Materials
a glass jar or clear drinking glass
vegetable oil
salt
water
food coloring
Content Provider
© The Exploratorium
www.exploratorium.edu
Student Procedure
Note: Tell students to make sure they are careful with the glass.
- Pour about 3 inches of water into the jar.
- Pour about 1/3 cup of vegetable oil into the jar.
- When everything settles, is the oil on top of the water or underneath it?
- Using your answer from the above question, explain why this happens.
- Add one to two drops of food coloring to the jar.
- What happens?
- Is the drop in the oil or the water?
- Does the color spread?
- Shake salt on top of the oil while you count slowly to five. WOW!
- What happens to the food coloring?
- What happens to the salt?
- You can continue to add salt to keep the action going as long as you want.
- After this experiment, give an explanation using the term density and explain why this happened.
Background Information
Lava Lites are lamps that were invented by an English man named Craven Walker in 1964. They are basically tall thin glass jars filled with liquid and a special kind of colored wax, set on top of a base with a light bulb. When the bulb is turned on, the lamp glows, the liquid heats up, and the wax begins to melt. Blobs of wax rise to the top of the lamp, then cool and sink back down--over and over again.
How a Lava Lite Works
Like the oil and water, the “lava” in a Lava Lite doesn’t mix with the liquid that surrounds it. When it’s cool, the “lava” is a little bit denser than the liquid surrounding it. When the “lava” rest on the bottom of the lava lite, the light bulb in the lamp warms it up. As it warms it up, the “lava” expands a little. When it expands, the “lava” stays the same weight but it takes up more space – so it is less dense than the surrounding liquid, and so it rises up to the top to float. The top is cooler, so the “lava” will cool, become denser, and sink to the bottom.
Description
This activity can be used during a unit on density. Students will mix together water and oil in a container along with some food coloring to see how a lava lamp works. This can also be made by a teacher during solids, liquids, and gases unit to show plasma. Use this as a mini-lab for the students and they will be able to take something home to show their parents!
Explanations for Students
Why does the oil float on the water?
Oil floats on water because a drop of oil is lighter than a drop of water the same size. Another way of saying this is to say that water is denser than oil. Density is the quantity of something per unit measure, especially per unit length, area, or volume. Things that are less dense than water will float in water. Things that are denser than water will sink.
Even though oil and water are both liquids, they are what chemists call immiscible liquids. That's a fancy word that means they don't mix.
What happens when I pour salt on the oil?
Salt is heavier than water, so when you pour salt on the oil, it sinks to the bottom of the mixture, carrying a blob of oil with it. In the water, the salt starts to dissolve. As it dissolves, the salt releases the oil, which floats back up to the top of the water.
This looks like a Lava Lite. How does a Lava Lite work?
Like your oil and water, the "lava" in a Lava Lite doesn't mix with the liquid that surrounds it. When it's cool, the "lava" is a little bit denser than the liquid surrounding it. When the "lava" rests on the bottom of the Lava Lite, the light bulb in the lamp warms it up. As it warms up, the "lava" expands a little. When it expands, the "lava" stays the same weight but it takes up more space-so it's less dense. When it's warm enough, the "lava" is less dense than the surrounding liquid, and so it rises up to the top to float. At the top of the lamp, it cools down, becomes denser, and sinks once again. This cycle repeats over and over as the "lava" warms up and rises, then cools down and sinks.
Assessment
Make sure that the students answer the questions on the lab as they go along. This will be the students assessment to make sure that they are doing the lab correctly and that they understand the concept of density.
Additional Credits
Exploratorium Teacher-in-Residence Eric Muller created this activity while playing with his food in a Chinese restaurant.
