A colloidal-solution experiment is an experiment that investigates the properties of particle-dispersion systems that differ from ordinary true solutions.
Colloidal particles are larger than molecules or ions and are too small to be seen directly with the naked eye, but they have characteristics such as scattering light and aggregating when electrolytes are added.
The Tyndall effect, Brownian motion, coagulation, and dispersion stability are important observation items for understanding colloidal solutions.
In a discussion of colloidal solutions, it is not sufficient simply to write that “the path of light was visible” or “a precipitate formed.”
It is necessary to explain why the path of light becomes visible, how true solutions differ from colloidal solutions, why the addition of an electrolyte causes coagulation, and how particle charge and hydration are related to dispersion stability.
In particular, using the concepts of particle size, light scattering, surface charge, and the electrical double layer makes the discussion more detailed.
This article clearly explains, as examples of discussions that can be used in laboratory reports on colloidal-solution experiments, the Tyndall effect, Brownian motion, particle dispersion, coagulation, the effects of electrolytes, protective colloids, hydrophilic and hydrophobic colloids, zeta potential, causes of error, and points for improvement.
Note:
This article is a reference intended to assist with discussions of observations obtained in colloidal-solution experiments in basic chemistry experiments, physical chemistry experiments, and colloid chemistry experiments at universities and similar institutions.
For the actual sample, type of colloidal solution, electrolyte added, concentration, light source, observation method, and safety precautions, always follow the instructions in your university’s laboratory manual and those given by your instructor or TA.
- What Is a Colloidal Solution?
- Main Items to Include in the Results
- What Is the Tyndall Effect?
- Relationship Between the Tyndall Effect and Particle Size
- Differences Among True Solutions, Colloidal Solutions, and Suspensions
- Discussion of Brownian Motion
- Why Colloidal Particles Remain Dispersed
- What Is Coagulation?
- Why Electrolytes Cause Coagulation
- Effect of Electrolyte Concentration
- Ionic Valence and Coagulating Power
- Difference Between Hydrophilic and Hydrophobic Colloids
- Discussion of Protective Colloids
- Zeta Potential and Dispersion Stability
- Effect of pH
- Effect of Temperature
- Discussion of Changes in Turbidity
- Causes of Error in Colloidal-Solution Experiments
- When the Results Can Be Considered Good
- Example Discussions When the Experiment 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 Colloidal Solutions
- Summary
What Is a Colloidal Solution?
A colloidal solution is a system in which fine particles are dispersed in a liquid.
Unlike a true solution such as salt water or sugar water, in which molecules or ions are uniformly dissolved, particles in a colloidal solution remain dispersed while retaining a certain size.
Because the particles are extremely small, they cannot be seen with the naked eye, but they can be observed through properties such as light scattering and aggregation.
Colloidal particles are generally larger than molecules but smaller than coarse suspended particles.
Because of this intermediate size, they are difficult to separate with filter paper, while showing behavior different from true solutions, such as scattering light.
Colloidal solutions are widely encountered in daily life and experiments, including milk, India ink, gelatin solutions, gold colloids, and sulfur colloids.
Example Discussion:
A colloidal solution is a system in which fine particles larger than molecules or ions are dispersed in a liquid.
Although the particles cannot be seen with the naked eye, they show properties different from true solutions because they scatter light and aggregate when electrolytes are added.
Therefore, observation of the Tyndall effect and coagulation provides clues for determining whether a sample is a colloidal dispersion.
Main Items to Include in the Results
In a colloidal-solution experiment, organize the type of sample, appearance, how it looks when illuminated, presence or absence of the Tyndall effect, changes after electrolyte addition, presence or absence of aggregation or precipitation, amount added, and changes over time.
If a true solution or suspension is used as a comparison, summarizing the differences in a table makes the discussion easier.
Main Items to Include in the Results
- Type of colloidal solution
- Color and transparency of the sample
- Appearance of the light path when illuminated
- Presence or absence of the Tyndall effect
- Results for the true solution used for comparison
- Results for the suspension used for comparison
- Type of electrolyte added
- Electrolyte concentration
- Amount of electrolyte added
- Presence or absence of coagulation
- Changes in precipitation and turbidity
- Time until coagulation
- Presence or absence of a protective colloid
- Temperature and pH conditions
- Causes of error and points for improvement
Example of How to Write the Results:
When light was directed into the colloidal solution from the side, the path of the light in the solution appeared white.
On the other hand, in a true solution such as salt water, the light path was hardly visible.
In addition, when an electrolyte was added to the colloidal solution, the turbidity increased and a precipitate formed over time.
From these results, light scattering by colloidal particles and coagulation caused by the electrolyte were considered to have occurred.
What Is the Tyndall Effect?
The Tyndall effect is the phenomenon in which the path of light becomes visible when strong light is directed into a colloidal solution.
Because colloidal particles scatter light, the light path appears white when viewed from the side.
In a true solution, the particles are extremely small and light scattering is weak, so the light path is hardly visible.
The Tyndall effect is a representative method for confirming the presence of colloidal particles.
However, if the concentration is too low, the light source is weak, or the particles are small, the Tyndall effect may be difficult to observe.
On the other hand, large particles such as those in a suspension also scatter light, but they differ from colloidal solutions in that they readily settle.
Example Discussion:
The path of light became visible when the colloidal solution was illuminated because the colloidal particles scattered the light.
In a true solution, the solute particles are molecular or ionic in size, so light scattering is extremely weak and the light path is hardly observed.
Therefore, observation of the Tyndall effect indicates that colloidal particles are dispersed in the sample.
Relationship Between the Tyndall Effect and Particle Size
The Tyndall effect is related to the size of the dispersed particles.
In a true solution, where the particles are as small as molecules or ions, visible light is hardly scattered and the light path is difficult to see.
On the other hand, colloidal particles are large enough to scatter light, so the path of light becomes visible.
Light scattering tends to become stronger as particle size increases, but particles that are too large become suspended particles and readily settle.
In other words, colloidal solutions contain particles that are “small enough to resist settling but large enough to scatter light.”
The Tyndall effect reflects this characteristic particle-size range.
Example Discussion:
The Tyndall effect was observed because the particles in the sample were large enough to scatter visible light.
In a true solution, the particles are molecular or ionic in size, so light scattering is weak and the light path is difficult to see.
On the other hand, colloidal particles are large enough to scatter light while not settling immediately, so they were observed as a dispersed system.
Differences Among True Solutions, Colloidal Solutions, and Suspensions
Colloidal solutions are easier to understand when compared with true solutions and suspensions.
In a true solution, the solute is uniformly dispersed as molecules or ions, and the Tyndall effect is hardly observed.
In a suspension, the particles are large and scatter light readily, but they also settle easily over time.
Colloidal solutions are intermediate between these systems, with particles large enough to scatter light but small enough not to settle readily over a short period.
By comparing the Tyndall effect and settling behavior in an experiment, it is possible to determine which type of dispersion system a sample most closely resembles.
| Type | State of Particles | Tyndall Effect | Tendency to Settle |
|---|---|---|---|
| True solution | Dispersed as molecules or ions | Hardly observed | Does not settle |
| Colloidal solution | Dispersed as fine particles | Observed | Difficult to settle |
| Suspension | Relatively large particles are dispersed | Often observed | Readily settles |
Example Discussion:
The light path was not visible in the true solution, whereas it was observed in the colloidal solution.
This is because the solute particles in a true solution are too small to scatter light strongly, whereas the particles in a colloidal solution are large enough to scatter light.
In addition, suspension particles are larger and settle more readily, which distinguishes suspensions from colloidal solutions.
Discussion of Brownian Motion
Brownian motion is the irregular movement of colloidal particles caused by the thermal motion of liquid molecules.
Liquid molecules are constantly moving and collide with colloidal particles from various directions.
Because these collisions are not perfectly balanced, the colloidal particles move irregularly.
Brownian motion is one reason colloidal particles do not readily settle.
When particles are very large, the effect of gravity becomes greater and they settle more easily, but for colloidal particles, the effect of thermal motion cannot be ignored.
Therefore, colloidal solutions can maintain a relatively stable dispersed state.
Example Discussion:
One reason colloidal particles are difficult to settle is Brownian motion.
Because the particles are moved irregularly by the thermal motion of liquid molecules, they are less likely to settle in only one direction under gravity.
Therefore, fine particles in a colloidal solution can remain dispersed for a relatively long period.
Why Colloidal Particles Remain Dispersed
Factors that allow colloidal particles to remain dispersed in a solution include particle-surface charge, the electrical double layer, hydration, and Brownian motion.
When particles carry charges of the same sign, they electrostatically repel one another and therefore are less likely to aggregate.
This repulsion maintains the dispersed state.
In hydrophilic colloids, water molecules bind strongly to the particle surfaces and form hydration layers that prevent the particles from approaching one another.
In hydrophobic colloids, repulsion caused by surface charge plays a major role in stability.
The stability of a colloidal solution is determined by the balance between repulsive and attractive forces between particles.
Example Discussion:
Colloidal particles maintain a dispersed state because their surfaces carry charges of the same sign and the particles electrostatically repel one another.
This repulsion suppresses the approach and aggregation of the particles.
In addition, in hydrophilic colloids, hydration layers also prevent particles from approaching one another, so dispersion stability is considered to be higher.
What Is Coagulation?
Coagulation is the phenomenon in which colloidal particles gather together, form larger particles, and appear as precipitate or turbidity.
When colloidal particles are dispersed, they are stabilized by repulsion between the particles.
However, when the repulsive force is weakened by adding an electrolyte or by another factor, the particles can approach one another and aggregate, making precipitation more likely.
When coagulation occurs, the transparency of the solution may change, precipitate may form, or the appearance of the Tyndall effect may change.
If turbidity or precipitation increases after an electrolyte is added in an experiment, the colloidal particles can be considered to have aggregated and increased in size.
Coagulation is the result of a loss of dispersion stability in the colloid.
Example Discussion:
The solution became cloudy and a precipitate formed after the electrolyte was added because the colloidal particles coagulated.
The electrolyte weakened repulsion caused by the charges on the particle surfaces, allowing the particles to approach one another more easily.
As a result, the particles gathered into large aggregates and were considered to have settled.
Why Electrolytes Cause Coagulation
When an electrolyte is added, ions in the solution neutralize the surface charge of the colloidal particles or compress the electrical double layer.
As a result, electrostatic repulsion between particles weakens and the particles can approach one another more easily.
When particles approach sufficiently closely, attractive forces act between them, causing aggregation and coagulation.
Hydrophobic colloids are particularly susceptible to electrolyte addition because repulsion caused by surface charge is a major factor in their dispersion stability.
Coagulation tends to become more likely as the electrolyte concentration increases and when multivalent ions with a charge opposite to that of the particles are present.
This is also related to the Schulze-Hardy rule.
Example Discussion:
When an electrolyte is added, ions in the solution neutralize the charge on the surfaces of colloidal particles and compress the electrical double layer.
Therefore, electrostatic repulsion between the particles becomes weaker and the particles become more likely to approach and aggregate.
As a result, the colloidal particles formed large aggregates and were observed as turbidity or precipitate.
Effect of Electrolyte Concentration
When the electrolyte concentration is low, the surface charge of the colloidal particles is sufficiently maintained and repulsion between the particles remains, so coagulation is unlikely to occur.
However, as the electrolyte concentration increases, the surface charge is more strongly shielded and the electrical double layer is compressed.
As a result, the particles become more likely to aggregate.
In an experiment, adding a small amount of electrolyte may cause little change, while turbidity or precipitation may suddenly appear once a certain amount is exceeded.
This is considered to occur because the limit at which dispersion stability can be maintained has been exceeded.
Comparing the minimum concentration at which coagulation occurs makes it possible to discuss differences in coagulating power among electrolytes.
Example Discussion:
Under conditions with a low electrolyte concentration, electrostatic repulsion between colloidal particles remained, so coagulation was difficult to occur.
On the other hand, when the electrolyte concentration was increased, the particle-surface charge was shielded and the particles became more likely to approach one another.
As a result, turbidity or precipitation was considered to have suddenly occurred above a certain concentration.
Ionic Valence and Coagulating Power
The valence of the ions in the added electrolyte greatly affects coagulation of colloidal particles.
In general, the greater the valence of the ion with a charge opposite to that of the colloidal particles, the greater the coagulating power.
This is because multivalent ions can more efficiently neutralize the particle-surface charge and more strongly compress the electrical double layer.
For example, in a negatively charged colloid, Ca2+ may cause stronger coagulation than Na+, and Al3+ may cause stronger coagulation than Ca2+.
This relationship is known as the Schulze-Hardy rule.
If ease of coagulation differs depending on the type of electrolyte in an experiment, the difference in ionic valence should be discussed.
Example Discussion:
If electrolytes containing multivalent ions caused coagulation with smaller amounts, the effect of ionic valence can be considered.
Ions with a charge opposite to that of the colloidal particles neutralize the particle-surface charge.
The greater the ionic valence, the more strongly the surface charge is neutralized, weakening repulsion between particles and making coagulation more likely.
Difference Between Hydrophilic and Hydrophobic Colloids
A hydrophilic colloid is a colloid in which particles or polymers with a high affinity for water are dispersed.
Gelatin, starch, and proteins are examples.
In hydrophilic colloids, hydration layers form on the particle surfaces and prevent the particles from approaching one another, making the dispersion relatively stable.
A hydrophobic colloid is a colloid in which particles with little affinity for water are dispersed.
Gold colloids, sulfur colloids, and metal hydroxide colloids are examples.
In hydrophobic colloids, dispersion is maintained mainly by repulsion caused by surface charge, so coagulation readily occurs when an electrolyte shields the charge.
Hydrophilic and hydrophobic colloids therefore differ in their susceptibility to coagulation.
| Type | Main Stabilizing Factor | Resistance to Electrolytes |
|---|---|---|
| Hydrophilic colloid | Hydration layers and polymer chains | Relatively difficult to coagulate |
| Hydrophobic colloid | Repulsion caused by surface charge | Readily coagulates |
Example Discussion:
Hydrophilic colloids form hydration layers on their particle surfaces and therefore do not readily coagulate even when a small amount of electrolyte is added.
On the other hand, hydrophobic colloids are dispersed mainly through repulsion caused by surface charge, so they readily coagulate when the charge is shielded by an electrolyte.
Therefore, even when the same electrolyte is added, hydrophilic and hydrophobic colloids differ in how readily coagulation occurs.
Discussion of Protective Colloids
A protective colloid is a highly hydrophilic polymer colloid that makes coagulation less likely when added to a hydrophobic colloid.
When polymers such as gelatin or starch cover the surfaces of the particles, hydration layers and steric barriers make it difficult for the particles to approach one another.
As a result, coagulation becomes less likely even when an electrolyte is added.
Protective colloids are also related to dispersion stabilization in foods, pharmaceuticals, inks, paints, and other products.
If a sample containing a protective colloid was less likely to precipitate in an experiment, the particle surfaces were considered to have been protected and the dispersion stability increased.
Not only surface charge but also steric hindrance and hydration layers are important for dispersion stability.
Example Discussion:
Coagulation became less likely in the sample containing a protective colloid because the hydrophilic polymer was considered to have covered the surfaces of the hydrophobic colloidal particles.
As a result, hydration layers and steric barriers caused by the polymer chains made it difficult for the particles to approach one another.
Therefore, the protective colloid has the effect of increasing the dispersion stability of colloidal particles.
Zeta Potential and Dispersion Stability
Zeta potential is the potential at the slipping plane when a colloidal particle moves through a solution and is used as an indicator for considering particle-surface charge and dispersion stability.
The larger the absolute value of the zeta potential, the stronger the electrostatic repulsion between particles and the more stable the dispersion is considered to be.
When an electrolyte is added, the electrical double layer may be compressed and the absolute value of the zeta potential may decrease.
As a result, repulsion between particles weakens and coagulation becomes more likely.
Even in experiments where zeta potential is not directly measured, changes in particle-surface charge and repulsive force can be discussed from the ease of coagulation.
Example Discussion:
The dispersion stability of colloidal particles is related to the charge on the particle surfaces.
When the absolute value of the zeta potential is large, electrostatic repulsion between particles is strong and aggregation is difficult.
When an electrolyte is added, the electrical double layer is compressed and the absolute value of the zeta potential decreases, so particles can approach one another more easily and coagulation is considered to occur.
Effect of pH
The surface charge of colloidal particles may change depending on pH.
If acidic or basic groups are present on the particle surface, the magnitude and sign of the charge change depending on the amount of H+ or OH-.
Therefore, when the pH changes, dispersion stability and the ease of coagulation also change.
At certain pH values, the surface charge of the particles may become small and repulsion between particles may weaken.
Under such conditions, aggregation becomes more likely.
In colloids such as proteins, the charge may become small near the isoelectric point, making precipitation more likely.
Example Discussion:
If the ease of coagulation changed when the pH was varied, this was considered to result from a change in the surface charge of the colloidal particles.
Under conditions where the surface charge becomes small, electrostatic repulsion between particles weakens and aggregation becomes more likely.
Particularly in amphoteric colloids such as proteins, aggregation and precipitation readily occur near the isoelectric point.
Effect of Temperature
Temperature affects the motion and dispersion stability of colloidal particles.
As temperature increases, the thermal motion of liquid molecules becomes more active and Brownian motion also increases.
On the other hand, the hydration state of particle surfaces or the structure of polymer colloids may change, making aggregation more likely in some cases.
In hydrophilic colloids such as gelatin and proteins, changes in temperature may alter structure and viscosity and change the dispersion state.
Heating may also cause denaturation, resulting in precipitation or aggregation.
The effects of temperature differ depending on the type of colloid.
Example Discussion:
Changing the temperature affects the Brownian motion and hydration state of colloidal particles and therefore influences dispersion stability.
In polymer colloids, heating may cause structural changes or denaturation and make the particles more likely to aggregate.
Therefore, it is important to compare samples under the same temperature conditions.
Discussion of Changes in Turbidity
The turbidity of a colloidal solution is related to light scattering by the particles.
When the particles are small and well dispersed, some light scattering occurs, but precipitation is unlikely.
When an electrolyte is added and the particles aggregate, the particle size increases and the turbidity may appear stronger.
As aggregation progresses further, the particles settle and the supernatant may become nearly transparent.
In other words, an increase in turbidity and the formation of precipitate appear differently depending on the stage of coagulation.
During observation, it is useful to record the turbidity immediately after addition separately from the precipitation observed after time has passed.
Example Discussion:
The increase in turbidity immediately after the electrolyte was added was considered to result from aggregation of the colloidal particles, which increased the particle size and strengthened light scattering.
If a precipitate subsequently formed and the supernatant became transparent, the aggregates were considered to have settled under gravity.
Therefore, changes in turbidity reflect the progress of aggregation and sedimentation of colloidal particles.
Causes of Error in Colloidal-Solution Experiments
Causes of error in colloidal-solution experiments include differences in sample concentration, variation in particle size, light-source intensity, observation angle, dirty containers, errors in electrolyte concentration, differences in amount added, insufficient mixing, changes in pH, temperature differences, and differences in standing time.
Because colloids are sensitive to conditions, even small differences in operation may change the observations.
If the Tyndall effect is difficult to observe, possible causes include low sample concentration, a weak light source, a bright background, or an inappropriate observation angle.
If coagulation results vary, the amount of electrolyte added, the mixing method, or the standing time may not have been consistent.
It is important to standardize the observation conditions.
Example Discussion:
Possible causes of variation in the results of the colloidal-solution experiment include differences in sample concentration, amount of electrolyte added, mixing time, and standing time.
In addition, during observation of the Tyndall effect, the intensity of the light source, observation angle, and brightness of the background also affect the results.
Therefore, in comparative experiments, the sample amount, amount added, and observation conditions must be standardized as much as possible.
When the Results Can Be Considered Good
A colloidal-solution experiment can be considered to have produced good results when the Tyndall effect is not observed in a true solution but the light path is clearly observed in a colloidal solution.
In addition, if turbidity or precipitation occurs when an electrolyte is added and the ease of coagulation changes depending on electrolyte concentration or ionic valence, the results clearly demonstrate the dispersion stability of colloidal particles.
If coagulation is suppressed under conditions where a protective colloid is added, dispersion stabilization by a hydrophilic polymer can be considered to have occurred.
If the observations correspond to the concepts of the Tyndall effect, charge neutralization, the electrical double layer, and protective colloids, the experimental results can be considered reasonable.
Example Discussion:
In this experiment, the Tyndall effect was observed in the colloidal solution, whereas the light path was hardly visible in the true solution.
In addition, when an electrolyte was added, the colloidal solution became cloudy and a precipitate formed, indicating that the particle-surface charge was neutralized and coagulation occurred.
These results clearly demonstrate the light-scattering properties and dispersion stability characteristic of colloidal particles.
Example Discussions When the Experiment Did Not Go Well
When a colloidal-solution experiment does not go well, possible causes should be considered from results such as failure to observe the Tyndall effect, failure of coagulation to occur, a small amount of precipitate, difficulty seeing differences among comparison conditions, or precipitation occurring earlier than expected.
Organizing the causes according to sample concentration, light source, observation method, electrolyte concentration, mixing, standing time, and temperature makes the discussion easier.
Example Discussion:
Possible reasons why the Tyndall effect was not clearly observed include a low colloidal-particle concentration, a weak light source, and an inappropriate observation angle.
In addition, the light path becomes difficult to see against a bright background.
Therefore, strong light should be directed from the side against a dark background and compared with a true solution under the same conditions.
Another Example Discussion:
One possible reason why sufficient coagulation did not occur even after the electrolyte was added is that the electrolyte concentration was too low to sufficiently neutralize the surface charge of the colloidal particles.
In addition, if a hydrophilic colloid or protective colloid is present, the hydration layer or polymer chains may prevent the particles from approaching one another, making coagulation less likely.
How to Write Points for Improvement
In a discussion of colloidal solutions, writing not only the causes of error but also points for improvement makes the report easier to organize.
Points for improvement can be organized according to sample preparation, observation of the Tyndall effect, coagulation experiments, and recording methods.
Improvements to Sample Preparation
- Keep the concentration of the colloidal solutions consistent
- Mix the sample thoroughly before use
- Keep the standing time before use consistent
- Remove contamination from the containers
- Keep pH and temperature constant
- Prepare the true solution used for comparison under the same conditions
Improvements to Observation of the Tyndall Effect
- Use light sources of consistent intensity
- Observe against a dark background
- View the light path from the side
- Use the same type of container
- Record the turbidity of the sample in advance
- Keep the observation angle constant
Improvements to the Coagulation Experiment
- Prepare electrolyte concentrations accurately
- Keep the amount added constant
- Standardize the mixing method after addition
- Set a fixed observation time
- Record changes in the amount of precipitate and turbidity
- Compare electrolytes with different ionic valences
Example of How to Write Points for Improvement:
To clearly observe the Tyndall effect, it is necessary to standardize the sample concentration, light source, observation angle, and background conditions.
In addition, in the coagulation experiment, the electrolyte concentration and amount added must be accurate, and the mixing method and observation time after addition should be standardized.
This makes it possible to compare the dispersion stability of colloidal particles and differences in coagulation caused by electrolytes more accurately.
Difference Between a Superficial Discussion and a Good Discussion
In a discussion of colloidal solutions, simply writing that “the light was visible” or “a precipitate formed” results in a superficial discussion.
A good discussion relates particle size, light scattering, surface charge, the electrical double layer, coagulation, and dispersion stability.
| Superficial Discussion | Good Discussion |
|---|---|
| The path of light was visible. | The colloidal particles scattered visible light, so the Tyndall effect was considered to have been observed as a white light path when viewed from the side. |
| It was not visible in salt water. | Salt water is a true solution in which the solute is dispersed as ions, so the particles are small and light scattering is weak, and the Tyndall effect was hardly observed. |
| A precipitate formed when the electrolyte was added. | Ions in the electrolyte neutralized the charge on the surfaces of the colloidal particles, weakening electrostatic repulsion between particles and causing the particles to aggregate and precipitate. |
| More precipitation occurred with multivalent ions. | Multivalent ions with a charge opposite to that of the particles strongly neutralize the surface charge, compressing the electrical double layer and making coagulation likely even in small amounts. |
Examples of Expressions That Can Be Used in Reports
The following expressions can be used when writing the results and discussion of colloidal-solution experiments.
Adjust the necessary parts according to your own experimental results.
- In a colloidal solution, fine particles are dispersed in a liquid.
- Colloidal particles scatter visible light and therefore show the Tyndall effect.
- In a true solution, the particles are molecular or ionic in size, so light scattering is weak.
- Because of Brownian motion, colloidal particles move irregularly and are difficult to settle.
- When particle surfaces carry charges of the same sign, electrostatic repulsion stabilizes the dispersion.
- When an electrolyte is added, the particle-surface charge is shielded and coagulation becomes more likely.
- Multivalent ions more strongly neutralize the particle-surface charge and have greater coagulating power.
- Hydrophilic colloids are relatively stable because of hydration layers.
- Protective colloids cover the particle surfaces and help prevent coagulation.
- Ease of coagulation is affected by particle-surface charge, electrolyte concentration, pH, and temperature.
Points to Check When Discussing Colloidal Solutions
Checking the following points before writing the report makes the discussion easier to write.
- Is the definition of a colloidal solution explained?
- Are the differences among true solutions, colloidal solutions, and suspensions organized?
- Is the Tyndall effect explained in terms of light scattering?
- Are particle size and the Tyndall effect related?
- Are Brownian motion and resistance to settling explained?
- Are particle-surface charge and dispersion stability considered?
- Is the reason electrolytes cause coagulation explained?
- Is the relationship between ionic valence and coagulating power considered?
- Are the differences between hydrophilic and hydrophobic colloids explained?
- Is the role of protective colloids considered?
- Are the effects of pH and temperature considered?
- Do the points for improvement correspond to the causes of error?
Summary
A colloidal solution is a system in which fine particles larger than molecules or ions are dispersed in a liquid.
Colloidal particles scatter visible light and therefore show the Tyndall effect, in which the path of light becomes visible when the sample is illuminated.
In a true solution, the particles are small and light scattering is weak, so the Tyndall effect is hardly observed.
Colloidal particles maintain a dispersed state through electrostatic repulsion caused by surface charge, hydration layers, and Brownian motion.
However, when an electrolyte is added, the particle-surface charge is shielded and repulsion between particles weakens, making coagulation more likely.
In particular, multivalent ions with a charge opposite to that of the particles may show strong coagulating power.
In a report, rather than simply writing that “the path of light was visible” or “a precipitate formed,” organize and discuss the Tyndall effect, particle size, light scattering, Brownian motion, surface charge, the electrical double layer, coagulation caused by electrolytes, hydrophilic and hydrophobic colloids, protective colloids, causes of error, and points for improvement.
Colloidal-solution experiments are important experiments for understanding the fundamentals of particle dispersion and interfacial chemistry.
