Chemistry Experiments
You Can Do In Your Kitchen
Laura Kosbar
Lee Park
Nilda Rivera
IBM Local Education Outreach
1993
Updated: 01-18-2010
CHEMISTRY EXPERIMENTS YOU CAN DO IN YOUR KITCHEN
Introduction
Chemistry is the study of what things are made of. Sometimes it's easy to tell when something is
made up of two or more different things - you can tell what a sandwich is made of just by
looking at it, for instance. But sometimes, it's a lot harder to tell what something is made of.
Suppose someone puts 3 glasses in front of you and tells you that one has plain water in it, one
has sugar water, and one has salty water. All of the liquids would look the same, right? How
would you tell which is which? You can tell the difference by tasting the three liquids. This is a
very simple kind of chemistry experiment: you've looked at a group of things that all look alike,
and you've been able to show that they are actually all different from each other.
Chemists try to find out if things are pure (made up of only one substance). If something is not
pure, then a chemist will try to find a way to purify it, or to separate it into all the parts that
make it up. In the example given above, you can tell that the three glasses of water are different
by tasting them, but how would you go about separating out the things that are in them?
One way would be to boil off the water in each of the glasses (or just let the water evaporate
slowly). Then, you'd be left with plain sugar in one glass, salt in the second,
and nothing in the last one.
Another thing that chemists try to do is to look at a collection of items and decide which things
are alike. To do this, you have to make a list of the properties of each of the items and compare
the lists carefully. For instance, suppose someone showed you 4 glasses filled with liquids as
shown below:
•
•
•
•
Glass 1: water + red food coloring
Glass 2: water + red food coloring + sugar
Glass 3: water
Glass 4: water + sugar
How can you group these 4 glasses? You could group them by color, putting the two glasses
with red liquid together, and the two glasses with colorless liquid together. But you could also
group them by sweetness by tasting them, and deciding which two glasses had the added sugar,
and which did not. This is another kind of simple chemistry experiment - you've tried to
put things in groups and figure out the ways that things can be alike - and sometimes, there's
more than one way to look at things.
Now there are a lot of situations where tasting or noticing what something looks like won't be
enough to tell you whether or not something is pure, or whether two things are alike. So you
need to have a lot of different ways to test things. Some tests are easy and you already know
how to do them: you can notice how things look (compare a glass of water with a glass of
colored water), how they smell (compare a glass of water with a glass of vinegar), or
how they feel (compare a glass of water with a glass of soapy water). But even all of these tests
don't always tell you enough about the things that you're looking at. So chemists have developed
a lot of other ways to test things.
In the experiments that are described here, you will learn about a way to see if something is pure
or not (Paper Chromatography). You will learn about how to make two things that are different
combine to look like one (Immiscible Liquids and Emulsions). You will do some experiments to
show that things that you think of as being "pure" are sometimes really made up of several
different things (Separating Milk, Cream, and Butter). You will also learn about a test that you
can use to group things together (Purple cabbage acid/base indicator).
PAPER CHROMATOGRAPHY AND CAPILLARY ACTION
Materials and Equipment Needed:
•
•
•
•
•
Coffee filters, cut into strips about 1 inch wide and 4 inches long
A bag of M&M's
Colored Felt tip markers (water soluble)
Paper towels
Several jars or glasses
Instructions:
For the M&M's:
Place an M&M in a glass and cover with water (add just enough water to cover the M&M).
Wait a few minutes so that the dye from the shell of the M&M has a chance to dissolve in the
water. Then place a coffee filter strip in the glass so that one end of it is in the colored water and
the other end sticks out the top of the glass. Wait a few minutes and see what happens as the dye
moves up the filter paper. Remove the filter paper and let it dry (the colors will show up better).
For the colored markers:
Choose a colored marker and draw a line on a strip of filter paper about 1/2 inch from one end
(make the line pretty dark by drawing over it a few times with the marker.) Place the strip of
filter paper in a glass containing about 1/4 inch of water (you don't want to put the colored line
below the surface of the water.) Watch to see what happens as the water moves up the filter
paper. Again, the colors will show up better after the paper dries.
Discussion/ Questions/ Things to try at home:
As the water moves up the filter paper, it carries the colored dyes with it. Different colored inks
will move up the paper at different speeds. So if you use an ink or dye that is make up of two
different colors, then you will see the ink separate into the colors that make it up. What do you
get when you chromatograph red ink? blue ink? purple? brown? This method of separating
dyes into their different parts by using liquid to carry the dyes along a piece of paper is called
paper chromatography.
Red, blue and yellow are known as primary colors. All of the other colors are made up of
combinations of these primary colors. You can test this by making up your own dyes using food
coloring. If you start with red, yellow, and blue food coloring, you can you can easily make
many other colors. Try making up some other colors and chromatographing them the way that
you did for the M&M's.
In these chromatography experiments, what makes the water move up the paper? The water
moves because of something called capillary action. You can see another example of capillary
action in how plants and flowers "drink" water from the ground (or a vase, if the flowers are cut)
and get it all the way up their stems up to their leaves and petals. If you think about how tall a
tree is, that's pretty impressive! Trees and plants have tiny hollow tubes that run up the length of
their trunks and stems. These tubes are used to carry the water, by capillary action, up from the
roots to the leaves.
Try this experiment: take a stalk of celery and make a fresh cut at the bottom, and put it in a glass
of colored water. Let the celery stand in the colored water for a day or two. Now if you cut the
stalk in half, you can see colored spots where the columns that carry the water up the celery stalk
are located. You can do the same thing with carrots and scallions too - be sure to choose a color
that will show up well against the color of the vegetable.
Another experiment you can try: buy a white carnation (the full size ones work better than the
miniature ones), cut the stem to about 6 inches in length with a knife (a knife is better than
scissors, since scissors will tend to crush the stem as you cut it). Place the carnation in colored
water. Wait a few days. You should be able to see the flower on the carnation turning color as
the water is pulled up the stem. You may have seen green carnations in the stores around St.
Patrick's Day - the florists prepare them essentially the same way that is described in this
experiment. (The only difference is that they water the carnation plants with green colored water
before they cut the flowers, which works a little better.)
IMMISCIBLE LIQUIDS AND EMULSIONS
Materials and Equipment Needed:
•
•
•
•
•
•
Two small clear glass jars with a tight fitting lid
Vegetable oil (liquid)
Vinegar
Liquid food coloring
Laundry detergent or dish soap
A small flashlight
Instructions:
Fill both jars about half full with water. Add a little vinegar to one of the jars, and shake the jar
(with the top on). Add some oil to the other jar and shake that jar. Add a couple of drops of
food coloring to the surface of the oil. Let the food coloring mix, or shake the jar. Add some
detergent and shake the jar. Take the flashlight and shine it through the jar from the side.
Discussion/ Questions/ Things to try at home:
When you add vinegar to water, they will completely mix with each other to form a solution.
When you add the oil to the water, however, they do not mix. Even if you shake them up, the oil
will eventually rise back up to the top of the water. You have probably seen a thin layer of oil
floating on water puddles in the road after it rains (the thin layer of oil is what produces the
pretty colors that you sometimes see), or maybe you have seen salad dressings (like Italian
dressings) that you have to shake up before you use them because they separate into two layers.
Liquids that don't mix are called immiscible. When you added the food coloring, did it mix with
the oil or the water? The food coloring is miscible with the water (it mixes with the water), but it
is immiscible in the oil.
But sometimes you want to get liquids to mix that normally don't mix. Can you think of any
examples of this? Two examples that you have probably seen are water based (latex or acrylic)
paints, and gravies. In both of these cases, an oily substance (either the paint base or the grease
from cooked meat) is mixed with water and a substance that keeps the oily substance mixed with
the water. That substance is called an emulsifying agent. You can use emulsifying agents to
form emulsions, too.
What happened when you added the detergent to the oil and water? The detergent helps the oil
and water to stay mixed together. Soaps and detergents are long molecules where one end is
very different from the other. One end of the detergent likes to dissolve in water, and the other
end of the detergent likes to dissolve in oil. The detergent molecules surround little balls of oil,
kind of like a sugar coating on a cookie. The end of the detergent that likes the oil is on the
inside, towards the oil, and the end that likes the water is on the outside, towards the water. That
way, the water doesn't think it is mixing with the oil, but it thinks it is mixing with the detergent.
The food coloring can mix with the detergent, too, so now the whole solution should be colored.
The oil is sort of hiding inside the detergent - but we can still tell that it is there! Shine the
flashlight through the side of the jar with the water and vinegar. The light goes right through the
solution. Now shine the light through the jar with the oil, water, and detergent. This solution
should look cloudy, and you should be able to see the beam of light as it passes through the
liquid. You can see the beam of light because it is reflecting off of the little oil droplets that are
surrounded by the detergent. This type of solution - when you are mixing two things that don't
normally want to mix, is called an emulsion.
When do you use emulsions at home? You use them every time you wash the dishes! What
happens when you wash greasy dishes with soapy water? What happens if you just try to wash
the dishes in plain water? It is really hard to get grease off dishes with just water, but the soap
forms an emulsion with the grease and allows you to wash it off. This is the same reason that
you use detergents when you wash clothes, and your parents tell you to use soap when you wash
your hands. There are a lot of things in dirt that don't dissolve well in water, but will form an
emulsion with soap.
You eat and drink emulsions, too. Milk is an emulsion. If you take milk fresh from the cow and
let it sit, the fat (which is like oil) will rise to the top, just like our oil did when we added it to the
water. To make the fat stay in the milk, they homogenize the milk. This just means breaking the
fat up into really tiny droplets. The tiny droplets can stay in solution in the watery part of the
milk. Butter is an emulsion, too, and so are gravy and mayonnaise. Instead of soap, gravy uses
flour and mayonnaise uses egg yolks to form the emulsion. To form an emulsion with egg yolks,
you have to mix the yolks very fast while you are adding the oil (like with an electric mixer).
You can try the following recipe to make your own mayonnaise.
Mayonnaise Recipe (from Science Experiments You Can Eat).
Materials and Equipment:
•
•
2 egg yolks
3 tablespoons vinegar
•
•
•
•
•
1/2 teaspoon salt
1/4 teaspoon prepared mustard
1 cup salad oil
A small bowl
An electric beater or a friend with an egg beater
Procedure:
Have all the ingredients at room temperature. Cold oil does not flow as quickly as warmer oil
and cold egg yolks will not emulsify as much oil as warmer egg yolks.
Put the egg yolks, mustard, salt, and 1 teaspoon of vinegar in the bowl and beat at a medium
speed until the egg yolks are sticky. The egg yolks are now thoroughly mixed with the water in
the vinegar and are ready to receive the oil.
Add the oil drop by drop while beating constantly. If you do not have an electric beater, you can
make the emulsion by hand with a friend. One of you does the beating with a rotary egg beater
while the other adds oil.
The idea in making mayonnaise is to spread tiny droplets of oil evenly through the egg yolks.
Egg yolk coats these droplets as they form and prevents them from coming together and forming
a separate layer, so the egg yolk is acting as the emulsifying agent. If you add the oil too fast, or
too much oil at one time, the droplets will come together before they can be forced into the egg
yolks and the mayonnaise will "curdle" or separate. If this happens, you can correct the situation
with a fresh egg yolk, but add the curdled mayonnaise to the yolk rather than the oil.
You can tell when the emulsion has formed because the mixture gets thick. This usually happens
after about 1/3 of a cup of oil has been added. Once the emulsion has formed, you can add the
oil slightly faster until the full cup has been beaten into the yolks. If the mixture gets too thick,
add a teaspoon of vinegar. Beat in the remaining vinegar at the end.
Homemade mayonnaise is thick, yellow, and glistening, though it won't be as stiff as the
mayonnaise that you buy at the store. It spoils easily and should be stored in the refrigerator.
Cover it so a skin doesn't form on top.
SEPARATING CREAM (MAKING BUTTER) AND SEPARATING
BUTTER
Materials and Equipment Needed:
•
•
•
•
•
1/2 pint of heavy cream
A plastic container (that holds at least one pint) with a tight
fitting lid
A marble
A small sauce pan
Instructions:
Put the cream and the marble in the plastic container. Put the lid on and make sure there are no
leaks. Shake the container in a figure-eight motion. At first, can you hear the marble moving?
What happens after a while?
Continue shaking the cream. It will become very thick. Then, all of sudden, the butter will form.
Drain the butter from the butter milk, wash the butter with cold water, and pack down. Store in
the refrigerator.
Take a little of the butter (a few tablespoons) and put it in the sauce pan. Put the sauce pan on
the stove on low heat until the butter completely melts. Turn off the burner, but leave the pan on
the burner to stay warm for awhile (10-30 min.). When you come back to look at the butter, you
will see a foamy layer on top. Spoon this layer off. Below it you should find a clear yellow
liquid with a layer of white sediment on the bottom. Carefully pour the yellow liquid off into a
small bowl, and put it in the refrigerator. This is called "clarified" butter.
Discussion/ Questions/ Things to try at home:
Milk is not a single substance, but is actually a mixture of many different things. The primary
ingredient of milk is water. Milk also contains fat, proteins, vitamins, minerals, and salts. Just
like with the oil/water mixtures, milk is an emulsion. It is possible to separate out several of the
substances in milk, and this is done to make many of the dairy products that you eat every day.
If you could get milk fresh from a cow (that hasn't been homogenized) you would notice that
after the milk sits for awhile, it separates into two parts - the cream rises to the top and the
skim", or low-fat, milk would be on the bottom. The cream rises to the top because it has a
much higher concentration of fat (which is like oil), and as we saw in the experiment with the oil
and water, oil is lighter than water. The cream has a lot more fat in it than regular milk does, but
it still has a lot of water and even some proteins, too.
Homogenized whole milk has just as much fat in it as it did when it came out of the cow, but it
doesn't separate - why not? When they homogenize milk, they make a stable emulsion of the
milk fat in the watery part of the milk. They do this by pushing the milk through very small
screens. This causes the fat to break up into very tiny droplets. The tiny fat droplets are too
small to rise to the top on their own, and they are surrounded with protein and other things so
that they don't get together with other droplets to form bigger drops (just like the soap keeping
the oil droplets apart in the experiment with oil and water). So, the fat stays mixed in the
milk, and you don't have to shake the milk up before you put it on your cereal in the morning.
Butter is made from the milk fat in the cream layer. The process of making butter takes
advantage of certain properties of fat. Cream is a fat-in-water emulsion. In other words, the fat
droplets are held in suspension by milk protein. When you shake the cream, you forced the fat
droplets to bump into each other, and some of them start to join together. They form larger and
larger globules until they separate from the water part of the mixture. The name of this process
is coalescing. Fat can coalesce because the fat globules are more attracted to each other than they
are to the water in which they are suspended. In the end, you have solid chunks of milk fat - or
what we call butter! The liquid that is left over is what we call buttermilk.
Most of the time, though, you don't want to make butter when you buy cream. Heavy cream
(which means that it has a high fat content) is usually used to make whipped cream. Whipped
cream is made by trapping lots of small air bubbles in the cream - making sort of a cream foam.
When you whip the cream, it forces the air to mix with the cream, and the fat in the cream
surrounds the little air bubbles and stabilizes them. When you make whipped cream, you want to
start with both the cream and the bowl you are going to use chilled. Why do you think that it is
important for everything to be cold? What is butter like when you take it out of the refrigerator?
What is it like if you let it sit on the table for a couple of hours in the summer? The fat in butter
melts (turns from a solid to a liquid) near room temperature. To keep the bubbles in your
whipped cream stable, you want to have the fat cold, so that it is stiffer. If the fat starts to get
warm and melt, the bubbles will pop and your whipped cream may turn into a puddle. If you
take whipped cream and keep whipping it, you will see it start to turn more yellowish, and
finally you will get little lumps of butter in it. As you keep beating the whipped cream it will
warm up and the fat will start sticking together into bigger and bigger droplets - just like when
we shook it in the jar.
But butter doesn't just contain fat. We can see this when we heat the butter up and melt it. As
the melted butter sits, it starts to separate, too. The foam on the top of the melted butter contains
mainly whey proteins. The clear yellow liquid is the pure butter fat, and the sediment at the
bottom is a mixture of casein proteins (curds), salts, and water (butter can be up to 1/5 water). If
you have ever fried anything in butter, you have seen evidence of these things in the butter.
When you put butter in a hot pan, it will usually sizzle and spit - that is the water droplets in the
butter turning into steam. If you get the butter too hot, it will turn brown - that is the proteins
burning. When cooks want to fry foods in butter, they will often use clarified butter because it
doesn't sizzle or turn brown since the water and proteins have been removed.
SEPARATING MILK
Materials and Equipment Needed:
•
•
•
•
•
•
1/2 cup of milk
a pot
~2 teaspoons of vinegar
cheese cloth
a hot plate or stove
a mold (such as a sugar or chocolate mold)
Instructions:
Heat milk to just below boiling then remove from heat. Add two teaspoons of vinegar and allow
to cool. Strain the white curdled material with the cheese cloth. Place it in a mold and allow to
dry (several days).
Discussion/ Questions/ Things to try at home:
As we mentioned in the experiment on butter, milk is not a single substance, but is actually a
mixture of many different things. When we made butter, we were able to separate out the milk
fat. Milk also contains other things that can be separated out to form useful (and tasty) foods.
For thousands of years, people have known how to separate some of the protein out of milk - and
they made cheese!
Milk has two main types of protein - casein (which produces curds) and whey - but even Little
Miss Muffet knew that! The whey is quite soluble in water, and is difficult to separate from
milk, but the casein is easy to separate if you add an acid to the milk.
What happened when we added the vinegar (an acid) to the hot milk? Lumps started to form in
the milk. The lumps are called curds, and they are made up primarily of the casein protein,
though they may have some of the milk fat in them, too. In the experiment on oil and water, we
saw that soap would help oil dissolve in water, now we see that the vinegar helps to "undissolve"
the curds from water. When you add the vinegar to the milk, it allows the little balls of protein in
the emulsion to start sticking together, forming bigger and bigger balls until they come out of
solution as the lumpy curds.
This "curdling" of milk is used to make many things that you eat - can you name some of them?
Have you ever eaten cheese, yogurt, sour cream, or buttermilk? All of these are made from
curdled milk. But these products are not generally made by adding an acid like vinegar directly
to milk. Instead, they add a bacteria to the milk and the bacteria produces the acid. Bacteria are
living one-celled organisms that need food to grow and multiply. One variety of bacteria, called
lactobacilli, feeds on milk and is the foundation of the dairy industry.
These rod-shaped microscopic organisms are ordinarily found floating in the air in an inactive, or
"latent", state. When they land in some milk, they spring to life, using the sugar in milk (called
lactose) as food. Lactose is similar to table sugar, but it doesn't taste as sweet. Bacteria
don't get "fatter” as they take in food; instead each organism divides in half, and their numbers
increase. In the process of growing the lactobacilli give off a waste product called lactic acid.
This acid not only makes milk taste sour (as any acid would), but it causes the protein in milk to
clump together, or become a semisolid mass as in our experiment. Yogurt, sour cream and many
cheeses, called "cultured milk products" are the end result of the work of living lactobacilli.
How can you get all of those different dairy products just starting with milk and bacteria? Well,
some of it has to do with the time and temperature used to grow the bacteria in the milk. To
make yogurt, the temperature of the milk is kept around 110 F for four hours, while buttermilk is
kept at 40 F for 12-14 hours, and sour cream may be cultured for 24 hours. The bacteria cultures
grow more slowly at cooler temperatures - which is why you keep dairy products in the
refrigerator. The colder temperatures in your refrigerator keep the bacteria from continuing to
grow, and so make the dairy products last longer.
Cheese seems very different from yogurt or sour cream, but it is made from a similar process.
Bacteria alone can be used to curdle the milk, but more often it is used with rennet (which comes
from calves stomachs). Once the curds form (like they did in this experiment), the curds are
separated from the liquid portion (which contains the whey) by adding salt and pressing on the
curds, and then the pressed blocks of curds are aged. During the aging, the bacteria will still
grow slowly, and they are part of what give different cheeses their distinctive flavors. It is often
the case that the cheeses with milder flavors (like muenster and mozzarella) aren't aged very
long, while stronger flavored cheeses (like cheddar) are aged for many months, so the bacteria
have a long time to affect the flavor of the cheese. I bet you never realized how helpful some
bacteria could be!
PURPLE CABBAGE ACID/BASE INDICATOR
Materials and Equipment Needed:
•
•
•
•
•
•
A blender
A coffee filter setup
A knife
One large jar and nine small jars or glasses
Paper towels
Several spoons
For the indicator:
• A couple of leaves from a purple cabbage
• 4 cups of water (or 3 cups water, 1 cup rubbing alcohol)
Acids and Bases:
• A lemon
• An orange
• A can of colorless soda (like 7-up or sprite)
• White vinegar
• Vitamin C tablet, crushed up
• Baking Soda
• A Rolaids or Tums tablet, crushed up
• Soap or laundry detergent (a colorless liquid will work best)
• Mr. Clean or a similar cleaner
• Ammonia
Instructions:
Tear up the cabbage leaves and place them in the blender with the water. Grind everything up
very well. Filter the mixture through the coffee filter, squeezing out all of the liquid into your
largest container. Pour some of the indicator solution into each of the smaller containers. Now
add a small amount of one of the acids or bases to each of the small jars.
Discussion/ Questions/ Things to try at home:
The material in the cabbage that gives it its purple color is called an acid/base indicator, which
means that it changes color when you add an acid or a base to it. You can tell by the color
changes whether something is an acid or a base. What color do you get when you use an acid? a
base? What color do you think the cabbage turns in your stomach when you eat it?
Many other foods contain materials that will change color if you add an acid or a base. Try
making a strong cup of tea. Now add some lemon to it - watch for a color change - you have to
watch carefully because the color change is not as dramatic as with the purple cabbage indicator.
What happens when acids and bases react with one another? They neutralize each other.
Rolaids or Tums are "antacid" tablets and are made of bases (you can test this by checking the
color you get when you add a tablet to the cabbage indicator solution) and they work because
they neutralize the acid in your stomach.
You can try some experiments to see how acids and bases react together:
Try taking the wishbone from a turkey or a chicken and putting it in a glass of vinegar overnight.
It will turn rubbery, and you can even tie it in a knot. This happens because bones contain a
compound called calcium carbonate, which is a base. This base reacts with the vinegar, which is
an acid, and is removed from the bone, so the bone loses its stiffness.
Take a raw egg and place it in a jar of vinegar overnight. The shell of the egg is also made of
calcium carbonate, a base, and it reacts with the vinegar. The shell will slowly dissolve, and
you will be left with the raw egg in the vinegar.
Usually, when an acid and a base react, you get gas bubbles forming. If you look carefully at the
egg experiment described above, you will be able to see bubbles forming on the eggshell. If you
add vinegar directly to baking soda, you'll see a lot of fizzing from the gas that is produced when
this acid and base react together. This gas is called carbon dioxide.
This is why cakes and muffins rise when you bake them. If you look at a recipe, you will see
that they usually contain an acid (usually in the form of lemon juice, or soda, or buttermilk...)
and a base (baking soda). As all the ingredients are mixed together and baked, the acid and base
react, giving off a gas, which is what makes the cake rise. If you forget to add either the acid or
the base, you'll end up with a pretty flat cake! (Baking powder is a little different because it
already has both an acid and a base in it - so recipes that use baking powder don't necessarily
have to have an acid in the list of ingredients to work properly.)
MATERIALS THAT CHANGE WHEN YOU STIR THEM
Materials and Equipment Needed:
•
•
•
1 cup catsup
a small plastic container with a tight fitting lid
2 marbles
•
•
1 lb box of cornstarch
water (about 1½ cups)
Instructions:
Pour the catsup into the plastic container, and let it sit for at least 5 minutes. Drop one of the
marbles into the catsup, and time how long it takes the marble to disappear into the catsup. Place
the lid on the plastic container, and shake it well. Take the cover back off, and drop another
marble into the catsup. How long does it take for this marble to disappear?
Add water slowly to the cornstarch, while stirring the cornstarch. The cornstarch will be rather
hard to stir. Add just enough water so that all of the cornstarch is wet, and it will flow if you tip
the container, but it should still turn more solid when you stir it. Try poking hard at the
cornstarch with your finger - what happens? Try slowly pushing your finger into the cornstarch now what happens? Try to quickly pull your finger back out of the cornstarch - is it hard or easy
to do this? Use a spoon to pick a clump of the cornstarch. Roll it between your hands to make a
ball. What happens when you stop rolling the cornstarch?
Discussion/ Questions/ Things to try at home:
What happens when you stir a glass of milk or water? Not much! Most materials don't change
much when you stir them - they may get more mixed together (like when you stir chocolate mix
into milk), but other than that, the behavior of the solution doesn't seem to change much. There
are a few materials that will change when you stir them, however. Catsup and mixtures of
cornstarch and water are two examples of these kinds of materials.
Catsup is a thixotropic material - that is a big word that means that this type of material is easier
to pour after you stir it or shake it than it was before you stirred it. Have you
ever opened a new bottle of catsup and tried to pour it on your French fries, but the catsup didn't
want to come out? Next time that happens to you, try putting the cap back on the bottle, shaking
the bottle, and then try pouring the catsup - it should come out much more easily! That is similar
to what we did in this experiment. When you drop a marble into catsup that has been sitting for
awhile, the marble disappears very slowly into the catsup - the catsup seems very thick, and the
marble has a hard time pushing through it. But then, after we shook the catsup up, the marble
disappeared into the catsup quite quickly - it was as if the catsup was thinner (just like the catsup
pouring onto your French fries more easily after you shake it up). Why does this happen?
Well, there are a couple of different ways that thixotropic materials may work. Some of these
materials are made up of big branched molecules. One way to think of them is like a lot of
bushes that are growing fairly close together. The branches of the bushes touch each other. If
you tried to walk through the bushes, it would be pretty hard to do, since you would always be
pushing the branches back. Now imagine the bushes if a strong wind were blowing through
them - the wind would bend the branches closer to the trunk of the bush, and there would be
room for you to easily walk through. When a thixotropic material sits around, the molecules
stretch out and touch each other like the branches of the bushes, but when you stir them, the
branches pull in, so the molecules can go past each other and the mixture can pour more easily.
Other thixotropic materials are made up of emulsions or colloids. Emulsions have little droplets
of one liquid (say oil) mixed up in another liquid (like water), and colloids have very small
solid particles (like flour) mixed in a liquid. The particles or droplets can usually pass by each
other, and the emulsion will pour easily. But sometimes the particle or droplets can stick
together - kind of like balloons when they have lots of static electricity on them - if they get
close together they will stick to each other, but they are still very easy to pull apart again (if you
have never tried this – blow up two balloons, tie the ends, then rub them a lot on your clothes
or hair and bring them close together to see what happens). So, you can think of a thixotropic
solution as a bunch of balloons with static electricity on them in a big container. All of the
balloons will stick to the other balloons that are close to them. If you tipped the container, the
balloons would all try to come out stuck together in one big chunk. But, if we mixed up the
balloons first, so that they weren't sticking together any more, and then tipped the container, the
balloons could flow out one at a time. This is exactly what happened when we stirred the
catsup - we mixed up the droplets so that they weren't sticking together any more and then the
catsup could flow. What happens if you let the catsup sit on the counter for 5-10 min. after you
stir it? All of the droplets have time to start sticking together again, and the catsup won't flow as
well again!
A few materials act in an exactly opposite way. Instead of flowing better when you stir them,
they get more solid when they are stirred. These are called dilatant materials. The cornstarch
and water react this way. That is why when you poke at it, it seems to get hard and you can't get
your finger to go into it, but if you gently push your finger into it, it slides right in. Once your
finger is in the cornstarch, though, if you try to pull it out quickly, it may seem like you are
pulling it out of partly dried cement! If you pull it out slowly, it will just seem like you are
pulling it out of a thick liquid. When you pick up a ball of the cornstarch, it will act like a solid
as long as you keep the ball moving, but as soon as you stop putting pressure on the ball, it
"melts" into a liquid again.
Can you think of any other materials that reacts this way? What happens when you walk on wet
sand? What happens if you pick up some wet sand and try to pour it from your hand? The wet
sand will sort of pour from your hand like a very thick liquid, but if you step on it, it acts like a
solid and you don't sink in! In the case of the sand and the cornstarch, the particles that are
mixed in the water are not attracted to each other at all - kind of like marbles in a big jar. If you
pour a lot of marbles into a big jar and jiggle the jar a little, the marbles will fit very close
together, with only small spaces between them. If you added water to the jar it would fit into the
small spaces between the marbles. Now imagine adding just enough water to cover the marbles.
What would happen if you stirred the marbles and water up?
When you stir up the marbles, they would get all mixed up, and wouldn't end up fitting as closely
together as they did at first. There would be bigger holes left between the marbles for the water
to fit into. This would make it look like the water level went down, and that the top of the
marbles got dry (just like you see with wet sand or with the cornstarch). As long as you keep the
marbles moving, they can't fit together as well, and there will be more room for the water
on the inside, so that the outside will stay "dry". But, what happens when you stop stirring
them? Then the marbles can fit back into the spaces, and be very close together again. The
water will get pushed out of the spaces, and you would see it covering the marbles again, so now
they would look "wet" again.
This is what happens with the cornstarch. As long as you are not stirring or moving it, more of
the water is on the outside, which allows the particles on the outside to flow with the water like a
liquid. When you are stirring the cornstarch, or rolling it in your hands, you are creating more
open holes in the middle for the water to go into, so the outside seems dry, and the cornstarch
acts like a solid. As soon as you stop stirring it, the cornstarch falls back into the open holes and
pushes the water to the outside, so the cornstarch acts like a liquid again.
REFERENCES USED FOR THE KITCHEN CHEMISTRY EXPERIMENTS
Vicki Cobb, Science Experiments You Can Eat, Harper Trophy, 1972, pp. 35-37.
Vicki Cobb, More Science Experiments You Can Eat, Harper Trophy, 1979, pp. 23, 54-55.
Harold McGee, On Food and Cooking,. Collier Books, NY, 1984, chapter 1.
Bassam Z. Shakhashiri, Chemical Demonstrations, Vol. 3, University of Wisconsin Press,
Madison, WI, 1989, pp. 364-367.