In this simple 4 step liquid density science experiment, your child will get to create a “volcanic” eruption in a jar. This is simple, easy for your child to play around with, and will remind you a lot of the lava lamp you probably had in your college dorm.
My 6-year-old son and I did this experiment a few times, and he was really into the cool effects he could create. It was easy for him to understand that different liquids have different densities just by watching what happened in the jar.
- Glass bottle or jar
- 1/4 cup vegetable oil
- 1 cup of water
- Salt (ideally rock salt if you have it; table or Kosher salt will also work)
- Food coloring (not essential, but it makes the experiment look much cooler)
Have your child make a prediction:
Explain to your child that if an object is heavy and compact, it has a high density. Different substances have different densities, and those with a higher density are going to sink, while those that are less dense will rise to the top.
If you want to be more precise, density describes how much space an object or substance takes up (volume) in relation to the amount of matter in that object or substance (mass).
Ask your child:
- Which substance is going to sink? Which do you think will float to the top? Why?
- What do you think’s going to happen when you sprinkle the salt on top? Why?
Your child can even write down a hypothesis if you’re feeling ambitious here.
My son is very into experiments lately, so I got him a science journal to record his predictions and findings. He’s in Kindergarten, so this is also a good way to get him to practice his handwriting!
Pour 1 cup of water followed by 1/4 cup of vegetable oil into a glass or jar. Do not stir them together!
Add food coloring.
Sprinkle salt onto the top of the liquid.
Let your child play around with the amount of salt to add, starting out with a little bit sprinkled on top, and then gradually pouring in more!
Watch what happens!
As the liquids begin to separate by their relative densities, you get a “lava lamp” effect in your jar!
Watch the bubbling at the surface – it looks like a mini volcanic eruption.
You can feel free to try different amounts of water, as well as different types of salt. See what you discover! It is an experiment, after all.
The Science Behind the Experiment
This science experiment is a great way to teach kids about relative density.
Oil and water don’t mix. And oil is less dense than water, which is why it floats to the top of a water/oil mix. Salt is more dense than water, which is why it sinks to the bottom of the mixture.
As the salt sinks, the oil will cling to it, getting dragged down in the process. But once the salt reaches the bottom of the jar it will begin to dissolve, and the oil that was clinging to it will be set free, rising back up to the top.
The released oil swirling to the top of the jar and bubbling at the surface creates a “volcano” effect, which is made even cooler by the presence of the food coloring.
- Rock salt is ideal for this experiment, but it’s fine to use table salt or Kosher salt as well. Kosher salt seemed slightly superior to regular table salt.
- You can play around with the timing of when to add the food coloring. It’s fine to add it right after you pour the water or after you pour the oil. Try both ways to see the different visual effects!
- If you don’t have food coloring this experiment will still work, it just won’t pack the same visual punch.
Why I Like This Experiment
This experiment readily demonstrates how the various liquids have different densities. Your child will learn something in a “that wasn’t painful, that was cool” way.
If you have a younger child, like I do, the experiment is especially dazzling. My 6-year-old loved watching the “lava” effect, and probably not coincidentally chose his book on volcanoes to read the same day!
It’s a neat little experiment, takes no time to pull off, and you already have what you need on hand.
Check out these other cool and easy science experiments for kids:
- Liquid Density Experiment: Color Explosion in a Jar
- Water Suspension Science Experiment for Kids
- 5 Minute Fizzy Fun Jar
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