A suspension is a heterogeneous mixture in which particles disperse within a liquid or gas (fluid) but do not dissolve. These particles are large enough that they are often visible and eventually settle out due to gravity if left undisturbed. Suspensions are typically cloudy or opaque because the particles scatter light. Filtration or decantation separates the components of most suspensions.
Key Points
- A suspension is a heterogeneous mixture that settles out upon standing.
- Everyday examples of suspensions include mud, flour in water, and dust in air.
- Both suspensions and colloids are heterogeneous mixtures, but the particle size in a suspension is larger. Colloid particles are not visible and they don’t settle out.
- In contrast, a solution is a homogeneous mixture where the solute dissolves into the solvent.
Properties of Suspensions
Suspensions are easily recognizable because their particles are often visible and they settle out over time:
- Heterogeneous Mixture: Suspensions are visibly non-uniform, with distinguishable solid particles within the dispersing medium.
- Particle Size: The particles in a suspension are larger than 1 micrometer (μm) in diameter. This distinguishes them from smaller particles found in colloids and solutions.
- Sedimentation: Due to the larger particle size, solid particles in suspensions settle out if the suspension is left undisturbed over time.
- Filtration: Most suspensions separate using simple filtration methods since the solid particles are large enough to be trapped by filter paper.
- Tyndall Effect: Some suspensions exhibit the Tyndall effect, where particles scatter light and make the path of light visible through the mixture.
- No Dissolution: The particles in a suspension do not dissolve in the medium. They maintain their distinct phase and chemical identity.
- Instability: Suspensions are inherently unstable because the particles eventually settle out or separate.
- Opaque or Cloudy Appearance: The mixture often appears turbid or opaque due to the light-scattering properties of the suspended particles.
- Reversible: Shaking or stirring a separated suspensions reforms it.
- Concentration Dependent: The concentration of particles affects the stability and appearance of a suspension; higher concentrations typically settle faster.
10 Examples of Suspensions
- Muddy Water: A mixture of soil or clay particles dispersed in water settles over time.
- Flour in Water: Flour particles suspended in water display the Tyndall effect and eventually settle.
- Sand in Water: Sand particles suspended in water quickly settle or separate by filtration.
- Chalk in Water: Chalk powder mixed with water forms a cloudy mixture.
- Paint: Pigment particles suspended in a solvent separate if left undisturbed.
- Orange Juice with Pulp: The pulp forms a suspension within the juice, requiring shaking to redistribute.
- Dust in Air: Solid particles of dust float in the air and slowly settle on surfaces.
- Snow Globe: The globe contains tiny solid particles suspended in a liquid medium.
- Medicinal Suspensions: Some liquid medications with undissolved active ingredients require mixing or shaking, such as amoxicillin suspension.
- Salad Dressing (e.g., vinaigrette): Some salad dressing contain oil droplets and seasonings that separate after mixing.
Types of Suspensions
Classification of suspensions is according to the state of the dispersed phase (solid, liquid, or gas) and the dispersing medium:
Slurries (Solid in Liquid)
- Definition: A slurry is a suspension of fine solid particles in a liquid, often water.
- Examples: Cement slurries in construction, coal slurries in mining, and food slurries in the processing of beverages.
- Properties: Slurries are typically thick, but settle if left undisturbed.
Aerosols (Solid in Gas)
- Definition: Aerosols are suspensions where solid particles or liquid droplets disperse in a gas, usually air.
- Examples: Smoke (solid particles in air) and dust storms.
- Properties: Aerosols have a small particle sizes that helps them remain suspended in the air for extended periods, though larger particles eventually settle.
Foams (Gas in Liquid)
- Definition: A foam is a suspension of gas bubbles in a liquid. While most foams are colloids, foams with larger bubbles behave more like suspensions.
- Examples: Whipped cream, fire-fighting foam, and soap suds.
- Properties: Foams are unstable and collapse as the gas bubbles merge and rise to the surface.
Suspensions with Gas Bubbles (Gas in Liquid)
- Definition: This type of suspension involves gas bubbles in a liquid without forming a stable foam.
- Examples: Carbonated drinks immediately after opening, where gas bubbles are temporarily suspended in the liquid.
- Properties: The gas bubbles are often large and rise quickly, separating from the liquid medium.
Suspension Emulsions
- Definition: A suspension emulsion combines characteristics of both suspensions and emulsions, where both solid particles and immiscible liquid droplets coexist in a medium.
- Examples: Certain types of paints or salad dressings containing both suspended solids and oil droplets in water.
- Properties: Highly complex and often require stabilizers to maintain the mixture.
Magma
- Definition: Magma is a term that describes suspensions of insoluble mineral particles in water or other liquids.
- Examples: Milk of magnesia, which consists of magnesium hydroxide suspended in water, and magma, which contains solid crystals and gas bubbles in molten rock.
- Properties: Magmas are thick, with particles that settle slowly.
Precipitate Suspensions
- Definition: A precipitate suspension forms when a chemical reaction produces a solid precipitate in a liquid medium.
- Examples: The formation of silver chloride when mixing solutions of silver nitrate and sodium chloride.
- Properties: The precipitate forms rapidly and settles unless agitated, typical in many chemical reactions.
Sedimentation Suspensions
- Definition: Suspensions where the particles are prone to sedimentation due to gravity, often in natural or industrial processes.
- Examples: River silt in water, mining tailings ponds.
- Properties: Sedimentation rates depend on particle size, density, and the viscosity of the medium.
Difference Between a Suspension and a Colloid
The key difference between a suspension and a colloid is the particle size. Otherwise, both are heterogeneous mixtures that contain undissolved particles and often display the Tyndall effect. The particles in a colloid don’t settle out and are not easily separated using filtration. Fog and milk are examples of colloids.
- Particle Size: Colloids have intermediate particle sizes (1 nm to 1 μm) that are smaller than those in suspensions but larger than those in solutions. These particles do not settle out like those in suspensions.
- Settling: Suspensions settle over time, while colloids remain stable without settling due to Brownian motion, which keeps the particles dispersed.
- Visibility: Particles in a suspension are often visible to the naked eye or under a microscope, whereas colloidal particles require a microscope for visibility.
- Tyndall Effect: Both suspensions and colloids sometimes exhibit the Tyndall effect. The effect is more pronounced in suspensions due to the larger particle size.
- Filtration: Colloidal particles pass through regular filter paper, whereas suspension particles cannot.
- Stability: Colloids are generally more stable than suspensions due to the small particle size and the presence of stabilizing agents that prevent coagulation.
Difference Between a Suspension and a Solution
Suspensions and colloids differ from solution in that a solution is homogeneous, contains dissolved particles, and is translucent. A solution consists of a single phase, while suspensions and colloids contain distinct phases. Sugar in water and sea water are examples of solutions.
- Particle Size: Suspensions have large particles (greater than 1 μm) that are visible and can settle out, while solutions contain dissolved particles (ions or molecules) smaller than 1 nm that do not settle.
- Homogeneity: Solutions are homogeneous at the molecular level. Their composition is uniform throughout, whereas suspensions are heterogeneous with visibly distinct phases.
- Stability: Solutions are stable, with particles evenly distributed indefinitely, whereas suspensions are unstable, with particles settling over time.
- Separation: Particles in a suspension separate by filtration, while solutions require evaporation or more complex chemical separation methods since the solute is dissolved.
- Appearance: Solutions are usually clear and transparent, while suspensions are cloudy or opaque due to light scattering by large particles.
Applications of Suspensions
Suspensions have various practical uses:
- Pharmaceuticals: Suspensions are useful in liquid medications where the active ingredient does not dissolve completely.
- Food and Beverages: Many foods, like salad dressings and chocolate milk, are suspensions that require shaking before use.
- Environmental Science: Suspended sediments in water bodies aid in understanding water quality and environmental impacts.
- Industrial Processes: Suspensions are important in paints, ceramics, and other manufacturing processes where solid particles disperse in a medium. Thickeners, stabilizers, and emulsifiers help keep components from separating.
Simple Experiment: Make a Suspension
This simple experiment provides a hands-on way of identifying suspensions and differentiating them from other mixtures such as colloids and solutions.
Objective: To prepare a suspension, observe its properties, filter it, and draw conclusions about its classification.
Materials Needed:
- Water
- Sand or flour (to act as the suspended solid)
- Beaker or clear glass container
- Stirring rod or spoon
- Filter paper or coffee filter
- Funnel
- Light source (flashlight)
- Glass of milk (optional: as a colloid for comparison)
Procedure:
- Preparation of the Suspension:
- Fill a beaker or glass with about 200 mL of water.
- Add 2-3 tablespoons of sand or flour to the water.
- Stir the mixture vigorously to disperse the particles evenly.
- Observation:
- Observe the mixture immediately after stirring. Note its cloudy or opaque appearance.
- Let the mixture sit undisturbed for 10-15 minutes and observe if the particles settle at the bottom.
- Shine a flashlight through the mixture to observe the Tyndall effect. Note the light scattering and compare it with the effect in a clear solution or a colloidal mixture.
- Filtration:
- Set up the funnel with filter paper over an empty container.
- Pour the suspension carefully into the funnel.
- Observe whether the filter paper retains solid particles while the liquid passes through.
- Compare the filtrate (the liquid that passes through) to the original suspension.
- Drawing Conclusions:
- Note that the sand or flour particles settle over time, confirming the instability characteristic of suspensions.
- The filtration step demonstrates that suspensions can be separated by simple filtration due to the larger particle size.
- The visible scattering of light confirms the Tyndall effect.
Conclusions:
- Classification: The mixture behaves as a suspension, evident by the settling of particles and the ability to separate them through filtration.
- Key Properties: The experiment shows that suspensions have larger particles that do not dissolve, settle over time, and are easily filtered.
- Comparison with Colloids: In contrast, the particles in a colloid to not settle or filter out.
References
- Brade, J.; Senese, F. (2004). Chemistry: Matter and Its Changes (4th ed.). Wiley. ISBN: 0-471-21517-1.
- Hiltner, P.A.; Krieger, I.M. (1969). “Diffraction of light by ordered suspensions”. J. Phys. Chem. 73 (7): 2306. doi:10.1021/j100727a049
- Mengual, O.; Meunier, G.; et al. (1999). “Characterisation of Instability of Concentrated Dispersions by a New Optical Analyser: The TURBISCAN MA 1000”. Colloids and Surfaces A: Physicochemical and Engineering Aspects. 152: 111-123. doi:10.1016/S0927-7757(98)00680-3