Edward Craven Walker, best known as the inventor of the lava lamp, passed away recently. He lived long enough to see his 1963 hit product make a comeback in recent years. What are the science principles at work in a lava lamp?

"It's like the cycle of life. It grows, breaks up, falls down, and then starts all over again." This is how Edward Craven Walker described his invention, the lava lamp. Walker first began manufacturing the lamps in 1963. By the late 1960s they were a popular item for college dorm rooms, doctors' offices, and hip living rooms. The lamps invite daydreaming, as the viewer watches the globs of "lava" float slowly through the liquid lamp, creating ever-changing shapes.

Lava lamps achieve this effect by using two liquids and a heat source.

• The liquids must be very close in density, yet insoluble in one another. For example, water and cola are similar in density (one liter of each weighs approximately the same), but they are not insoluble in one another—they mix together into a solution when put in the same container. Oil and water are insoluble in one another, but they have different densities, since a liter of water weighs considerably more than a liter of oil. The liquid contents of official lava lamps are a patented, well-guarded secret.
  
  
• Heat must be applied to the bottom of the liquid mixture. In a lava lamp, the heat is supplied from a light bulb in the lamp base. Let's call the two liquids in the lamp A and B. At room temperature, A is just slightly more dense than B, so A sinks to the bottom of the container. When the light bulb is turned on, liquid A absorbs the heat, causing A to expand. As it expands, it becomes less dense. Heated A is now lighter than B, causing A to rise. As it rises away from the light bulb, it cools, making it become denser again. When A reaches the point that it is again heavier than B, it sinks. The liquids move very slowly because the heat absorption and dissipation are slow processes, and the density changes involved are extremely small.

Walker died last month at the age of 82. Students can read the CNN.com article, "Lava lamp inventor dies in London."

Teaching the Problem

There are several important chemistry principles at work in a lava lamp. It is recommended that students have learned about states of matter before continuing with this problem. (All SimLibrary activities mentioned in this section require Logal Express. Get a free trial subscription.)

1. Density is defined as mass per unit of volume: D = m/V. Changes in temperature change the volume of a substance. An increase in temperature increases the volume. Ask students: Based on the equation, how does an increase in temperature affect the density? Students can study the relationship between volume and temperature for gases in the Chemistry Gateways activity, Charles's Law.
   
   
2. Liquids have intermolecular forces that are strong enough to hold molecules together, so they do not expand to fill their containers as a gas does. But these intermolecular forces are weak enough to flow past one another, so a liquid assumes the shape of its container.
   
   
3. Temperature affects not only the volume of a substance, but also the kinetic energy of its molecules. Students can learn about the relationship between temperature and kinetic energy in the Chemistry Gateways activity, Temperature and the Kinetic-Molecular Theory.

Analyzing the Problem

Challenge your students to draw the connections between the different concepts: Based on the kinetic-molecular theory, how does an increase in temperature affect the density? Explain your answer.

1. The actual ingredients of the official lava lamp line are patented and have been kept a well-guarded secret. Students can read various attempts to determine the ingredients: Lava Lamp Materials.
   
   
2. Students can do a hands-on experiment to see that objects change size when heated or cooled: Exploratorium's Cool Hot Rod.
   
   
3. www.chem4kids.com's States presents the basics of the states of matter, appropriate for elementary school to middle school students. Students can view an animation of the molecular structure of the three states at Harcourt Brace's States of Matter.
   
   
4. Students can examine substances that seems to have properties of more than one state in the activity, Wondernet State Debate.
   
   
5. The name "lava lamp" is an appropriate one because of the similarity between the lamps and the inner workings of a volcano. Magma—the hot molten rock beneath Earth's surface—rises through the denser, cooler surrounding rock. When magma breaks through the surface, it becomes lava. Since the lava is removed from the heat source, it begins to cool immediately.