insufficient.
Causes of Distorted Crystal Shape
Causes of distorted or irregular crystal shapes include rapid crystallization, contamination by impurities, stirring or vibration, temperature changes, and irregular solvent evaporation rates.
Crystals develop well-defined shapes through regular growth, but unstable growth conditions make irregular shapes more likely.
Example Discussion:
One possible reason the crystal shape became irregular is that the temperature or concentration changed rapidly during crystal growth.
When precipitation proceeds rapidly, the crystal faces cannot grow uniformly and the shape becomes distorted.
In addition, if impurities adsorb onto the crystal surface, growth of particular crystal faces may be inhibited, producing an irregular shape.
Causes of Fine Crystals
Major causes of fine crystals include rapid cooling, a sudden increase in supersaturation, excessive stirring, and increased nucleation caused by impurities or dust.
When many nuclei form, many crystals grow simultaneously, making each individual crystal smaller.
| Cause | What Happens | Result |
|---|---|---|
| Rapid cooling | Degree of supersaturation rises suddenly | Many small crystals form |
| Excessive stirring | Nucleation or crystal breakage occurs | More likely to become powdery |
| Impurities or dust | More nucleation sites are introduced | Crystals are more likely to become fine |
| Concentration too high | Rapid precipitation occurs | Crystal growth becomes uneven |
Example Discussion:
One possible reason the obtained crystals were almost powdery is that cooling was rapid and many nuclei formed simultaneously.
When many nuclei form, the solute is distributed among many crystals, making it difficult for each crystal to grow large.
In addition, if the crystals were broken by stirring or vibration, this may also have caused many fine crystals to be observed.
Conditions for Producing Large Crystals
To obtain large crystals, it is important to suppress nucleation and allow crystal growth to proceed slowly.
Conditions such as slow cooling, slow evaporation of the solvent, avoiding vibration, and minimizing impurities and dust may therefore be effective.
However, large crystals are not necessarily always highly pure.
Mother liquor or impurities may become trapped inside large crystals, so it is important not to determine purity from appearance alone.
Example Discussion:
One possible reason large crystals were obtained is that the number of nuclei formed was small and the nuclei that did form grew over time.
Slow cooling allows the degree of supersaturation to increase gradually and suppresses the simultaneous formation of many nuclei.
As a result, solute is incorporated into a limited number of nuclei and the crystals are considered to have grown relatively large.
Discussion When Seed Crystals Are Used
A seed crystal is a small crystal added as a starting point for crystal growth.
When a seed crystal is added, solute can more readily be incorporated onto its surface, promoting growth of the desired crystal.
Adding a seed crystal to a supersaturated solution may reduce random nucleation and make it easier to grow a particular crystal.
Example Discussion:
By using a seed crystal, the solute is considered to have been incorporated regularly onto the surface of the seed crystal, promoting crystal growth.
Without a seed crystal, many nuclei may form irregularly in the solution and produce many small crystals.
Therefore, the use of a seed crystal is one effective method for obtaining large, well-shaped crystals.
Crystal Loss During Filtration
During filtration of crystals, the recovered amount decreases if crystals remain on the filter paper or apparatus or if fine crystals pass through with the filtrate.
Losses during filtration are particularly likely to be large when the crystals are fine or powdery.
Example Discussion:
One possible reason the yield was low is that some crystals were lost during filtration.
When crystals are fine, they may pass through the filter paper or remain attached to the apparatus without being recovered.
As a result, the recovered amount becomes smaller than the amount of crystals actually formed, and the yield is considered to have decreased.
Crystal Loss Caused by Washing
Washing crystals is necessary to remove mother liquor and soluble impurities.
However, if the target compound dissolves in the washing solution, some of the crystals are lost during washing.
If the type or amount of washing solution is inappropriate, the yield decreases.
Example Discussion:
If some of the target crystals dissolved in the washing solution during washing, the amount of crystals recovered would decrease.
In particular, if the target compound has even slight solubility in the washing solution, the loss becomes greater as the amount of washing solution increases.
Therefore, washing should be performed using the minimum amount necessary to remove impurities.
Reduced Purity Caused by Insufficient Washing
If washing is insufficient, mother liquor and impurities remain on the crystal surface.
As a result, the mass after drying may become larger and the color and purity of the crystals may be affected.
Even if the yield appears high, the sample may actually contain impurities.
Example Discussion:
If washing was insufficient, mother liquor and soluble impurities may have remained on the crystal surface.
In this case, the measured mass includes components other than the target compound, so the yield is overestimated.
In addition, remaining impurities may also affect the crystal color, melting point, and spectra.
Errors Caused by Insufficient Drying
If the crystals are not sufficiently dried, solvent or moisture remains on the surface or in gaps between the crystals.
If they are weighed in this state, the measured mass becomes larger than the actual crystal mass and the yield is overestimated.
Wet crystals also affect evaluation of appearance and purity.
Example Discussion:
If moisture remained in the crystals after drying, solvent or water may have remained on the crystal surfaces.
In this case, the measured mass includes the mass of solvent in addition to that of the target crystals, causing the yield to be overestimated.
Therefore, to determine an accurate yield, the crystals must be sufficiently dried before weighing.
Discussion of Crystal Water and Solvates
Crystals of inorganic salts and metal complexes may contain crystal water or solvent molecules.
When crystal water or solvent molecules are included in the chemical formula, they may be treated as part of the target compound.
On the other hand, water or mother liquor adhering to the surface is regarded as an impurity.
If the drying conditions are too severe, crystal water may be lost and the color, mass, or structure may change.
Conversely, insufficient drying leaves surface water and causes the measured mass to become too large.
Example Discussion:
If the obtained crystals are a hydrate, the crystal water is treated as a component included in the chemical formula.
However, water or mother liquor remaining on the crystal surface is not part of the target compound and causes the mass to be overestimated.
If drying is too strong, crystal water may be lost and the color or structure of the crystal may change, so the drying conditions must be controlled appropriately.
Evaluating Purity by Melting-Point Measurement
For crystals of organic compounds, purity may be evaluated by melting-point measurement.
Highly pure crystals often show a narrow melting-point range and a value close to the literature value.
When impurities are present, the melting point may decrease or the melting-point range may broaden.
Example Discussion:
One possible reason the melting-point range of the obtained crystals was broad is that impurities were present.
When impurities are mixed in, the crystal lattice becomes disturbed and the sample becomes less likely to melt at a constant temperature, broadening the melting-point range.
In addition, if the melting point is lower than the literature value, melting-point depression caused by impurities may also be considered.
Relationship Between Yield and Purity
In crystal growth experiments, a high yield does not necessarily mean high purity.
If mother liquor or impurities remain because of insufficient washing or drying, the mass increases and the yield appears high, but the purity decreases.
On the other hand, recrystallization or thorough washing tends to improve purity but may lower the yield because some target compound is lost.
| Condition | Yield | Purity |
|---|---|---|
| Insufficient washing or drying | Appears high | Tends to decrease |
| Excessive washing | Tends to decrease | May increase |
| After recrystallization | Tends to decrease | Tends to increase |
Example Discussion:
In this experiment, it is necessary to evaluate not only the yield but also the purity of the crystals.
If washing or drying is insufficient, mother liquor or moisture other than the crystals is included in the mass, so the yield appears high while the purity decreases.
Conversely, thorough washing or recrystallization improves purity but may also cause loss of some target compound and reduce the yield.
Discussion When No Crystals Form
If no crystals form, possible causes include the solution not becoming supersaturated, too much solvent being used, insufficient cooling, the target compound being highly soluble in the solvent, or nucleation not occurring.
Impurities or inappropriate solvent selection may also inhibit crystallization.
Example Discussion:
One possible reason no crystals precipitated is that the solution did not reach a supersaturated state even after cooling.
If too much solvent was used or a solvent in which the target compound has high solubility was selected, the target compound tends to remain dissolved in the solution.
In addition, if nucleation did not occur, visible crystals may fail to precipitate even when the solution is supersaturated.
Discussion When Recrystallization Is Performed
Recrystallization is an operation commonly used to increase the purity of crystals.
The target compound is dissolved in a hot solvent and crystallized again upon cooling, leaving impurities in the mother liquor while recovering the target compound as crystals.
In recrystallization, purity increases while the yield tends to decrease because some target compound remains in the mother liquor.
In a report, it is useful to discuss the balance between yield and purity.
Example Discussion:
Recrystallization is considered to have left impurities in the mother liquor and produced crystals of higher purity.
On the other hand, because some of the target compound also remained dissolved in the mother liquor, the recovered amount decreased and the yield was reduced.
Therefore, recrystallization is effective for increasing purity but is accompanied by a decrease in yield.
When Crystals Can Be Considered Good
Crystals can be considered good when their shapes are relatively regular, their color and transparency are uniform, they are well dried, and confirmation results such as melting point and spectra do not contradict the target compound.
However, even crystals that appear clean may contain impurities, so purity cannot be determined completely from appearance alone.
Example Discussion:
The obtained crystals had a uniform color and relatively regular shape, so crystal growth is considered to have proceeded generally well.
In addition, no noticeable moisture remained after drying, suggesting that little mother liquor remained.
However, because purity cannot be completely determined from appearance alone, the result must be evaluated together with melting-point measurements, spectra, and other data.
Example Discussion When Crystal Growth Did Not Go Well
When crystal growth does not go well, possible causes can be considered from observations such as no crystals forming, fine crystals, distorted shapes, uneven coloration, low yield, or low purity.
It is easier to write the discussion by separately considering cooling rate, degree of supersaturation, amount of solvent, impurities, washing, and drying.
Example Discussion:
One possible reason the obtained crystals were fine and irregular in shape is that cooling was rapid and the degree of supersaturation increased in a short period of time.
Rapid supersaturation produced many nuclei, and precipitation proceeded before each crystal could grow sufficiently, possibly producing crystals that were nearly powdery.
In addition, if impurities adsorbed onto the crystal surfaces, growth of the crystal faces may have been inhibited and the shapes become distorted.
How to Write Points for Improvement
In a discussion of a crystal growth experiment, including points for improvement as well as sources of error makes the report easier to organize.
Improvements are easier to organize when divided into methods for obtaining large, well-shaped crystals, methods for increasing purity, and methods for improving yield.
Methods for Obtaining Large, Well-Shaped Crystals
- Cool the solution slowly rather than rapidly
- Avoid vibration and excessive stirring
- Use seed crystals when necessary
- Avoid contamination by dust and impurities
- Allow the solvent to evaporate slowly
- Avoid increasing the degree of supersaturation too rapidly
Methods for Increasing Purity
- Select an appropriate solvent
- Remove insoluble impurities by methods such as hot filtration
- Remove mother liquor appropriately by washing
- Avoid loss of the target compound caused by excessive washing
- Perform recrystallization when necessary
- Evaluate the crystals only after sufficient drying
Methods for Improving Yield
- Do not use too much solvent
- Cool sufficiently to promote crystallization
- Reduce the amount of target compound remaining in the mother liquor
- Avoid losing crystals during filtration and transfer
- Use an appropriate amount of washing solution
- Recover the crystals sufficiently before drying
Example of How to Write Points for Improvement:
To obtain larger and better-shaped crystals, it is important to suppress nucleation by cooling the solution slowly rather than rapidly.
In addition, because impurities may interfere with crystal growth, the solution should be kept clean and insoluble impurities should be removed when necessary.
To improve the yield, it is effective to avoid using too much solvent, cool sufficiently to promote crystallization, and prevent crystal loss during filtration and washing.
Difference Between a Superficial Discussion and a Good Discussion
In a discussion of a crystal growth experiment, simply writing that “crystals formed” or “they were small” results in a superficial discussion.
A persuasive discussion can be produced by relating the results to supersaturation, nucleation, crystal growth, cooling rate, impurities, and solvent amount.
| Superficial Discussion | Good Discussion |
|---|---|
| Crystals formed. | Cooling reduced the solubility of the target compound in the solution, causing the solution to become supersaturated, after which nucleation and crystal growth occurred and crystals precipitated. |
| The crystals were small. | Rapid cooling caused the degree of supersaturation to increase sharply and many nuclei to form simultaneously, so the solute was distributed among many nuclei and each crystal was unable to grow large. |
| The purity was low. | Possible causes include incorporation of impurities during crystal growth and residual mother liquor on the crystal surfaces because of insufficient washing. If impurities are present, the melting-point range may broaden and uneven coloration or cloudiness may occur. |
Examples of Expressions That Can Be Used in Reports
The following expressions can be used when writing the results and discussion of a crystal growth experiment.
Adjust the necessary parts according to your own experimental results.
- Cooling decreased the solubility and caused the solution to become supersaturated, resulting in crystal precipitation.
- Crystal size is affected by the number of nuclei formed and the rate of crystal growth.
- Rapid cooling may have produced many nuclei, resulting in fine crystals.
- Slow cooling allowed a small number of nuclei to grow over time, resulting in larger crystals.
- If too much solvent was used, the target compound may have remained in the mother liquor and the yield may have decreased.
- If impurities adsorb onto crystal surfaces, the crystal shape may become distorted.
- Uneven coloration or cloudiness may indicate contamination by impurities or residual mother liquor.
- If washing is insufficient, mother liquor and soluble impurities may remain and reduce purity.
- If washing is excessive, the target compound may dissolve in the washing solution and reduce the yield.
- Because yield and purity do not necessarily correspond, crystal appearance, melting point, and other results must also be evaluated.
Points to Check When Discussing a Crystal Growth Experiment
Checking the following points before writing the report makes the discussion easier to write.
- Have you recorded the conditions under which crystals precipitated?
- Have you specifically described the size, shape, and color of the crystals?
- Have you explained supersaturation, nucleation, and crystal growth?
- Have you discussed the effect of cooling rate?
- Have you considered the effects of solvent type and amount?
- Have you written about the effects of impurities on crystal shape and purity?
- Have you considered losses and errors caused by filtration, washing, and drying?
- Have you evaluated yield and purity separately?
- If melting point or spectral results are available, have you related them to the discussion?
- Have you avoided determining purity from appearance alone?
- Have you considered causes when no crystals formed?
- Do the points for improvement correspond to the sources of error?
Summary
In crystal growth experiments, a solution is brought into a supersaturated state and crystals are precipitated through nucleation and crystal growth.
Crystal size and shape are affected by cooling rate, degree of supersaturation, number of nuclei, rate of crystal growth, impurities, amount of solvent, and other factors.
To obtain large, well-shaped crystals, it is important to avoid rapid nucleation and allow crystal growth to proceed slowly.
On the other hand, rapid cooling or sudden changes in concentration make the formation of many small or powdery crystals more likely.
Impurities and residual mother liquor can cause uneven coloration, cloudiness, distorted crystal shapes, and reduced purity.
In a report, do not simply write that “crystals formed,” but explain why the crystals had their particular size, shape, and purity by relating the results to supersaturation, nucleation, crystal growth, solvent, impurities, filtration, washing, and drying.
Separately evaluating yield and purity makes it possible to produce a persuasive discussion of a crystal growth experiment.
