Making Mixtures: How Do Colloids Size Up?
AbstractHave you ever wondered why some things disappear when they are put in water but other things do not? For example, you may have seen that salt disappears, or dissolves, when it is mixed in a glass of water. But when you throw a rock in a stream it will not usually dissolve, and instead it will just sink to the bottom. And then there are some things that do not act like the salt or the rock. These are called colloids. If you have made Oobleck out of cornstarch and water, then you have seen that when a colloid (the cornstarch) is mixed with water it acts like a solid and a liquid at the same time! For this science project get ready to make some different types of fascinating mixtures and identify each type of mixture based on its appearance and behavior.
Make mixtures of sand, sugar, and cornstarch with water and determine whether each mixture is a true solution, a colloidal solution, or a suspension.
When a spoonful of salt is mixed with water, it behaves very differently than when a handful of gravel is mixed with water. You can probably predict that the salt would disappear, or dissolve, in the water while the gravel would just sit in the water. Each piece of gravel would stay the same size and shape. Why does this happen?
It has to do with the size of the salt compared to the size of the gravel. The salt is made up of many tiny particles. Because the particles are so small, they dissolve in water. When they do this, you can no longer see them. If particles of something are dissolved in water, the mixture is called a solution. A piece of gravel is much larger than a salt particle and is too large to dissolve in water. When particles are so big that they settle out, or separate, from water like this, the mixture is called a suspension. In a suspension, you can easily see the particles with your eye, and it is not hard to physically separate them from the water. For example, think of how easy it would be to pick a piece of gravel out of water compared to how impossible it would be to pinch some salt dissolved in water.
Solutions can be further divided into two groups: true solutions and colloidal solutions. In true solutions, the dissolved particles are so tiny that they cannot be seen with a microscope! When salt is dissolved in water it creates a true solution. The particles in colloidal solutions, however, are a little bigger, big enough that they can be seen using some microscopes. But they are not as big as the particles that are in suspensions.
This brings us to a riddle: What do ketchup, Oobleck, and quicksand have in common? They are all colloidal solutions. Colloidal solutions have very interesting physical properties. This is mostly because they are non-Newtonian fluids. Non-Newtonian fluids break the rules of ideal fluids described by Isaac Newton in the 1700's. To understand this, we have to first understand how ideal fluids behave. Ideal fluids, or Newtonian fluids, move out of the way when you exert force against them, such as by sticking your finger in them or stirring them with a fork. If you press against them slowly, they will move out of the way slowly, and if you press against them more forcefully, they will move out of the way faster. Water, and most fluids, are Newtonian fluids and behave this way. However, non-Newtonian fluids, like colloidal solutions, do not behave this way. A fast, hard force will cause a colloidal solution to appear solid, but a slow, even force will cause the colloidal material to flow like a liquid. Phases, such as liquid, gas, and solid, are physical properties of matter.
As it turns out, colloidal solutions are very common. Even though they have such strange physical properties, those same properties make them very useful products and materials. Foam, gel, glue, and clay are all examples of colloidal solutions. There are many colloidal solutions found in food products, like: marshmallows, mayonnaise, pudding, milk, butter, and jelly. Building materials like cement, stucco, plaster, and paint are colloidal solutions. Even our bodies and other living organisms are made of colloidal solutions! They are everywhere!
In this materials science project, you will make a true solution, a colloidal solution, and a suspension and you will use you knowledge of these types of mixtures to identify which is which. Specifically, you will mix cornstarch, sand, or sugar, as shown in Figure 1, each with water. You already know that when you mix cornstarch with water, you can create a colloidal solution. But what kind of mixture do you make when you mix sugar with water? What about sand with water? Get ready to make some mixtures and find out!
Terms and Concepts
- True solution
- Colloidal solution
- Non-Newtonian fluids
- Phases of matter
- Physical properties
- What is a solution?
- How is a true solution different from a colloidal solution?
- What is a suspension?
- What is a non-Newtonian fluid?
Do further research by visiting the following websites, which give information on Oobleck (also called Ooze), types of mixtures, and non-Newtonian fluids:
- Exploratorium. (1998). Outrageous Ooze. Retrieved February 22, 2013.
- Math and Science Activity Center. (n.d.). Types of Mixtures - Solutions, Suspensions, and Colloids. EdInformatics.com. Retrieved February 22, 2013.
Materials and Equipment
- Cup, mug, or drinking glass
- Small bowls (3)
- Measuring spoon
- Medicine dropper
- Forks (3)
- Optional: Camera
- Lab notebook
In this science project you will make three different mixtures. One will be a true solution, one will be a colloidal solution, and one will be a suspension. After making some observations you will be able to figure out which mixture is which.
- In your lab notebook, create a data table like Table 1. You will be recording your observations about the mixtures in this data table.
|Mixture||Did particles dissolve?
|Would it be easy to separate the particles from the water?
|When you mixed it, did it ever become a solid, and then turn back into a liquid when you stopped mixing?
|Do you think it is a true solution, a colloidal solution, or a suspension?|
|Water and cornstarch|
|Water and sand|
|Water and sugar|
Table 1. In your lab notebook, make a data table like this one to record your observations in.
- Fill an empty cup, mug, or drinking glass with water.
- To one small bowl, add 1 tablespoon (tbsp.) of cornstarch.
- Use the medicine dropper to add water from the cup to the small bowl with the cornstarch. Add the water one drop at a time, counting as you go, trying to sprinkle it evenly across the cornstarch.
- After you have added 20 drops, stir the cornstarch with a fork. Break up any clumps that formed.
- Repeat steps 4 to 5 until you have added 100 drops of water total.
- After you have added 100 drops total, repeat steps 4-5 until you reach 200 drops total, but now stir the mixture after every 10 drops instead of every 20 drops.
- After adding 200 drops total, look at your mixture and answer the questions in the data table in your lab notebook.
- If you want, you can wait until you have made all three mixtures before answering the questions in your data table. Comparing the mixtures to each other may make it easier for you to answer the questions and determine which mixture is which.
- To determine whether particles have dissolved, see if it looks like there are fewer particles visible than there were at the beginning or you can no longer make out individual particles in the mixture.
- Tip: If too much water is added to a colloidal solution, it may not behave like a colloidal solution anymore. To determine if the mixture is a colloidal solution, think about whether it ever behaved like one while you were adding water and mixing it.
- Hint: If you are having trouble determining whether a mixture is a true solution, a colloidal solution, or a suspension based on your other answers, try re-reading the Introduction in the Background tab.
- To a new small bowl, add 1 tbsp. of sugar.
- Repeat steps 4-8 with the bowl of sugar.
- Be sure to answer the questions in the data table in your lab notebook about this mixture.
- To a third small bowl, add 1 tbsp. of sand.
- Repeat steps 4-8 with the bowl of sand.
- Be sure to answer the questions in the data table about this mixture.
- When you are done making all three mixtures, compare each of them. Make sure to answer all of the questions in the data table in your lab notebook.
- Which mixture do you think is the true solution? Which mixture is the colloidal solution? Which one is the suspension? Why do you think this is?
- Hint: Try to use your answers from the other questions in the data table to answer the question of which mixture is mixture. If you are having trouble doing this, try re-reading the Introduction in the Background tab.
- What do you think this tells you about the size of cornstarch, sand, and sugar particles? Hint: Try re-reading the Introduction to find out.
Ask an Expert
- Experiment with the different properties of colloidal solutions. Are they solid or liquid? Do they respond differently to different kinds of pressure? Push slowly into a colloidal solution with your finger. How does it respond? Poke it hard with your finger. Does it respond differently? You can make other colloidal solutions using different kinds of starches (like potato, rice, or corn starch) or using gelatin and investigate whether different colloidal solutions respond similarly and have similar properties. Can you find other ways of testing the physical properties of colloidal solutions?
- Starches are often used to make gels. What happens if you increase the temperature of your colloidal material? Try heating up the colloidal starch solutions. How do they change? Are there new physical properties that you can observe? Do starches from different plant sources gel at the same temperature and have the same consistency? For more on gels, try the Science Buddies project Are You Gellin'?®
- Clay earth behaves like a colloidal solution when it has just the right amount of water in it. Can you find the critical amount of water needed to make a colloidal clay soil solution? Colloidal solutions can appear solid against strong downward forces, but are very weak against lateral forces that push sideways. Try testing your colloidal solution against these two different directional forces. Which direction is your colloidal solution the weakest? If your colloidal solution were a clay soil, how would this contribute to landslides or earthquakes? Can you engineer a way to make colloidal soils resistant to lateral forces during earthquakes and landslides?
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