Question

(a) Do colloids made only of gases exist? Why or why not? (b) In the 1850s, Michael Faraday prepared ruby-red colloids of gold nanoparticles in water that are still stable today. These brightly colored colloids look like solutions. What experiment(s) could you do to determine whether a given colored preparation is a solution or colloid?

Short answer

(a) Colloids made only of gases do not exist because gas particles freely mix with each other, rather than forming suspensions with a dispersed phase and dispersion medium. (b) To determine if a colored preparation is a solution or colloid, perform experiments like (1) observing the Tyndall effect, (2) filtration, and (3) sedimentation or centrifugation.

Step-by-step solution

Understand What Colloids Are

Colloids are a type of mixture that consists of two or more substances uniformly distributed throughout a powdery or granular substance or a liquid, but in which the individual particles do not dissolve, and instead are merely suspended (unlike solutions where the solute is dissolved). The size of particles in a colloid generally ranges between 1 nm and 1000 nm. The substance that is dispersed in a colloid is called the dispersed phase, and the medium in which it is dispersed is called the dispersion medium.

Determine if Colloids of Gases Exist and Explain Why

Colloids made only of gases do not exist because, in a gas, the particles are already in a state of continuous, random motion with large spaces between them. As a result, gas particles freely mix with each other, rather than forming a colloid in which particles remain suspended without dissolving. In a colloid made up of gas particles only, there would be no distinction between the dispersed phase and the dispersion medium because the particles would simply mix together instead of forming a suspension.

Identify Experiments to Determine if a Colored Preparation is a Solution or Colloid

To determine whether a given colored preparation is a solution or a colloid, one can perform the following experiments: 1. Tyndall effect: Shine a beam of light through the colored preparation and observe if the light path is visible or dispersed. In a colloid, the light path will be visible due to the scattering of light by the suspended particles (this is called the Tyndall effect). In a solution, the light path will not be visible, as the particles are dissolved and do not scatter light. 2. Filtration: Filter the colored preparation through a suitable filter paper. If the colored substance passes through the filter paper, it is a solution, as the dissolved particles are too small to be trapped in the filter. If the colored substance is retained by the filter paper, then it is a colloid, as the particles are larger and can be trapped in the filter. 3. Sedimentation and centrifugation: Allow the colored preparation to stand undisturbed for a suitable period of time. If the particles settle at the bottom, then it is a colloid. For faster results, centrifuge the sample. If particles are separated by centrifugation, it is a colloid. However, if the particles remain uniformly distributed and do not settle or separate, then it is a solution.

Key concepts

Tyndall effect

Imagine you're in a dim room with a flashlight. When you shine the light through a solution like sugar water, the light goes right through and you see nothing special. But when you have a colloid, like milk, the light makes its path visible. This phenomenon is called the Tyndall effect. It's due to the way particles in colloids scatter the light.

The Tyndall effect is a handy way to distinguish between colloids and true solutions. In solutions, particles are too small to scatter light effectively, so you won't see the light beam.

• If you shine a light and see the beam, you're likely dealing with a colloid.
• If there's no visible light path, it's probably a solution.

It's a simple and effective method for identifying which type of mixture you're examining.

Gas mixtures

Can gas colloids exist? To answer this, let's look at how gases behave. Gas particles move randomly and mix freely. This motion doesn't leave room for forming a colloid, which requires particles to be suspended within a medium. In gases, such suspension isn't possible due to the natural mixing of gas molecules.

In other words, all particles in gases act as both the dispersed phase and the dispersion medium, mixing uniformly. This is why gas-only colloids do not exist. The concept of suspension is key when it comes to colloids, something that gas mixtures simply cannot achieve.

Particle suspension

Particle suspension is at the heart of what defines a colloid. Unlike solutions where particles dissolve completely, suspensions feature particles that remain distributed throughout the mixture without dissolving.

Suspended particles are typically larger than those in solutions, ranging in size from 1 nm to 1000 nm. This size allows them to interfere with light, creating the Tyndall effect.

• The dispersed phase consists of these larger particles.
• The dispersion medium holds and spreads these particles.

Aggregation doesn't occur easily in colloids due to the stability provided by this suspension, unlike in solutions where particles dissolve completely.

Filtration

Filtration is an experiment that helps us decide if a mixture is a solution or a colloid. Imagine pouring a liquid through filter paper.

In a solution, dissolved particles are so small that they pass right through the filter. However, in a colloid, the larger particles are intercepted by the filter paper. This method effectively separates out the colloidal particles.

• If the particles are trapped by the filter, you have a colloid.
• If they aren't, then it's likely a solution.

Filtration serves as a practical tool in distinguishing between different types of mixtures.

Sedimentation and centrifugation

Sedimentation and centrifugation are systematic ways to separate mixtures. In sedimentation, a colloid is left undisturbed. Over time, gravity might cause larger particles to settle at the bottom. This process is not usually seen in solutions as dissolved particles stay evenly spread.

Centrifugation speeds up sedimentation using a spinning force to push the particles to the bottom. It's particularly effective for colloids, where particles are heavier than the surrounding medium.

• If particles eventually settle, the mixture was likely a colloid.
• If they remain distributed, it might be a solution.

These techniques highlight the physical properties of colloids, making them distinct from solutions.