Solutions, Suspensions, and Colloids: Seventh Grade Science
Mixtures exist along a physical spectrum defined by particle size, ranging from transparent molecular solutions to cloudy colloidal dispersions and settling suspensions.
The Spectrum of Chemical Mixtures
When substances are physically combined, the physical behavior of the resulting mixture depends on the diameter of the dispersed particles. Physical chemistry categorizes fluid mixtures into three primary classes: solutions, colloids, and suspensions.
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| MIXTURE CLASSIFICATION BY PARTICLE DIAMETER |
+---------------------+-------------------+-----------------+-----------------+
| Attribute | Solution | Colloid | Suspension |
+---------------------+-------------------+-----------------+-----------------+
| Particle Diameter | < 1 nanometer | 1 to 1000 nm | > 1000 nm |
| | (Atoms, ions) | (Aggregates) | (Visible grains)|
+---------------------+-------------------+-----------------+-----------------+
| Appearance | Clear, transparent| Translucent / | Opaque, cloudy, |
| | uniform single ph.| turbid milkiness| non-uniform |
+---------------------+-------------------+-----------------+-----------------+
| Settling Over Time | Never settles out;| Does not settle;| Settles under |
| | stable forever | stable over time| gravity |
+---------------------+-------------------+-----------------+-----------------+
| Filtration Action | Passes through | Passes through | Particles trapped|
| | filter paper | filter paper | on filter paper |
+---------------------+-------------------+-----------------+-----------------+
| Tyndall Effect | Negative (No beam)| Positive (Scat- | Positive (Scat- |
| | visible in liquid)| ters light beam)| ters light) |
+---------------------+-------------------+-----------------+-----------------+
| Real-World Examples | Saltwater, vinegar| Milk, fog, | Muddy water, |
| | brass, clear soda | gelatin, blood | Italian dressing|
+---------------------+-------------------+-----------------+-----------------+
The Anatomy of a Solution
A solution is a homogeneous mixture composed of two or more substances in a single physical state:
- Solute: The substance that is dissolved and is present in the smaller quantity (e.g., sodium chloride salt in seawater).
- Solvent: The dissolving medium that does the dissolving and is present in the greater quantity (e.g., water in seawater). Water is known as the universal solvent because its polar molecular structure allows it to dissolve a wide variety of ionic compounds and polar covalent molecules.
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| MOLECULAR DISSOLUTION MECHANISM |
| |
| Water Molecule Dipole: Hydration Shell around Sodium Cation: |
| (-) O (-) (H) (H) |
| / \ \ / |
| (H) (H) ( O ) (-) |
| (+) (+) | |
| (O)(-) --- [Na+] --- (-) (O) |
| Polar water molecules surround / \ / \ |
| and stabilize charged ions. (H) (H) (H) (H) |
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The Dissolution Mechanism: Hydration Shells
When an ionic crystal like table salt (NaCl) is placed in water, dissolution occurs through solvation (hydration). Water molecules are electric dipoles: the oxygen atom carries a partial negative charge, while the hydrogen atoms carry partial positive charges. The negative oxygen ends attract positive sodium cations (Na+), and the positive hydrogen ends attract negative chloride anions (Cl-). Water molecules pull ions away from the crystal lattice, surrounding each ion in a protective hydration shell that keeps them dissolved throughout the solvent.
The Tyndall Effect: Diagnostic Tool for Mixtures
In 1869, Irish physicist John Tyndall identified an optical technique to distinguish solutions from colloids:
- In a true solution, solute particles are smaller than 1 nanometer, too small to reflect visible light photons. A laser beam passing through a true solution is invisible from the side.
- In a colloid or suspension, the dispersed particles (1 to 1000 nm) are comparable in size to the wavelengths of visible light (400 to 700 nm). The particles scatter the light beam in all directions, making the beam's path visible through the fluid. This scattering is the Tyndall effect, explaining why automobile high beams produce glare in fog (a colloid of water droplets in air) but shine through clear air.
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| THE OPTICAL TYNDALL EFFECT |
| |
| LIGHT SOURCE TRUE SOLUTION COLLOID |
| [===] +-------------+ +-------------+ |
| Laser Pointer | | | * * * | |
| =====> | (Invisible | ======> | * (BEAM) * | |
| =====> | Beam) | ======> |* SCATTERS * | |
| | | | * * * | |
| +-------------+ +-------------+ |
| No light scattered Visible light path! |
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Solubility Dynamics and Saturation States
Solubility is the maximum mass of solute that can dissolve in a given volume of solvent at a specific temperature, usually expressed in grams of solute per 100 grams of water (g / 100 g H2O):
- Unsaturated Solution: Contains less solute than the maximum amount the solvent can hold at that temperature. Adding more solute will result in it dissolving.
- Saturated Solution: Contains the maximum equilibrium amount of dissolved solute at that temperature. Any additional solute added will not dissolve and settles to the bottom.
- Supersaturated Solution: An unstable state where a solution temporarily holds more dissolved solute than the equilibrium saturation limit. Produced by saturating a solution at high temperature and cooling it slowly without vibration. Introducing a single seed crystal triggers crystallization of the excess solute.
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| FACTORS INFLUENCING THE RATE OF DISSOLUTION |
+---------------------+-------------------------------------------------------+
| Physical Factor | Kinetic Molecular Mechanism |
+---------------------+-------------------------------------------------------+
| Agitation (Stirring)| Moves fresh unsaturated solvent into direct contact |
| | with solute surfaces, sweeping away hydration shells |
+---------------------+-------------------------------------------------------+
| Temperature | Increases particle kinetic speed and solvent collision|
| | frequency; breaks solute lattice bonds faster |
+---------------------+-------------------------------------------------------+
| Surface Area | Grinding a solute into fine powder exposes more |
| (Particle Size) | surface area to solvent collisions |
+---------------------+-------------------------------------------------------+
Factors Affecting Solubility
- Temperature: For most solid solutes (e.g., sugar, potassium nitrate), solubility increases with rising temperature. For gases dissolved in liquids (e.g., oxygen or carbon dioxide in water), solubility decreases as temperature rises because higher kinetic energy allows gas molecules to escape the liquid surface.
- Pressure: Pressure has negligible effect on solid or liquid solubility, but strongly affects gases. Henry's Law states that the solubility of a gas in a liquid is directly proportional to the partial pressure of that gas above the liquid. Opening a soda can releases pressure, causing dissolved carbon dioxide to come out of solution as bubbles.
Exercises and Solutions
Exercise 1: Interpreting Solubility Curves Across Temperature Gradients. The solubility of potassium nitrate (KNO3) in water is 32 grams per 100 grams of water at 20 degrees Celsius, and 85 grams per 100 grams of water at 50 degrees Celsius. A laboratory student adds 60 grams of KNO3 to 100 grams of water at 50 degrees Celsius and stirs until completely dissolved. (a) Classify this initial solution at 50 degrees Celsius as unsaturated, saturated, or supersaturated; (b) The student cools the solution slowly to 20 degrees Celsius. Assuming no solute precipitates during cooling, classify the resulting solution state at 20 degrees Celsius; (c) If the student drops a single seed crystal of KNO3 into the 20-degree solution, describe what physical event will occur and calculate the exact mass of solid KNO3 that will precipitate.
Solution 1: (a) At 50 degrees Celsius, water can dissolve up to 85 grams of KNO3 per 100 g H2O. Because the student dissolved only 60 grams, the solution contains less than the maximum limit, making it an unsaturated solution. (b) When cooled to 20 degrees Celsius, the saturation limit drops to 32 grams per 100 g H2O. Because the solution holds 60 grams in dissolved form without precipitating, it temporarily holds more solute than the saturation limit, making it an unstable supersaturated solution. (c) Dropping a single seed crystal into the supersaturated solution disrupts the metastable equilibrium, providing a nucleation template that causes excess solute to crystallize out of solution. The mass of solid precipitate formed equals the excess solute beyond the 20-degree saturation limit: Mass of Precipitate = 60 g (dissolved) - 32 g (saturation limit) = 28 grams of solid crystalline KNO3 will precipitate out, leaving a stable saturated solution containing 32 grams of dissolved solute.
Exercise 2: Diagnosing Fluid Mixtures in Clinical Laboratory Testing. A medical technologist evaluates three liquid samples: Sample A is clear and pale yellow; when a red laser is directed through it, no light beam is visible from the side, and centrifuging the tube produces no sediment. Sample B is translucent white; when a laser is directed through it, a distinct beam is visible through the liquid, and no sediment settles after twenty-four hours. Sample C is cloudy red; when allowed to stand for one hour, dense red particles settle to the bottom, leaving a clear fluid above. Classify Samples A, B, and C as a solution, a colloid, or a suspension, providing two diagnostic justifications for each.
Solution 2: Sample A is a true solution because it is clear and transparent, does not scatter a laser beam (negative Tyndall effect), and contains molecular-sized particles that do not separate under centrifugation. Sample B is a colloid because it has a translucent milky appearance, exhibits a positive Tyndall effect by scattering the laser beam, and maintains stable dispersed particles that do not settle over time under gravity. Sample C is a suspension because it is cloudy and opaque, contains large cellular particles that settle under gravity into distinct layers over time, and can be physically separated by standard filtration.