Why do things float or sink?
This is one of the easiest experiments we’ve done, and everyone loved it. The end result is a liquid rainbow that appears to magically suspend objects in its different layers.
Here’s what you’ll need:
- A tall clear glass
- Liquid honey
- Cooking oil
- Food coloring (blue works well)
- A marble
- A grape
- A lego brick
- A ping pong ball
Find a space where a little spill isn’t a big deal. Like your kitchen floor, or a table with a plastic cover. This isn’t a messy experiment, but just in case.
1. Fill about 1/3 of the glass with water. Add food coloring (optional, but cooler) and stir it up.
2. Squeeze in some liquid honey, about another third of the glass. Which is denser, the honey or the water? (hint: the denser one goes to the bottom)
3. Add the cooking oil. We used olive oil because that’s what kitchens in Barcelona have. But any oil should do the trick. Is the oil more or less dense than the water and honey?
You should now have some pretty neat looking liquid layers. Your kids, like mine, may ask if they can drink it. Saying something like “sure, as long as you clean the floor after you throw up” will probably deter them.
4. Drop objects into the glass carefully, one by one.
Ping pong ball…
Pet fish… kidding
You should now have something that looks like this:
Pretty cool, right?
5. Explore some more. Drop a few more objects into the glass. Try a raisin, a marshmallow, your homework. But wait! Before you drop, hypothesize. Where do you think the object will rest—on the oil, water, honey? Why? Take your best guess, then drop away.
So what’s going on here? To understand, we’re going to need to dive deep inside each object.
Why do things float or sink?
Everything around us is made of atoms. Water, honey, grapes, legos, tables, trees, clouds, dogs, people, and ping pong balls—all full of atoms.
To understand why things sink or float, you have to compare how packed together the atoms are inside of things.
Let’s start with water and a lego brick. If you look deep inside these objects with a special microscope, you’ll see their atoms. But look closely. Notice that the atoms in the water (on the left) are more tightly packed together than the atoms in lego (on the right).
In the same amount of space, the water has more atoms. That’s what density is: how packed atoms are together in a given space. And the thing with more tightly packed atoms is the denser object.
If the lego is denser than the water, it will sink to the bottom. If it’s less dense than the water, it will float. So if we throw our lego brick into the water and…
It floats, of course, because it’s less dense than the water.
Now let’s go back to the colored layers in our glass, Why does the honey sink to the bottom of the glass while the oil floats at the top? Why does a grape fall through the water but sit on top of the honey?
You guessed it: density. The different liquids and objects all have different densities, so they all stack up in different places. The grape is denser than water but less dense than honey. The lego brick is denser than oil but less dense than water. And so on.
Let’s grab our special microscope and have a look through different eyes.
Pretty cool, right?
Expand your thinking
Think about other things that float or sink. Any surprises come to mind?
Here’s one: how does a gigantic Disney cruise ship, carrying 4,000 passengers and weighing up to 130,000 tons stay afloat?
There’s a secret in the belly of the boat: lots of open spaces.
The average density of the ship includes the weight of the ship plus all the empty spaces on board. So engineers design cruise ships to have lots of open spaces filled with air, making the overall ship less dense.
There’s a related thing going on that helps ships stay afloat, and it has to do with a force called buoyancy. Whereas gravity wants to pull an object down to the bottom of the ocean, the buoyant force of the water pushes the object back up.
Basically the ship is designed to push water out of its way while it moves forward. When the water gets pushed out of the way, it quickly rushes back in to try to fill the space it’s been pushed out of. And as it does, it creates an upward lift that pushes up on the boat—buoyancy. As long as water pushed away by the ship weighs more than the boat, it stays afloat.
You can see a similar trick in action in your own belly. Next time you’re in the swimming pool take a huge gigantic breath of air and hold it in. Now lay back in the water. Do you float or sink? Now blow all the air out of your lungs and lay back again. What happens? Down you go, glub, glub, glub.
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