Free Night? Make a Quick “Lava Lamp”

  by Erik Wolgemuth

Odds are, the topic of density doesn’t often occupy conversations with your kids. And, in the rare occasion that it does, it’s likely used to explain the wonders of that two-inch square yet 12lb. Cosmic Brownie, which Little Debbie has perfected. This experiment, however, provides opportunity to turn that edible descriptor into scientific understanding.

Here’s your supply list:

  • a tall glass cup
  • water
  • vegetable oil
  • salt
  • (food coloring also helps here, though you can do without)

Fill your glass with water about ¾ full (or 1/4 empty if that strikes your personality better) and, if you’re up for the color, a few drops of the coloring(s) of choice. Then add in ¼ cup of oil and enjoy the liquid separation that results. Once you’ve noted and described the basics of oil/water densities (in brief, water is denser than oil – that’s why the oil rises to the top) and enjoyed the observation thoroughly, it’s lava time.

Pour a healthy amount of salt (a tablespoon or so should do) into the glass. You’ll immediately see the newly introduced salt team up with globules of oil, increasing the density of the oil blobs and rushing them straight to the bottom of the glass. Before too long the salt dissolves, the oil’s density returns to normal, and the oil rises once again. Depending on how liberally you’ve dispensed the salt, you should see the oil continuing to rise and fall for a while following the initial introduction of salt.

Years down the road, there will come a physics instructor who will guide your child through density calculations; now’s likely not the time for this. Perhaps in summary you can explain that water is denser than oil, causing it to float on top of the water. And that you can change this temporarily by the increasing the density of the oil with salt…or, you can simply explain that water loves being water, oil loves being oil, and they don’t much like mixing with each other…that is until Mr. Salty comes along.


When you’ve got a wide-open evening that needs some fun activities, dadcraft + science = great. Kitchen science (cupcakes and pizza, for instance) and science science (salt and ice and pennies, vinegar, and salt) for the win.

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Picture by Jo Christian Oterhals; Used via Creative Commons license.