Buoyancy

Buoyancy the upward force on an object in a liquid

 Buoyancy is an important concept in many fields, if you are interested in becoming a meteorologist, a ship captain, a marine biologist, or any number of other amazing things a basic understanding of the buoyant force is essential.

Note: This experiment is designed to follow the basic lab notebook setup for an advanced lab notebook based on our Sample Lab Notebook Setup document. The section titles in red correlate to the sections in your lab notebook. Please refer to this document when setting up a lab notebook for the first time and for younger children.

Experiment Title: 

Can an egg float in water?

Objective:

The objective of this experiment is to learn about buoyancy (the upward force exerted on an object when placed in a fluid). This will be done by seeing how an egg when placed in water floats (first objective) then if it is possible to get that egg to float or sink in water by changing the density of the water (this is done by adding salt to the water).

Background on Buoyancy:

Buoyancy is the upward force acting on an object in a liquid. What? Seems confusing but think about it this way we have a force called gravity constantly pushing on us and keeping us on our planet, without gravity everything would float away (including you!). When you drop something in water (let’s say your parents cell phone) what happens? It sinks to the bottom of the container the water is in, in this case the buoyant force is less than the gravity force (i.e. the gravity is pushing harder on the object than the upward force of the fluid). Now instead of your parents phone toss a ball filled with air (solid balls are a little more complicated to explain). The ball will initially sink slightly but will stay on the top of the water, in this case the buoyant force is greater than the force of gravity (the buoyant force is stronger than the gravity force keeping the ball from submerging in the liquid). Pretty cool isn’t it?

Overview of Buoyancy

Positive Buoyancy

In this type of Buoyancy the buoyant force is greater (larger) than the gravity force. This causes the object to float in top of the liquid. Think about a beach ball, does a beach ball float, sink or submerge but not go to the bottom when it is tossed in water? It floats this is because it has positive buoyancy.

Neutral Buoyancy

In this type of buoyancy the force of gravity and the buoyant force are the same. This means that the object will sink into the liquid but will not fall to the bottom. Think about a bottle filled with water that is placed into a container of water as long as there is a little bit of air in the bottle the bottle will float in the water instead of sinking to the bottom creating neutral buoyancy.

Negative Buoyancy

In this type of buoyancy the gravity force is greater than the buoyant force. This causes the object to float to the bottom of the container that the liquid is in. Think about sunken treasure. The gravity force is greater than the buoyant force causing the treasure to sink to the bottom of the ocean.

So far we have talked about what buoyancy is and three different types of buoyancy, but we have not discussed why one object would float and one object would sink in a liquid. The reason for this has to do with density (and I am not talking about your parents being dense). Density boils down to how compact are the atoms/molecules that make up something. Water is a great way to understand this. Pure Water is made up of molecules that have one oxygen atom and two hydrogen atoms.

When water is in liquid form these molecules can squish altogether and jumble up. But what happens when you freeze water? When water is frozen it creates a lattice (think about a brick house as you build the house the bricks line up and overlap each other, this is what happens when water freezes but that means that areas where molecules of water used to be are now empty space). In the lattice you have less molecules in the same area as the liquid took up making the frozen water less dense (having less molecules in the same space) and the liquid water more dense (having more molecules in the same space).

In buoyancy you have two (or more but let’s focus on two for now) densities interacting, the first is the liquid and the second is the object that is being put in the liquid. If the density of the liquid is high (lots of molecules or atoms in a small space) and the density of the object is low (very few atoms or molecules in a small space) the object should float in the liquid under most conditions. Now that we know the basics let’s have some fun and see if we can work to change the buoyancy of a single object in water!

Materials

• Safety Glasses
• Nitrile Gloves
• Stir stick
• Clear Plastic Cup
• 1 egg (uncooked and in its shell, do not throw the egg, it will break and you will not be able to do your experiment)
• water
• salt
• Lab notebook
• pencil

Method

Prepare Experiment 

     Eggs are pretty cool, if fertilized they could become a chicken, if not they are a nutritious food for many animals (including humans). Make some Observations about your egg, what color is it? How big is it? Does it have anything odd with it, crackles, speckles, etc.?

      

      Also look at the container you are going to put the water in. Can you see through it to make observations? Is it big or small? How much water are you adding to it (make sure you do not put more water than fills your container half way up (otherwise you might get some egg-citing disasters with your experiment (bad parent joke sorry can’t help it)).

      

      Run Experiment

      Now, we are going to see what happens when we drop (and I mean gently drop… spiking an egg into the water might seem fun but will result in a broken egg and a messed up experiment). What Happens to the egg? Write down your observations (observations are things that you see happening in your experiment or around you, if you do not know how to write go ahead and draw what you see happening, many scientists write and draw their observations when doing an experiment).  

     

     Using your lab gloves gently remove your egg from the glass and set it aside where it will not roll away and break.

     

     Now, let’s see if we can change what the egg does when dropped (gently) into the water. We have some standard table salt (the stuff you put on food to make it taste better). Let’s try adding some salt (make sure you write down or draw how much salt you are adding initially) to our water and mixing it (use a stir stick not your hands, you don’t want to taste delicious do you?) in then re-adding the egg. What happens? Does the egg still do what it did last time?

      

      Pull the egg out again and add some more salt and see what happens again. Can you get the buoyancy of the egg to change over time in the water?

Analyze Data

The cool thing about observations and experiments is that everyone has their own unique way about going about making and doing them. Let’s all share our observations of what happened to our egg as we add more and more salt over time. Realize that there are no wrong answers here this is what you did and you observed (and I must say you are all fantastic scientists!).

In just water the egg sunk directly to the bottom, as more salt was added the egg sunk more slowly to the bottom until the egg floated in the water at one point and then did not sink at all (took a lot of salt)!

Conclusions

In this experiment we used a glass of water, an egg, and some table salt to demonstrate how the buoyancy of an object can change when the density of a fluid that the object is in is changed. By adding the egg to water with nothing added to it we were able to observe an egg (which has a higher density than plain water) sink to the bottom of the glass. By adding a lot of water we were able to change the density of the water (making the water denser than the egg) and watch the egg float in the water.

Don’t worry if your egg did not float. We left so much of the procedure open to you amazing scientists that each of you each did a different experiment resulting in different outcomes. Differences between eggs (even those of the same size) can cause some to float and some to sink. Be proud of yourself for working hard and learning something new.

Make It Your Own

This experiment was designed for all of our young scientists to be able to do with adult supervision, now think about what you did and make it your own at home with your parents. Think about the experiment, if you were to add something other than salt would that change the outcome? What if you used a liquid other than water would that change the outcome? Would a different egg change the outcome (using a white egg vs a brown egg)? What if the egg was hard boiled first would it still sink, could you get it to float by adding salt to the water? Would something entirely differen, but with an egg’s shape change the outcome?

Extension Activities to do at home

Buoyancy is tricky in that an object that is actually more dense than the liquid it is in can float on that liquid provided it has a large enough surface area). Think about it this way we tear two pieces of aluminum foil from a roll. They are both the same size and initially the same shape. Since they are the same size and from the same material they should have the same weight and overall density. Now crumple one of the pieces into a ball and gently fold up the sides of the other (to form a type of boat). Gently set both into a container and observe what happens. The ball will sink and provided you do not have a leaky edge the boat shape will float. This is an advanced way of looking at buoyancy and how to make an object buoyant enough to float on any liquid even though it may be far denser than the liquid it is on. Taking this into account, see if you can figure out a way to make the egg float in water (no salt added) just by using a single sheet of aluminum foil (note that the egg will probably be under the water line, but shouldn’t get wet). Learn more about Buoyancy and Archimedes Principle: https://en.wikipedia.org/wiki/Buoyancy