Question

How do colloids differ from suspensions?

Short answer

Colloids have smaller particles that remain distributed without settling, exhibit Brownian motion, and cannot be filtered, while suspensions have larger particles that settle and can be filtered.

Step-by-step solution

Identify Key Concepts

First, we need to understand what colloids and suspensions are. Colloids are mixtures where very small particles (typically between 1 and 1000 nanometers) are evenly dispersed throughout another substance. In contrast, suspensions are heterogeneous mixtures where larger particles (generally larger than 1000 nanometers) are dispersed in a liquid or gas but can settle over time.

Examine Particle Size

Colloids have particle sizes that range between 1 and 1000 nanometers. This small size allows them to remain evenly distributed without settling. Suspensions, however, have larger particles typically greater than 1000 nanometers, which tend to settle over time because of gravity.

Consider Particle Movement and Suspension

In colloids, the small particle size allows for Brownian motion, which keeps the particles dispersed and prevents settling. Suspensions lack this because their larger particles are affected by gravity, causing them to settle if not constantly agitated.

Understand Optical Properties

Colloids can scatter light, a phenomenon known as the Tyndall effect, because their particle size is close to the wavelength of visible light. This makes colloids appear translucent. Suspensions, on the other hand, may appear opaque and can scatter light if the particles are large enough, but generally, they scatter less than colloids when settled.

Explore Separation Methods

Colloids are stable and cannot be separated by simple filtration due to their small particle size. In contrast, suspensions can be separated by filtration because the particles are large enough to be caught by filter paper.

Key concepts

Suspensions

In chemistry, a suspension is a type of mixture where solid particles are dispersed throughout a liquid or gas. These particles are usually larger than 1000 nanometers and are visibly distinct. Suspensions are often considered heterogeneous mixtures as the components are not uniformly distributed.
One key characteristic of suspensions is that the particles tend to settle over time under the influence of gravity. This is why you may see the solid component settling at the bottom of a container if left undisturbed.

• Particle size: Greater than 1000 nanometers.
• Mixture type: Heterogeneous.
• Separation: Can be separated by filtration or decanting.

Sometimes, to keep the particles suspended, it requires agitation or stirring, as seen in certain medications or paints.

Tyndall Effect

The Tyndall effect is an optical phenomenon observed in colloids and some suspensions. It happens when light is scattered by particles smaller than the wavelength of visible light. This scattering causes the light beam to become visible as it passes through the mixture, much like a flashlight beam in fog.
The Tyndall effect is useful in distinguishing between different types of mixtures:

• Colloids generally exhibit the Tyndall effect, making them appear cloudy or opaque.
• True solutions do not scatter light and hence, do not show the Tyndall effect.

This phenomenon helps scientists and students identify colloids and differentiate them from true solutions or suspensions.

Particle Size

Particle size in mixtures determines many of their properties, such as stability and how the particles interact with light. For colloids and suspensions, particle size is a key distinguishing characteristic.
In a colloid, the particles are typically between 1 and 1000 nanometers. This range allows the particles to remain stable and not settle out over time, thanks to forces like Brownian motion. In contrast, particles in suspensions are larger than 1000 nanometers. This larger size causes them to settle out due to gravity, unlike smaller colloid particles.

• Colloids: 1 - 1000 nm.
• Suspensions: Typically larger than 1000 nm.

The size not only affects the stability but also impacts how each mixture can be separated or filtered.

Brownian Motion

Brownian motion is a random movement of particles suspended in a fluid (liquid or gas), a phenomenon seen significantly in colloids. It was discovered by observing pollen grains jittering in water. This motion results from particles colliding with the fast-moving molecules of the fluid.
In colloids, Brownian motion helps to keep particles evenly distributed, preventing them from settling.

• Result of random collisions with molecules in the fluid.
• Helps maintain stability in colloids.

Suspensions do not exhibit significant Brownian motion due to their larger particle size that overcomes the random collisions, allowing gravity to dominate and lead to settling.