Homemade Lava Lamp

Concepts – density, convection, miscibility, liquid volume measurements

Materials – 1-liter bottle, water, vegetable oil, food coloring, Alka-Seltzer tablet, flashlight (like a Maglite), tape

Figure 30: What you need to make a homemade lava lamp.
Procedure – add 200 mL of cold water to the liter bottle. Add ½ mL or 10 drops of food coloring to the water to darken it. Add 750 mL of oil to the liter bottle. It is important to leave space at the top of the liter bottle – you do not want to fill it up to the top with oil. Break the Alka-Seltzer tablet into 4 similar-sized pieces. In preparation of the next step, cut off a long piece of tape and turn the flashlight on. Now drop the pieces of the Alka-Seltzer in the bottle, quickly put the flashlight on top of the bottle so the light shines down on the liquid mixture, and tape the flashlight to the bottle. If you are able, turn the lights off in the classroom and have students make observations of what is occurring in the bottle.

Discussion – questions to ask the students include: 1) why does the oil float on top of the water? 2) why does the oil not mix with the water? 3) why does the Alka-Seltzer not react with the oil but it does react with the water? 4) why do some of the particles in the bottle rise to the top and then fall back to the bottom?.

Explanation – a general rule when hypothesizing whether different substances will mix is “like dissolves like”. Oil and water do not mix because the two substances are so dissimilar. Water is a short, polar molecule while oil is a long, non-polar . We say oil and water are not miscible (they don’t mix). Similarly, Alka-Seltzer is unlike the oil in the bottle so they don’t mix either. However, the Alka-Seltzer is similar enough to the water to cause the tablet to first dissolve in the water, and following this the citric acid and baking soda in the tablet reacts to make tiny bubbles of carbon dioxide gas. These bubbles attach themselves to the colored water and the pair floats to the surface. Look carefully at the colored water bubbles – it’s supercool to see the little air bubbles latch on to the water and carry them up to the top. When the bubbles pop, the colored water sinks back to the bottom of the bottle.

figure31.jpg
Figure 31: Oil and water do not mix.

The light shining down on the mixture in the lava lamp is a good example of scattering. Light scattering is the deflection of a ray from a straight-line path by particles in the medium. The different materials suspended in the oil, such as water and carbon dioxide gas, cause the scattering of the light. Additionally, some refraction of light is evident at the air/oil interface as well as the oil/water interface.

Figure31b.jpg

*This activity was adapted from “Bubbling Lava Lamp” on SteveSpanglerScience.com.

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