Coffee: A Solution, Colloid, or an Emulsion?

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Have you ever thought about why you have to throw away coffee beans after brewing them? Have you ever wondered where instant coffee goes when it gets mixed with hot water? Well, coffee is a solution (a homogeneous mixture involving a solute and a solvent), but it could also be what’s called a colloid.

Colloids are a type of solution that differ in a number of ways. The main difference is that solutions don’t reflect light (in most cases), and can pass through semipermeable membranes, and other neat stuff.

Coffee is an interesting drink when you try to classify it as a solution, a mixture, or even something else. Coffee can go from a solution to a heterogeneous mixture just by adding cream (yeah, it’s weird). Let’s look at what a colloid is exactly and how coffee fits into this category.

What is a colloid?

A colloid is a type of solution, or mixture, where the substances are regularly suspended in a fluid. The particles in a solution are fully dissolved and are mixed, in a colloid, they’re not. In fact, the particles in a colloid range from 1 nm to around 1000 nm.

Colloids can come in multiple forms, but here are the four main forms of a colloid:

  • Sol- This colloid has solid particles suspended in a liquid in a uniform manner
  • Emulsion- A colloid between two liquids
  • Foam- This happens when many gas particles are trapped in a liquid
  • Aerosol- This is a colloid where small liquid or solid particles are dispersed in a gas

As you can probably tell, colloids can be in various states of matter. They can be between a solid and liquid, two liquids, a gas in a liquid, etc.

The substances that make up a colloid can also be separated by passing the mixture through a semipermeable membrane. The solid or larger particles can’t pass through a semipermeable membrane but the liquid or smaller ones can. You can also use the Tyndall effect to see if a mixture is a colloid.

Examples of colloids

You may have used a colloid and not even realized it. Here are some examples of colloids that you might have used before:

  • Mayonnaise
  • Hairspray
  • Milk
  • Medical dressings
  • Fog
  • Smoke
  • Toothpaste

The above examples should cover all forms of matter and their combinations. Fog is a colloid composed of air and water particles. Milk is a colloidal mixture of butterfat and a water-based solution. So, how can you tell if something is actually a colloid or not?

The Tyndall Effect

This is a very simple test that can determine if a solution is a colloid or not. First, you have to shine a light on the solution. Now, if the light passes through, it’s most likely a solution, if the light reflects in multiple directions, then it’s a colloid.

Now think about this—when was the last time you saw light pass through coffee? Probably not, but why does light react in a colloid in this way? It’s simple, light gets scattered by the particles that are dispersed in the colloid. In a solution, this doesn’t happen.

How is coffee a colloid?

This is pretty simple—coffee is a colloid because the particles in a cup of black coffee fall within 1 nm to 1000 nm and are uniformly dispersed in liquid. The trouble with classifying coffee between a solution and a colloid comes from technicalities. The tyndall effect on black coffee shows that it’s a colloid, but what type of colloid?

Could coffee be an emulsion or a sol?

We know that coffee isn’t a gas, but its two separate parts are a solid (the actual coffee beans) and a liquid (the hot water) in its most basic form (black coffee). An emulsion, as stated before, is a colloid between two liquids. This could be true.

When you brew coffee, small particles of oil are released in the brewing process and are small enough to pass through the paper filter. This water and oil mixture is a form of an emulsion called a water/oil emulsion.

When you start to add things like milk foam (a foam colloid), or milk (another emulsion), classifying coffee gets more complicated. So, could coffee be a sol as well?

Coffee as a sol colloid?

Coffee isn’t a sol because, even if you do add a solid to it like ice cubes, it doesn’t fit the definition of a sol colloid. Remember, a colloid has particles that range in size from 1 nm to about 1000 nm. Ice cubes are way bigger than that!

 Adding solids to coffee would make it a heterogenous mixture instead of a colloid. Now, does coffee stay as a colloid? No, not really, you can change the mixture type of coffee in various ways.

Can you change coffee from a colloid to something else?

Yes, absolutely! When you start adding things like milk foam, ice, creamer, etc, then this changes the mixture. You end up with a heterogeneous mixture made up of multiple colloids, pure substances, or other mixtures.

Think of your favorite coffee drinks like espressos, lattes, iced coffees, etc. They all start out as black coffee but change as soon as you add something. It can be a bit confusing but once you understand how mixtures (including solutions and colloids) work, then it all makes sense.

Summary

Coffee is a magical drink. It helps us to stay awake during those difficult parts of the day, helps us start out the morning just right, and has a great culture around it. When it comes to classifying coffee, then it gets a bit complicated. What we do know is that coffee is a solution that is classified as a colloid.

We know that a colloid is defined as particles, ranging from 1 nm to 1000 nm, is dispersed in a solution in a uniform manner. Colloids come in all states of matter and are found in everyday household items, like hair spray.

Coffee is an emulsion colloid in its most basic form. This is because coffee contains coffee oils suspended in water. So go ahead and enjoy your delicious cup of emulsion and caffeine bliss!