Question

What is a suspension, and how does it differ from a colloid?

Short answer

A suspension is a heterogeneous mixture with larger particles suspended in a continuous phase, whereas a colloid is a mixture where smaller particles are evenly distributed in a continuous phase. The main differences between them are particle size, stability, appearance, and their response to light and separation methods. Suspensions have larger, visible particles that settle over time, appear cloudy, and can be separated through filtration or sedimentation. Colloids have smaller particles that do not settle, can appear cloudy or transparent, and exhibit the Tyndall effect when light is passed through them.

Step-by-step solution

Define Suspension

A suspension is a heterogeneous mixture in which small particles of a solid, called the dispersed phase, are suspended in a liquid or gas, called the continuous phase. The particles in a suspension are larger than those in a solution and are typically visible to the naked eye. Over time, these particles will settle to the bottom due to gravity unless they are constantly agitated or mixed.

Define Colloid

A colloid is a mixture of two or more substances where one substance, called the dispersed phase, is distributed evenly throughout another substance, called the continuous phase. The particles in a colloid are smaller than those in a suspension, but larger than those in a solution, typically ranging from 1 to 1000 nanometers in size. Due to their small size, these particles do not separate out of the mixture and the mixture appears homogeneous, even though it is not a true solution.

Differentiate between Suspension and Colloid

The main differences between a suspension and a colloid are: 1. Particle size: In a suspension, the dispersed particles are larger (usually visible to the naked eye) than those in a colloid. 2. Stability: Suspensions are unstable, and the dispersed particles will eventually settle due to gravity, whereas colloids are stable and the particles do not settle out over time. 3. Appearance: Suspensions are generally cloudy or opaque, while colloids can appear cloudy, translucent or transparent, depending on the size and nature of the dispersed particles. 4. Separation: Suspended particles can be separated from the continuous phase through filtration or sedimentation, while colloidal particles usually cannot be separated easily by these methods. 5. Tyndall effect: When a beam of light is passed through a colloid, the dispersed particles scatter the light, causing the path of the light to be visible (known as the Tyndall effect). In suspensions, the effect is generally less pronounced or not observed. In summary, both suspensions and colloids are mixtures consisting of a dispersed phase distributed in a continuous phase. The key differences between them are particle size, stability, appearance, and how they react to light and separation methods.

Key concepts

Heterogeneous Mixture

Understanding the differences between suspensions and colloids starts with recognizing that both are types of heterogeneous mixtures. This means they consist of two or more substances that are not evenly mixed on a molecular level. Unlike homogeneous mixtures, where the substance is uniform throughout, the components of a heterogeneous mixture can be visibly and easily distinguished.For instance, when oil is combined with vinegar to make a vinaigrette, the two substances separate into distinct layers. This characteristic of heterogeneity is shared by suspensions, where larger particles are seen floating or settling in the liquid, and colloids, where smaller but still discrete particles are dispersed throughout. However, the distinction between these two mixtures lies in the stabilisation of the particles and their interaction with light, which we'll explore in further sections.

Particle Size in Mixtures

The size of particles in a mixture is crucial in determining its classification as a suspension or a colloid. In suspensions, the particles typically exceed 1000 nanometers, and this large size is why they are often visible to the naked eye. Due to gravity, these large particles will eventually settle at the bottom if left undisturbed, revealing the heterogeneous nature of the mixture.Colloids, on the other hand, have smaller particles, ranging from 1 to 1000 nanometers. These particles are too small to be seen individually without a microscope but are large enough to scatter light. This smaller particle size in colloids prevents them from settling out, maintaining a more even distribution throughout the continuous phase, which can sometimes give the appearance of a homogeneous mixture.

Tyndall Effect

The Tyndall effect provides a unique way to distinguish colloids from other mixtures, particularly suspensions. This optical phenomenon occurs when light passes through a colloid and is scattered by its dispersed particles, making the light beam visible inside the mixture. For example, if you've ever seen a beam of sunlight filtering through a dusty room, you've witnessed the Tyndall effect. The scattered light makes the path of the beam apparent. In colloids, this effect allows us to detect the presence of those tiny particles. Suspensions may show a less pronounced Tyndall effect or may not show it at all because their larger particles do not scatter light in the same way. It's this interaction with light—not just the size of the particles or how they settle—that further differentiates colloids from suspensions, making the Tyndall effect a practical and revealing test.