Question

Given an unknown mixture consisting of two or more substances, explain how you could determine whether that mixture is a true solution, a colloid, or a suspension.

Short answer

Observe physical appearance, conduct the Tyndall effect test, check for sedimentation, use filtration, and if possible, centrifuge the mixture.

Step-by-step solution

Observe the Mixture

Look at the physical appearance of the mixture. Check for any obvious particles that may be settling at the bottom, which might indicate it's a suspension.

Conduct the Tyndall Effect Test

Shine a light through the mixture. If the light scatters and becomes visible as it passes through, it is likely a colloid. If the light passes through without scattering, the mixture is most likely a true solution.

Check for Sedimentation

Let the mixture sit undisturbed for a period. Observe if particles settle down at the bottom. If they do, it confirms the mixture as a suspension.

Filtration Test

Try to filter the mixture using a fine-filter paper. If the particles are retained on the filter paper, the mixture is a suspension. If it passes through but is still cloudy, it could be a colloid. If it passes through and is clear, it is a true solution.

Use of Centrifuge (if available)

Spin the mixture in a centrifuge. In a suspension, particles will separate out and form layers. In a colloid, the particles may separate but will remain dispersed when not centrifuged. A true solution will have no separation.

Key concepts

Tyndall Effect

The Tyndall Effect allows you to distinguish between a colloid and a true solution. Simply shine a beam of light through your mixture. If the light scatters and becomes visible as it passes through the mixture, it confirms the presence of colloidal particles. This scattering happens because the particles in a colloid are large enough to disrupt the light. If the light beam passes through without any noticeable scattering, your mixture is most likely a true solution, which means it has molecules or ions too small to scatter the light. Observing the Tyndall Effect is straightforward and can be done easily at home with a flashlight.

Filtration

Filtration helps separate the components of a mixture based on particle size. Use fine-filter paper to filter the mixture:

• If particles are retained on the filter paper, it's likely you have a suspension.
• If the mixture passes through but remains turbid (cloudy), it might be a colloid.
• If the filtering results in a clear liquid, the mixture is a true solution.

Filtration is a simple and effective method often used in both daily life and scientific practices. Even if colloidal particles are very small, they might still pass through ordinary filter paper, distinguishing them from suspensions.

Centrifuge

Using a centrifuge can improve your separation process by applying centrifugal forces much stronger than gravity. When you spin your mixture fast enough:

• In a suspension, heavier particles will settle at the bottom of the tube, creating layers due to their greater mass.
• In a colloid, the smaller particles may also separate but will suspend again once you stop the centrifuge. Colloidal particles are small enough to remain dispersed.
• A true solution will show no separation regardless of how fast you spin it.

This method is powerful and efficient, but centrifuges are typically available only in labs. Nonetheless, it offers a clear, reliable means of distinguishing different types of mixtures.

Sedimentation

Sedimentation allows gravity to do the work for you. Let your mixture sit undisturbed for several hours or even days:

• If you observe visible particles settling at the bottom, the mixture is a suspension. The particles are large enough to be affected by gravity.
• In colloids, you won’t see particles settling like you would in a suspension. Colloidal particles are too small to settle out under the force of gravity alone.
• True solutions will show no change; the solute particles are molecular size and remain uniformly distributed.

Sedimentation is a passive process and easy to observe, requiring no special equipment other than time and patience.