Question: Can you make water float on oil?
KIDS’ LAB LESSONS
In this experiment, you will first demonstrate how it is possible to make water “float” on oil by cooling it to the point that it freezes. You see, when water freezes, it expands and becomes less dense. This change is usually enough to make the density of ice less than that of the cooking oil. You will pour cooking oil and colored water into a jar and allow them to settle until one floats on top of the other. Then you will place the jar in the freezer for several hours. What you see when you remove the jar from the freezer may surprise you.
Your next task will be to test other liquid combinations to see if freezing them changes their density enough to make “heavy” liquids float.
The factor that determines whether or not something will float is its density. For example, a piece of wood floats in a lake because the wood's density is less than that of water. However, if you were to drop a large boulder into the lake, it sinks quickly to the bottom. The boulder's density is greater than that of water.
If you were to pour some cooking oil into a jar containing water, the oil wouldn't mix with the water. It would appear to “float” or sit on top of the water. This is because the density of cooking oil is less than that of water. This is also why you tend to see rainbows of colors where oil or gasoline has spilled on wet pavement. The oil stays on top of the water because of its lower density.
When you freeze the water, however, its density decreases to the point that the ice will actually float on top of the oil, as if it were an ice cube in a very large drink.
Large, clear, canning jar without a lid
1 cup of tap water
1 cup of cooking oil
Other liquids for testing, such as:
1. Pour one cup of cooking oil into the canning jar.
2. Mix a few drops of food coloring into one cup of water until the color is easy to observe.
3. Slowly pour the colored water into the jar containing oil. Watch carefully to see how the water and oil interact. Do they mix, or do they stay separate?
4. Take note of which liquid “floats” on top of the other.
5. After clearing a spot in the freezer, place the jar in the freezer for several hours. Resist the temptation to sneak a peek while you wait.
6. When sufficient time has passed, remove the jar from the freezer and observe any changes you see.
Questions for the Scientist
1. Why don't you think the oil and water mixed when you placed them in the jar?
2. Which liquid appeared to have the lowest density? How could you tell?
3. What did you observe about your liquids when you removed the jar from the freezer? Did they look the same as when you placed them in the freezer?
4. What do you predict will happen as the ice begins to melt? Let the jar sit in the room for a period of time and observe the changes in the two liquids as the ice melts. Describe what you observe.
5. Repeat this experiment with other pairs of liquids. Think about some of these questions as you perform your experiment:
a. Which pairs of liquids mixed together and which ones did not?
b. What does this tell you about the densities of these pairs of liquids?
c. Did you find any other liquids that changed positions like the water did?
d. What characteristics do those liquids share that makes them do this?