Salad dressing is an emulsion, which is a type of mixture made up of small droplets of one liquid dispersed throughout another liquid. Emulsions are examples of colloids, so salad dressing can be classified as a colloid.
What is a Colloid?
A colloid is a mixture in which one substance is dispersed evenly throughout another substance. The dispersed substance consists of tiny particles or droplets that are between 1 and 1000 nanometers in size. These particles are large enough that they do not truly dissolve but remain suspended within the dispersing medium.
Some examples of colloids include milk, paint, butter, and salad dressing. In a colloid, the substance that is broken down into the tiny, dispersed particles is called the dispersed phase. The substance that makes up the bulk of the mixture is called the dispersion medium.
Properties of Colloids
Colloids have the following key properties:
- The particles remain suspended and do not settle out over time.
- The mixtures are homogeneous, with the particles distributed evenly throughout.
- The dispersed particles scatter light, causing colloids to appear cloudy or opaque.
- The particles are small enough to remain dispersed but too large to pass through filter paper.
- Colloids are reversible; the dispersed particles can be separated from the medium if enough force is applied.
What is a Suspension?
A suspension is a heterogeneous mixture containing solid particles dispersed in a liquid medium. The key difference between a suspension and a colloid is that the particles in a suspension are larger, typically greater than 1000 nanometers.
The large particle size means that the particles in a suspension will eventually settle out if left undisturbed. Suspensions appear cloudy or opaque because the dispersed particles refract and scatter light. Paint that has not been thoroughly mixed and contains pigment particles settled at the bottom is an example of a suspension.
Comparing Colloids and Suspensions
| Property | Colloid | Suspension |
|---|---|---|
| Particle size | 1 – 1000 nm | > 1000 nm |
| Dispersed phase | Solid, liquid or gas | Solid |
| Settling | Does not settle | Settles over time |
| Filterability | Cannot pass through filter paper | Can be filtered |
| Appearance | Cloudy, opaque | Cloudy, opaque |
| Example | Milk, fog | Muddy water, chalk in water |
This table summarizes the key differences between colloids and suspensions. The smaller particle size and lack of settling are distinguishing characteristics of colloids.
What Makes Up Salad Dressing?
A typical salad dressing contains oil, vinegar or lemon juice, and emulsifiers. Common emulsifiers used in salad dressing include egg yolks, honey, mustard, and mayonnaise. These help the oil and vinegar combine into a smooth emulsion.
In an emulsion, one liquid is dispersed as tiny droplets throughout the other liquid. In salad dressing, the dispersed phase is the oil, while the continuous phase is the aqueous vinegar or lemon juice. So salad dressing consists of oil droplets suspended in the surrounding vinegar/lemon juice.
Role of Emulsifiers
Emulsifiers are substances that help oil and water mix together and stay combined in an emulsion. They have a hydrophilic (water-loving) end that attaches to the vinegar and a hydrophobic (water-fearing) end that attaches to the oil droplets.
This allows the emulsifier molecules to act as a bridge between the oil and vinegar components. They surround each oil droplet and prevent the droplets from coalescing back into larger oil pools, keeping the salad dressing evenly mixed.
Why Salad Dressing is a Colloid
With oil droplets dispersed throughout the vinegar/lemon juice, salad dressing mixtures have the defining qualities of a colloid:
- Oil droplets remain suspended throughout the liquid and do not separate or settle out over time.
- The mixture appears cloudy and opaque.
- The oil and vinegar components remain mixed evenly.
- The droplets are between 1-1000 nm in size, within the colloid range.
Additionally, the emulsion can be destabilized by adding enough acid, cooling it, or shaking vigorously. This demonstrates the reversible nature of colloids.
Therefore, with its dispersed phase as tiny oil droplets suspended stably throughout the continuous vinegar/lemon juice medium, salad dressing qualifies as a colloid.
Why Salad Dressing is Not a Suspension
Salad dressing does not meet the criteria to be classified as a suspension because:
- The particle size is much smaller, in the nanometer range rather than micrometers.
- The dispersed phase is liquid oil droplets, not solid particles.
- The oil droplets remain dispersed rather than settling out over time.
Suspensions contain large solid particles that will visibly separate from the liquid if left standing. Salad dressing does not have particles large enough to settle out or be filtered off.
Conclusion
Based on its structure as tiny oil droplets uniformly dispersed in vinegar/lemon juice liquid, salad dressing matches the defining qualities of a colloid. The nanometer-scale droplet size along with the stability of the emulsion over time distinguishes it from a suspension. Therefore, salad dressing is classified as a colloid mixture, not a suspension.