Buoyancy with 2 Liquids

Contributed by Merlin Ling

Introduction

This experiment tries to build an understanding of buoyancy from the ground up, all the way to the interesting and somewhat unintuitive behavior of floating between two liquids.

What is buoyancy?

When you push on water, it pushes up. Why does water do that? To understand this, we must first understand the phases of matter, and what things are made out of.There are three main phases of matter around us: solid, liquid, and gas. Pencils, paper, pillows, or anything that you cannot put your hand in are solids. On the other end, gas is anything you can put your hand in easily, like the air around us, or steam above hot water. Liquid is in the middle. You can put your hand in water at the swimming pool, but when you try to move your hand through the water, you feel something pushing against your hand. Why?

This is because all matter are made out of tiny things called atoms. They are so small you cannot see them with your naked eye. And in solids, there are a lot of these atoms in a small volume. This is why you cannot put your hand inside solids: there are so many of these atoms in the space that they block out your hand. On the other hand, there are very few atoms in gas, so when you push your hand in the air, you are not touching very many atoms: all you are touching are empty space. It is like trying to push your hand through a large window frame versus trying to push your hand through an iron fence or tennis net — when there is not enough empty space, you simply cannot push your hand through.

Liquid is in between solid and gas. There are not as many atoms in liquid compared to in solids. And because you are strong, when you push on water, you push the water atoms away from your hand. But there is not enough empty space for the water atoms to simply move around, so they try to go back to where they were. Hence, when you push liquid, it pushes back. This is the buoyant force — when things push on liquid, whether they are outside or inside, water pushes them back, trying to make the float.

Heavier things push on liquid harder than lighter things, so when you drop a feather on water, it floats. But when you drop a metal spoon in, it sinks. This is the same as pushing on water: you can push water away because you are strong, and if you are weak, you cannot push on water. This is like stretching a rubber band. If you are strong, you can stretch it very far. If you are not as strong, you can only stretch it a little bit.

Liquid on Liquid

Because solid can float or sink on liquid, and both solid and liquid are matter, it is only natural to think about putting liquid on liquid. Do you remember ever seeing oil floating on water? Do you remember accidentally dropping oil on your cup of water, and they form little bubbles on top of the water? What happens when you pour a lot of oil on water? In fact, what happens is that the oil floats on top of the water. This is because oil is lighter than water. We call how light something is by talking about their “density,” which is a measure of how many atoms of a thing are inside the same amount of space. Oil is less dense than water, meaning there are less atoms of oil in the same amount of space compared to atoms of water. Since oil is lighter than water, it floats on top of water. Try it out! Get a cup, pour some water in it, then pour the same amount of oil in it. Water and oil do not mix, so the oil stays on top of the water. Better yet, try pouring oil first, then pour water in. You will see that the water will eventually sink down to the bottom.

Something even cooler…

You would not know this if you have not seen it before (I certainly did not), but baby oil and vegetable oil have different densities. Baby oil is lighter than vegetable oil. Most interestingly, ice is lighter than vegetable oil, but heavier than baby oil. So what happens when you put ice in a cup with vegetable oil and baby oil? Watch the video!

The white liquid on top of the cup is baby oil, and the yellow liquid on the bottom is vegetable oil. As you can see, the ice floats in between vegetable oil and baby oil! More interestingly, water is heavier than vegetable oil, so when the ice melts, it drips down to the bottom. This is a very slow experiment, so if you want to actually see the water drop down, you can put your bottle near a heat source. Make sure to not burn anything in the process.

Materials

• A transparent bottle (or if you have it, a glass beaker)
• Vegetable oil, baby oil, and ice (You can also do interesting experiments with other liquids like honey, maple syrup, milk, etc.)
• Some small solids with different densities if you want (blueberry, metal bolt, etc.)

Procedure

1. Make sure the bottle you are using does not have any water in it.
2. Pour vegetable oil in the bottle.
3. Pour baby oil in the bottle.
4. Drop an ice in the liquids, and watch as it melts and the water sinks to the bottom.

If you do not have baby oil or vegetable oil at home, you can do variations of this experiment with different liquids.

1. Look up the densities of the liquids you have and organize them from highest to lowest density
2. Pour the denser liquid in first, then the less dense in sequence. Be careful to pour slowly because some liquids mix, and you do not want them to float on top of each other.
3. Once you build your density tower, drop some solids inside to see where they float.

For a detailed procedure for this variation of experiment, check out this link. You do not have to use all the liquids in their experiment.

Physics Concepts and Questions

• I explained that density is how many atoms there are inside a certain amount of space, but does that idea make sense? Can you think about the world as being made from small atoms you cannot see, with each thing having different densities of atoms?
• How does buoyancy work? I said it is water pushing back on things that push on it, but how does this work?
  • This may be hard to understand, but buoyancy is all about how water wants to stay where it is, and so when something pushes on water, it tries to push back so it can return to where it was. This sounds ridiculous because it makes it seem like water is alive, but if you are an aspiring physicist, try to build an intuition for that. You will eventually realize this is exactly how the world works: water does try to push back so it can go back to where it was. There is no magic here.
• Buoyancy works not only on liquids but also gas. As such, normally when an object is floating on top of a liquid, they also experience a negligible buoyancy force from air.

Conclusions and Further Investigations

• Since we know that buoyancy also works with gas, can you imagine an experiment that uses two different gases instead of liquids? The experiment may be difficult to conduct since it is much harder to contain gases than it is to contain liquids, so be sure to consult a science teacher if you want to do an experiment with it.
• Try doing the experiment with two liquids that mix, and pour them in slowly so they do not mix at first. If you have an object floating in between these two liquids, what happens when you mix them instead of keeping them stacked on top of each other? Will the object float at the same height?