You’ve probably played with a beach ball at the pool or the beach, and you know that if the ball hits the water, it will float. You may have gone on a hike and seen sticks and leaves floating in rivers and puddles. The beach ball, the leaves and the sticks all have one thing in common—they all float on the water.
Of course not all things will float on water such as coins or car keys! Why do the leaves and sticks float, while the keys sink? To understand why some things float on water, we have to understand buoyancy.
Buoyancy (pronounced BOY-an-see) is the upward push of water (or other liquid) on an object placed in it. If the water can push upward on the object with enough force, then the object will float. This means that the objects which float in water push down on the water less than the water is able to push back.
Of course not all things will float on water such as coins or car keys! Why do the leaves and sticks float, while the keys sink? To understand why some things float on water, we have to understand buoyancy.
Buoyancy (pronounced BOY-an-see) is the upward push of water (or other liquid) on an object placed in it. If the water can push upward on the object with enough force, then the object will float. This means that the objects which float in water push down on the water less than the water is able to push back.
Think of it like this: if we get a big, empty cardboard box, and ask a small person to hold it over their head, they should not have any difficulty holding the box up. After all, a cardboard box is not too heavy, especially if there is nothing in it.
But now what will happen if we fill the box with heavy books? Now that one person cannot hold the box over their head.
Water is a little like the person we got to hold the box. The box with nothing in it is like the sticks or the beach ball: there is not much matter (‘stuff’) in it, so it is easy for the water to hold up. This is why they float. The keys however have a lot more matter in them, so they are like the box full of books. The water under the keys cannot hold up that much ‘stuff’, so they sink.
So how can we know if something has too much matter in it for it to float on water? The amount of matter in an object is called its mass, and the amount of mass something has per unit of volume (size) is called its density. So, if something is denser than the water (that is, it has more mass per unit volume than the water does), it will sink. If it is less dense than water, it will float.
Here’s an example. Let’s say we have one gallon of pure water. This water has a weight of 8.3 pounds. So we say that the density of the water is 8.3 pounds per gallon. Now, let’s say we have one pint of rubbing alcohol, like the kind you buy at the drug store. The volume of the water and the rubbing alcohol are the same, but this much alcohol only weighs 6.6 pounds. So the alcohol is less dense than the water. On the other hand, if we had a gallon of ethylene glycol (a major ingredient in antifreeze) it would weigh 9.2 pounds. So the ethylene glycol is denser than water. If we place one tablespoon of each, ethylene glycol, rubbing alcohol, and water in a test tube very carefully, they will stay separated: the rubbing alcohol will float on the water, and the ethylene glycol will sink.
So how can we know if something has too much matter in it for it to float on water? The amount of matter in an object is called its mass, and the amount of mass something has per unit of volume (size) is called its density. So, if something is denser than the water (that is, it has more mass per unit volume than the water does), it will sink. If it is less dense than water, it will float.
Here’s an example. Let’s say we have one gallon of pure water. This water has a weight of 8.3 pounds. So we say that the density of the water is 8.3 pounds per gallon. Now, let’s say we have one pint of rubbing alcohol, like the kind you buy at the drug store. The volume of the water and the rubbing alcohol are the same, but this much alcohol only weighs 6.6 pounds. So the alcohol is less dense than the water. On the other hand, if we had a gallon of ethylene glycol (a major ingredient in antifreeze) it would weigh 9.2 pounds. So the ethylene glycol is denser than water. If we place one tablespoon of each, ethylene glycol, rubbing alcohol, and water in a test tube very carefully, they will stay separated: the rubbing alcohol will float on the water, and the ethylene glycol will sink.
Clearly, if something floats on water that means it is less dense than the water. We could also say the water is denser than the object. But does all water have the same density? As it turns out, the answer is no! Seawater (that is, water with a lot of salt in it) is denser than pure water. This is because when we dissolve salt in water, this water now has more matter—more ‘stuff’—in it, but the volume is the same. This means that if something does not float in pure water, it may float in water that has a lot of salt dissolved in it—if we can add enough salt that we raise the density of the water above that of the object.
TRY IT!!
Here’s what you’ll need:
1. Two raw eggs
2. Two large, clear drinking glasses
3. Water from a sink
4. About 1 cup of salt
Here’s what you need to do:
1. Fill both of your glasses about half full with the water from the sink.
2. Stir one cup of salt into one of the glasses of water. Stir the water and salt for at least 2-3 minutes until as much salt dissolves as possible. Don’t worry if not all the salt dissolves.
3. Gently lower one egg into the glass with plain water, and the other egg into the glass with the salt water. Observe what happens, and be sure to write down all your observations.
What happened to the egg in the plain water? What about in the salt water? Why did this happen?
BE SURE TO WRITE DOWN EVERYTHING YOU DO AND EVERYTHING YOU OBSERVE IN YOUR NOTEBOOK!
References for further reading:
Buoyancy. Wikipedia. 2014, Mar 17. Retrieved 3-17-2014. (http://en.wikipedia.org/wiki/Buoyancy)
Buoyancy. HyperPhysics. Retrieved 3-17-14 (http://hyperphysics.phy-astr.gsu.edu/hbase/pbuoy.html)
TRY IT!!
Here’s what you’ll need:
1. Two raw eggs
2. Two large, clear drinking glasses
3. Water from a sink
4. About 1 cup of salt
Here’s what you need to do:
1. Fill both of your glasses about half full with the water from the sink.
2. Stir one cup of salt into one of the glasses of water. Stir the water and salt for at least 2-3 minutes until as much salt dissolves as possible. Don’t worry if not all the salt dissolves.
3. Gently lower one egg into the glass with plain water, and the other egg into the glass with the salt water. Observe what happens, and be sure to write down all your observations.
What happened to the egg in the plain water? What about in the salt water? Why did this happen?
BE SURE TO WRITE DOWN EVERYTHING YOU DO AND EVERYTHING YOU OBSERVE IN YOUR NOTEBOOK!
References for further reading:
Buoyancy. Wikipedia. 2014, Mar 17. Retrieved 3-17-2014. (http://en.wikipedia.org/wiki/Buoyancy)
Buoyancy. HyperPhysics. Retrieved 3-17-14 (http://hyperphysics.phy-astr.gsu.edu/hbase/pbuoy.html)
