Key concepts
Matter
Colloidal solutions
Physical properties
States of matter

Introduction
Have you ever wondered what whipped cream, jelly, and milk have in common? Aside from all being tasty, they are also all made up of tiny, solid particles that are dispersed, or distributed, in water. This type of mixture is called a colloidal solution. Colloidal solutions have some very interesting physical properties, such as acting like a solid and a liquid at the same time! In this activity you'll get to create a colloidal solution that's made using cornstarch and water, and then explore these properties firsthand.

Background
Colloidal solutions (also called colloidal suspensions) contain little particles, ranging from one to 1,000 nanometers in diameter. (A nanometer is very small—a human hair is about 100,000 nanometers wide!) In this activity the particles used are cornstarch, and they're evenly dispersed throughout a quantity of water. These particles are so small that you can't see them with the naked eye—and when you look at the colloidal solution it appears homogenous, or uniform. If you put it under a microscope, however, it would look heterogeneous, or as if things had been mixed together.

Having the right particle size is essential for making a colloidal solution. If smaller particles are used, they will dissolve in the water and not be visible, even with a microscope. For example, think of how sugar dissolves in water, making a homogenous solution. On the other hand, if larger particles are used, such as grains of sand, they will not dissolve in the water. Instead, they separate from the water, and are so large they form a heterogeneous mixture of particles that can be seen with the naked eye. Particles of cornstarch are just the right size (about 100 to 800 nanometers in diameter) to make a colloidal solution with water.

Materials
• Small bowl or cup
• Cornstarch
• Cup, mug or drinking glass
• Water
• Medicine dropper
• Fork

Preparation
• Add one tablespoon of the cornstarch to the small bowl or cup.
• Fill the empty cup, mug or drinking glass with water.

Procedure
• Use the medicine dropper to add water from the cup to the small bowl that holds the cornstarch. Add the water one drop at a time, counting as you go. What happens as the water touches the cornstarch?
• After you have added 20 drops, stir the cornstarch with the fork. Break up any clumps that formed.
• Keep adding drops one at a time, stirring with the fork after every 20 drops. What happens as you add more water?
• Once you have added 100 drops of water total, stop for a moment and observe the cornstarch. How does it look? How has it changed?
• Continue adding drops, but now mix the cornstarch after every 10 drops.
• Stop adding water when all of the cornstarch has flowed together like a liquid. It will probably take around 150 to 170 drops of water total. How many drops did it take for you? How would you describe the solution’s appearance?
• Play around with the solution in the bowl. Poke it with your finger and put some on your fork or in your hand. How does it move? What does it feel like? What do you think this tells you about colloidal solutions and their physical properties? Tip: If it seems to get chalky while you're investigating it, try adding and mixing in a few more drops of water.
Extra: Colloidal solutions can be made out of other common products aside from cornstarch, such as other starches (for example, potato, tapioca and rice starches) as well as gelatin. Try making colloidal solutions out of other substances. How do these compare with the colloidal solution made using cornstarch?
Extra: Starches are often used to make gels. Try heating the solution and see what happens. How does it change? Are there new physical properties that you can observe?
Extra: Clay soil behaves like a colloidal material when it has just the right amount of water in it. You could try making a colloidal clay soil solution and test the effect of different forces on it. Colloid solutions can appear solid against strong downward forces, but weak against lateral forces that push sideways. In which direction is your colloid solution the weakest? If your sample were clay soil in the real world, how could this contribute to landslides or earthquakes?

Observations and results
Did the cornstarch solution resemble a sluggish white liquid? Did it become a hard solid when you pressed on it, but turn into a liquid when it was allowed to freely move?

Colloidal solutions can act like both a solid and a liquid. The colloidal solution you made does not behave like a typical solid or a typical liquid. Phases of matter such as liquid, gas, solid and plasma are physical properties of matter. The cornstarch solution is a non-Newtonian fluid, which means it does not behave like a Newtonian fluid. When most liquids experience a sideways shearing force, such as by pushing your finger down against them, they move out of the way. The liquids also have a proportionate response, meaning if you push harder, they move out of the way faster. The liquids' viscosity, or resistance to flow, doesn't change. Water, and most fluids, are Newtonian and behave in this way. Because the cornstarch solution is non-Newtonian, however, you should have seen that it does not behave like this. When you put stress on it, such as by pressing down on it or poking it with the fork, it responded by becoming harder, changing to a solid state. When you did not put stress on it, for example by simply holding it in your hand, it again changed its state, this time becoming a runny liquid.

More to explore
Colloids, from University of California, Davis, ChemWiki
Outrageous Ooze, from Exploratorium
Non-Newtonian Fluid, from Discovery Communications
Particle Sizes, from The Engineering ToolBox
Making Mixtures: How Do Colloids Size Up?, from Science Buddies

This activity brought to you in partnership with Science Buddies