Key concepts
Physics
Chemistry
Density
Liquids

Introduction
You probably know that when solid objects are placed in liquid, they can sink or float. But did you know that liquids can also sink or float? In fact, it is possible to stack different layers of liquids on top of one another. The key is that all the different layers must have different densities. You can stack them by picking several liquids with a range of densities or by varying the density of one liquid by adding chemicals such as sugar or salt to it. If you choose colored liquids or add food coloring to each layer, you can even create a whole rainbow of colors in one single glass! Want to see for yourself? In this science activity you will stack several liquids—one by one—and create a colorful density column!

Background
Whether an object sinks or floats depends on its density compared with the density of the liquid into which it is dropped. All types of matter—solids as well as liquids—are made up of many different atoms. Depending on the mass of these atoms, their size and the way they are arranged, different substances will have different densities. The density is characteristic for each individual compound and defined as the mass of a compound divided by its volume. In other words, the more matter there is in a certain amount of volume, the denser a substance is. One cubic centimeter of rock, for example, is much heavier than a cubic centimeter of wood. This is because there is much more matter in the same volume of rock compared with the wood.

Liquids can also have different densities. Freshwater, for example, has a density of about one gram per cubic centimeter at room temperature. Any compound—liquid or solid—that has a higher density than water will sink whereas substances with a lower density than that will float. You can test that for yourself by gathering several liquids that you have in your kitchen such as vegetable oil, corn syrup, dishwashing soap, water, rubbing alcohol and more. Which one of these do you think will sink or float in water? Find out in this activity!

Materials

• Glass or cup
• Water
• Food coloring
• Scissors
• Rubber band (wide)
• Small piece of wax
• Popsicle stick
• At least two small, clear jars or transparent mini cups (two ounces) with lids
• Permanent marker
• Three tablespoons
• Dark corn syrup
• Vegetable oil
• Penny
• Paper towels
• Rubbing alcohol, dishwashing soap and other liquids (optional)
• Sugar (optional)

Preparation

• Gather all your materials at a workspace that can tolerate spills of all the liquids.
• Fill a cup with tap water and add a couple of drops of food coloring to the water.
• Ask an adult to help you cut and prepare small pieces (about 0.5 by 0.5 centimeter) of the rubber band, the popsicle stick and the wax.
• Place the two empty jars in front of you (without the lids) and label them “1” and “2” with a piece of tape and the permanent marker.

Procedure

• Take a clean tablespoon and pour two tablespoons of corn syrup in jar 1.
• With the second tablespoon, carefully pour two tablespoons of colored water into jar 1 on top of the corn syrup. What happens to the water on top of the corn syrup? Does it mix or stay separate?
• Use the third clean tablespoon to pour two tablespoons of vegetable oil in jar “1” on top of the colored water. Do you see mixing of the liquids? What happens to the oil? Does it float on top or sink to the bottom?
• Now take empty jar 2 and add the same liquids—but in reverse order. Start by pouring in two tablespoons of vegetable oil.
• Next, add two tablespoons of colored water on top. What happens to the water this time? Does it stay on top or sink to the bottom?
• With a fresh tablespoon, carefully pour two tablespoons of corn syrup into jar 2, on top of the water. Does the corn syrup float on top of the other liquids? Where does it settle compared with the water and oil?
• Wait one or two minutes to let all the liquids settle in jars 1 and 2. Then look at both jars and compare how the different liquids are layered. Is there a difference between jar 1 and 2? Did you expect this result? Why do you think the layers turned out this way?
• Now take jar 1 and close it with a lid. Carefully turn it upside down and set it on the table again. Observe the different liquids. What is happening to each of the liquids? How are they layered once the jar is upside down? Did you know this would happen?
• Compare the upside-down jar 1 with jar 2 after a couple of minutes. Do you see any differences in layering now? What happened to each of the liquids?
• Get all your cut-up objects and your penny ready, then drop the penny into jar 2. What happens to the penny?
• Next, take the piece of rubber band and drop it into jar 2. With the popsicle stick push the rubber band all the way to the bottom of the jar. Make sure it does not stick to the bottom and can float freely. Then let go. Where does the rubber band settle? Does it float on top, in the middle or sit on the bottom?
• Now drop the piece of wax into jar 2. Again, push the wax all the way down into the jar but make sure it does not stick to the jar’s bottom or sides. It should be able to float freely. What happens to the wax piece? Where does it settle compared with the rubber band?
• Last, take the piece of popsicle stick and drop it into jar 2. Submerge it with a spoon or popsicle stick and wait for it to settle in one of the layers. On top of which layer does the popsicle stick piece float?
• Look at all four objects that you dropped into jar 2 and compare their locations. Did they all settle in the same or in different liquid layers? If they are in different layers, why do you think this is the case?
• Finally, take jar 1, which is still upside down, and shake it really hard. Then put it back on the table (this time, right side up). Observe what happens for about five to 10 minutes. Did any of the layers mix while you shook the jar? How does the layering look after five to 10 minutes?
• Extra: With three liquids (vegetable oil, corn syrup and colored water) there are six different combinations for adding the different layers on top of one another. Try all the combinations and change the order of adding the different liquids each time. Will you always end up with the same layering result?
• Extra: In this activity you had three layers of different liquids. Can you find other household liquids that might make a fourth or fifth layer? Some liquids to try are rubbing alcohol or dishwashing soap. Where do you think they will settle in your density column?
• Extra: You can even make a density tower using the same liquid, such as water. You just have to change its density by adding different amounts of sugar to it. Try three different layers of water—each layer with the same volume of water, but one with one teaspoon of sugar, one with two teaspoons of sugar and one with three teaspoons of sugar. Add different colors to each liquid to see the layers more clearly. Which layer is on the top and which layer on the bottom? How does the amount of sugar change the density of the water? What will happen this time if you shake the jar or turn it upside down?
• Extra: Try to find different objects that might sink or float in the different layers of your density column. In which layer will they settle?
• Extra: You can find out how much the densities of the three liquids vary by pouring the same volume of each liquid into a different cup and then weighing each of them. How much do the masses differ? Do these results agree with your findings of how the liquids are layered?

Observations and results
Did you successfully stack all three liquids on top of one another to create a colorful density column? It actually does not matter in which order you add the three different liquids into your jar; the layers will always end up being the same: The corn syrup settles on the bottom, the colored water is in the middle and the vegetable oil floats on the top. This is because corn syrup has the highest density of all the liquids, about 1.4 grams per cubic centimeter whereas the density of water is about one gram per cubic centimeter at room temperature. Vegetable oil is lighter than water with a density of about 0.9 gram per cubic centimeter and thus floats on top of the water. Even if you turn the jar upside down, the layers rearrange to the original order due to their different densities. If you did the extra activity and weighed the same volume of each liquid, you should have found that corn syrup was the heaviest, followed by water and then the vegetable oil.

The objects that you dropped into jar 2 settled in the density column depending on their own densities. The rubber band probably settled on top of the corn syrup whereas the penny fell all the way to the bottom of the jar. The wax should have rested on top of the water layer and the popsicle stick should have floated all the way on top of the vegetable oil. Finally, if you shook the jar really hard, you might have observed that the vegetable oil still separated and floated on top, but the corn syrup and the water layer started to mix and did not separate very easily anymore. This is due to the miscibility of the different liquids. If liquids have a very different chemical structure that makes one polar and the other one unpolar, they will not mix and always stay separate. This is true for oil, which is an unpolar liquid, and water, which is a polar liquid. Corn syrup, on the other hand, has the ability to mix with water and thus can dissolve in it. This is what happens when you shake the jar really hard.

You can try many more objects and liquids to create even more layers; it is just a matter of density that will determine where they settle in your density column!

Cleanup
You can pour all the liquids from the jars down the drain. Clean any spills.

More to explore
Density of Matter, from Ron Kurtus’s School for Champions
Ocean Density, from Science Learning
Can Water Float on Water?, from Science Buddies
Science Activity for All Ages!, from Science Buddies

This activity brought to you in partnership with Science Buddies