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What is a Colloid?

By Paul Scott
Updated: May 21, 2024

A colloid is the end result of dispersing one substance evenly and microscopically in another without any changes in the structure of either. Colloids are distinct from other combinations such as solutions due to the fact that none of the constituent parts of the union dissolve or experience any structural changes during dispersal. Colloids may exist in combinations of all the matter states including gaseous, liquid, and solid unions. In an emulsified state, colloid unions may consist of one or more immiscible constituents where surfactants maintain even distribution of the individual parts. Some well known examples of colloids are milk and fog.

Mixtures or unions of different materials and substances exist in a variety of forms. In most, one or more of the individual parts of the mixture undergo some sort of physical or chemical change. Unlike combinations such as solvents and alloys, colloid mixtures feature an even distribution of all constituent parts in their original, unchanged form. This means that colloids deliver the individual characteristics of all their parts and not a reactive combination of qualities. In this way, active ingredients can be suspended in inert carrier mediums without changing or negatively affecting their characteristics.

Colloid mixtures consist of two distinct parts: a dispersed phase or substance and a dispersal medium. Milk, as an example, is a colloid consisting of a liquid butterfat dispersal phase distributed in a water based dispersal medium. Colloids may consist of combinations of all matter states with common unions including liquid/air mixtures such as fog and mist and solid/air unions such as smoke or cloud masses. Other common combinations include liquid/gas mixes such as whipped cream, liquid/solid gel combinations such as gelatin, and solid/liquid dispersals which include blood and inks. Decorative cranberry glass is an example of a solid/solid colloid and is a combination of red glass and gold chloride.

The individual parts of liquid/liquid colloids are not always miscible or mixable and would gradually separate after combination if no additional additives were introduced. These additives are known as surfactants and alter the surface tension characteristics of the mixture which encourages the immiscible components to remain evenly dispersed. Milk is one of the best known examples of these unions or emulsions. Colloids may also be formulated to exhibit unstable, fractional qualities which allow them to flow when exposed to stress, and then regain cohesion once the stress is ceased. One example of this phenomenon is toothpaste, which flows when the tube is squeezed and then remains stable on the toothbrush.

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Discussion Comments
By orangey03 — On Jul 28, 2012

@feasting – My cousin actually makes yogurt from scratch, and she told me that the liquid is nothing to be afraid of. I had the same question as you at one time, and since she was the yogurt expert, I knew that she was the person to ask.

She said that yogurt is a colloidal gel. She also said that it holds liquid inside of a solid, but there is a limit to how much liquid it can hold. Once it becomes saturated, the liquid oozes out and rests on top.

So, if you see a carton of yogurt with a wet topping, it simply means the gel is too full to hold it. Don't toss it out, because it is perfectly good yogurt.

By feasting — On Jul 28, 2012

I've heard that yogurt is a colloid. I have seen it separate in the container at times.

The first time that I opened a container of yogurt and saw some water sitting on top, I was afraid to eat it. I thought this meant that it had gone bad. However, my husband simply poured the water down the drain and ate it himself, and since nothing happened to him, I'm guessing it was safe.

Does anyone know why water comes out of yogurt? Should I throw it away just to be safe, or was my husband's case not just a fluke?

By Oceana — On Jul 27, 2012

@Kristee – Yes, acrylic paint is a colloid. I learned how to make my own paint in college, and I remember learning that acrylic paint is made of bits of color suspended in water and a liquid plastic.

Oil paint is also a colloid, but instead of water and plastic, the pigments are suspended in linseed oil. Then there is egg tempera paint, where you use an egg yolk as the emulsifier and suspend it in water.

I loved learning how to make my medium from scratch. It made the paintings I did so much more meaningful, because I felt like I really had more of a hand in the whole process.

By Kristee — On Jul 26, 2012

My heavy body acrylic paint has the same colloidal quality as toothpaste. Once I squeeze it from the tube onto the brush, it retains the same thickness and doesn't drip off.

Can acrylic paint be termed a colloid? I have no idea what it is made of, but it acts like toothpaste and whipped cream.

I think I even remember a really old tube of it separating. Clear fluid ran out when I squeezed it, and it was followed by chunks of solids the shade of the paint.

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