Density, Mass & Volume
For example, a suitcase jam-packed with clothes and souvenirs has a high density, while the same suitcase containing two pairs of underwear has low density. Size-wise, both suitcases look the same, but their density depends on the relationship between their mass and volume.
Density is calculated using the following equation: Density = mass/volume or D = m/v.
Let’s compare three familiar substances to explore the concept of density. If we take the same volume (one cubic centimetre) of foam, wood and concrete, we can see that each has a different mass.
Less Dense, More Dense
A pebble is heavy for its size, compared to a piece of popcorn which is light for it’s size. Imagine a big bowl of popcorn, compared to a big bowl of pebbles, which would feel heavier?
It is easy to estimate relative densities if you keep either the volume or the mass of two objects the same.
If you filled one bag with a kg of feathers and another with a kg of lead you would see that the feathers take up much more room, even though both bags have the same mass. This because feathers are less dense, they have less mass per volume. If you made a copper cube and an aluminum cube of the same volume and placed one in each hand, you would be able to feel that the copper cube would be heavier. Copper has more mass per volume than aluminum.
How can one substance have more mass per volume than another? There are a few possibilities:
Any one or a combination of these explanations could be the reason why one substance has a higher density than another. In the case of copper and aluminum, their atoms are arranged similarly, but copper atoms are smaller and have more mass than aluminum atoms, giving it a higher density.
Density, Sinking and Floating
You can really see relative densities at work when you look at a heavy object floating and a lighter one sinking. For example, imagine putting a small piece of clay and a large, heavy wax candle in a tub of water. Even though it’s lighter, the piece of clay has a higher density than water and therefore sinks. Even though it’s heavier, wax has a lower density than water, so the big candle floats.
Sinking and floating applies to liquids too. For example, if you add vegetable oil to water, the oil floats on top of the water because the oil has a lower density than the water.
Buoyancy and Archimedes’ Principle
The water pushes upward against the object with a force (buoyancy) equal to the weight of water that is displaced.
Let’s explore Archimedes’ principle by dropping a bowling ball into a tub of water. When the ball is submerged in the water, it displaces its volume in water. According to Archimedes’ principle, the water can “push back” with a force equal to the weight of the water that has been displaced.
A litre of water has a density of 1 kilogram per litre (1 kg/L), so a bowling ball’s worth of water (4.5 L) can push back on the bowling ball with a force equal to 45 newtons (N). That’s the weight of a 4.5 kg mass. However, the weight of the ball is more like 55 N. That’s more than the buoyant force of the water it displaced, so it sinks.
A beach ball may have the same volume as a bowling ball, but it has a much smaller mass. When you a beach ball in a tub of water, it displaces the mass of water equal to its own mass—about 0.01 kg. If you were to try to push the beach ball down and displace more water, the water would push back with a force greater than the weight of the beach ball. The push of the water keeps the beach ball afloat.
Buoyancy is the upward force we need from the water to stay afloat. Buoyant forces are why we feel so much lighter when we are in a swimming pool. Our bodies are mostly water, so our density is fairly close to that of water. Because of this, an average person needs only a little bit extra buoyancy to float. A life jacket provides this extra lift.
Archimedes: Greek mathematician, physicist, engineer, inventor and astronomer (c. 287 BC–c. 212 BC).
BrainPOP | Science | Matter & Chemistry | Measuring Matter
EDinformatics | Mass, Volume, Density
WatchKnowLearn.org | Buoyancy and Density
ProTeacher Collection | Density
Today’s Wonder of the Day was inspired by Tianna. Tianna Wonders, “why do icecubes float in water?” Thanks for WONDERing with us, Tianna!
When you've been playing outside all afternoon, there's nothing better than relaxing at the kitchen table with a tall glass of ice water. There's something about the clink of the ice cubes as they hit the bottom of the glass. As you pour water over them, they hiss and crackle a bit as they slowly float to the top of the glass.
Have you ever thought about what a curious concoction ice water is? It's literally solid water floating in liquid water. How cool is that? (Pun totally intended.) But why does the ice float in water?
Scientists will tell you it has to do with density, which is a measure of mass per unit of volume. Ice floats because it is less dense than the water.
Something denser than water, like a rock, will sink to the bottom. To be able to float, an object must displace fluid with a weight equal to its own weight.
The fact that ice floats in water is a bit strange, because most substances are denser when they're solids. Water, however, reaches its maximum density at 40º F (4.4º C). As water cools and freezes, it becomes less dense due to the unique nature of hydrogen bonds.
Each molecule of water consists of one oxygen atom strongly bonded to two hydrogen atoms with covalent bonds. This fact is reflected in the chemical formula for water: H2O.
Water molecules tend to be attracted to each other by weaker hydrogen bonds. These form between the positively-charged hydrogen atoms and negatively-charged oxygen atoms in nearby water molecules.
As water temperature decreases, the weaker hydrogen bonds begin to hold the negatively-charged oxygen atoms apart, forming a rigid crystal honeycomb structure we call ice. The water molecules in ice take up about 9% more space than liquid water, which means ice is about 9% less dense than water.
If you have a gallon of ice and a gallon of water, the gallon of ice will weigh less than the gallon of water. When you put the ice into the water, the denser water pushes the ice to the top where it will float.
This unique property of water is especially beneficial for fish that live in bodies of water that freeze in the winter. Because ice floats, bodies of water freeze from top to bottom. This allows fish to survive deep underwater even when the surface freezes!