All matter has mass and takes up space. The amount of matter in a given space is referred to a the objects DENSITY. It is the mathematical relationship between the mass and volume variables.
Nature orders itself by densities. Substances that have more matter per space are pulled by gravity with greater force than less dense substances. We can make a “density column” of different liquids like the one pictured below.
Ice sinks in rubbing alcohol but in vegetable oil, it floats. Notice that when the ice melts into liquid water, it sinks below the oil. Fluids order themselves according to their densities.
Pure water has a density of 1 g/ml. This is because 1 milliliter of water has a mass of 1 gram. Water is the most common substance on the planet and it is no coincidence that it has a simple, easy to remember density. The metric system was designed with water in mind.
Sink or Swim?
Densities GREATER than 1.0 g/ml will SINK in water, while those with densities LESS than 1.0 g/ml will FLOAT.
Not all things sink at the same speed. We can estimate their densities by comparing how quickly they sink. The cubes on the right are more dense than the cubes on the left, but they are all greater than 1.0 g.ml.
How much of these objects is floating BELOW the surface of the water? If we estimate the percent of the object under the water it will tell us the object’s density.
Wood floats so HALF is below the water. That’s 50%. So it should have a density of .5 g/ml.
Styrofoam floats much higher on the water than wood typically does. How much do you see below the surface of the water? What would the density be?
The hippo sinks because the weight of the animal is MORE than the weight of the water the hippo pushes aside (displaces).
What would you expect from a rock and a piece of wood?
Typically on water, rocks sink and wood floats. The rock pictured below is pumice. It’s an igneous rock made from lava that got cooled by air bubbling through it. As a result, it is less dense than 1 g/ml. It floats.
The wood used is called ironwood. It’s a type of tropical plant that produces extremely compacted cellulose because grows so slowly. As a result, the wood has a density greater then 1 g/ml. It sinks.
Mercury is a liquid at room temperature like water but it has a density of 13.59 g/ml. That means a bottle filled with Mercury would be 13.59 times heavier than a bottle filled to the same level with water.
Iron has a density of 7.87 g/ml so while it sinks quickly in water much less than Mercury. This iron anvil floats with ease on a pool of Mercury.
video from Cody’sLab on YouTube
Gases have a density as well.
It’s easy to think about the density of solids and liquids because we can usually see and touch the substance. Many gases are colorless, odorless, and tasteless and are more difficult to observe.
Some gases are “lighter than air”.
We often fill balloons up with the element Helium to have them rise above the birthday cake or on the mailbox to show where the party is. Some people say that Helium is “lighter than air” and while we know what they mean, it is more accurate to say it has a lower density than air. There are fewer particles in a balloon of Helium than there would be in the same sized balloon filled with regular air.
We can change the density of air by heating it up. The particles take the energy and use it to speed up and spread out. Imagine the shape of a hot air balloon. When heated, there are fewer particles of air in the balloon because some of the particles have “spread out” and left the space. Having less air than normal inside the balloon make is less dense than the cool air outside and makes it want to rise.
We use this idea of hotter air spreading out to do the “egg in a bottle” demonstration. Heating the air inside the bottle means that there is less air physically in the bottle. Less air means there is less air pressure holding the egg up. Normal air pressure in the room naturally pushes into that space to make the pressures equal. The egg just happens to be in the way and gets PUSHED inside.
One year a student asked if it would work with an OSTRICH egg…so we tried it!
Some gases are “heavier than air”
Carbon dioxide (CO2) is a gas as well but has the opposite property . CO2 is MORE dense than regular air and can be poured out like any other fluid.
For this demonstration we used Alka-seltzer tablets to generate the CO2 gas in the container. It is so dense that it stays there. We can prove how much gas is in the container by lowering a candle inside. When the flame burns out we have reached the level of CO2 gas in the container.
Notice that the level of CO2 goes down after the candles have been put out.