All About Mixtures: Sweet Solutions, Silty Suspensions & Creamy Colloids

Mix a spoonful of sugar into a glass of water and you can’t see the sugar, because it goes into solution. But if you stir cornstarch into water, no matter how much you stir the mixture remains cloudy. Cornstarch forms a suspension in water that settles out over time. However, if you heat the cornstarch-water mixture (or better yet, heat a mixture of cornstarch, milk, sugar, cocoa and vanilla), you end up with a creamy (and, in the latter case, yummy!) colloid.

In the following experiments kids can make, play with and eat three kinds of mixtures: solutions, suspensions and colloids.

Ages 6 and up; Download Printable PDF

Experiment 1: Sweet Solution, Silty Suspension

You will need:
Two clear glasses
Measuring cup
1 Tsp Measure
spoon
sugar
cornstarch
printer paper
funnel
bottle



Instructions
Step 1 Fill each of two glasses with 1 cup of hot tap water. Add 2 teaspoons of sugar to one glass and 2 teaspoons of cornstarch to the other and stir (hot tap water works better than cold water, because more sugar dissolves in warm water; if the sugar doesn’t dissolve, heat it in the microwave for a bit.) Take a look at your two mixtures: does the sugar mixture look different than the cornstarch mixture? Does the sugar water taste sweet?

Step 2 Fold a piece of paper into a cone shape, place it in the funnel and put the funnel in the bottle. Pour the sugar water through the paper filter, and let it drain into the bottle. Look at the paper. Can you see any sugar on it? Taste the filtred water. Does it taste sweet? Make a new cone-shaped filter, and repeat with the cornstarch mixture. Can you see any cornstarch on the paper filter? What about the liquid in the bottle- does it look cloudy or clear?


Notes: (1) Printer paper seems to work better than coffee filters as coffee filters are too porous and the cornstarch seeps through. (2) It takes a while for the liquid to drain; while you're waiting you can go on to experiment 2.

What’s Happening? 
Sugar water is a special kind of mixture called a solution.  The solid in a solution interacts very strongly with the liquid. Solutions are clear, and cannot be separated by a filter. Cornstarch forms a suspension in water. The solid in a suspension doesn't interact very strongly with the liquid, and can be separated from it by a filter. Suspensions typically look cloudy.

Experiment 2: Fun with Gooblek

You Will Need:
cornstarch
food coloring
measuring cups
a bowl
a spoon
newspapers to cover table



Instructions
To make your Gooblek, mix together 1/2 cup cornstarch and 1/3 cup water in a bowl and a couple of drops of food coloring.  Now, play with your goo. If you try to stir it quickly what happens? What happens if you stir slowly? Note: Be careful- Gooblek can clog the drains!

What’s Happening?
If you stir the Gooblek quickly it seems like a solid because the grains of cornstarch get crammed together and can’t move.  If you stir slowly, however, water has time to ooze between the grains of cornstarch so they are free to move and the Gooblek seems liquidy.

Experiment 3: Make a Yummy, Edible Colloid

You will Need:
instant pudding mix
milk
mixing bowl
whisk
measuring cup
spoons and bowls



Instructions
Make instant pudding according to package directions. Now, scoop your pudding with a spoon. How does it behave?  Take a bite. What kind of texture does it have?

What’s happening?
The ingredient that causes pudding to thicken, modified food starch, is very similar to the solid ingredient in Gooblek, cornstarch. So why is one smooth a creamy while the other is course and gritty? Cornstarch is made up of long, stringy molecules called starch. In the Gooblek suspension, the starch molecules form large clumps that can’t interact very well with the water, and settle out over time.
Modified food starch is cornstarch that has been treated so that the starch molecules interact more easily with water. 

In instant pudding, the interaction of the starch molecules with the water tends to immobilize the water, causing the mixture to thicken. With heating, unmodified cornstarch can be made to interact directly with water. Flour also contains starch, and both cornstarch and flour can be used to thicken sauces with heating.

Did you know?
There are four different kinds of colloids:
A gel is a colloid of a solid in a liquid; pudding is a gel of starch and water.
An aerosol is a colloid of a liquid in a gas; fog is an aerosol of water in air.
A foam is a colloid of a gas in a liquid; whipped cream is a foam of air in cream.
An emulsion is a colloid of liquid in a liquid; mayonnaise is an emulsion of egg yolk in oil.