This is a part of the What is … Wednesdays feature, where we take scientific topics that might seem convoluted to some and make them more understandable.
You’ve probably noticed that some things float on water and others sink. For instance, rubber duckies float and bars of soap usually sink. This is due to the relative densities of the objects and the liquid they are immersed in. Density is defined as the mass per volume of a substance.
Density is synonymous with concentration in chemistry. Densities change with pressure and temperature fluctuations. Generally, density increases with increased pressure or lower temperature. These increases are more pronounced in gases than in liquids. Water is an exception to this generalization.
Water has a density of 1.00 g/cm³. Ice is at a lower temperature than water, so it should theoretically be more dense. However, ice is indeed less dense than water, which is why ice floats on water. Also, salt water is heavier and therefore denser than fresh water. What other instances of density have you noticed in daily life?
Density is an example of an intensive property, meaning its value does not change with weight. This is because changing the weight of something involves adding or taking away a substance, which in turn changes both mass and volume of a sample.
Make a density tower
One way to observe density is by making a layered mix of liquids. For the layers to separate, you must use liquids that have different densities. Before doing research on the density values of any liquid, make a guess as to what order the layers will end up in.
Look up the densities of the liquids you have available in your home to determine which are suitable to create layers.
To complete this experiment, you’ll need at least two liquids with different densities, a tall clear or translucent container, and an eye dropper or baster. If you plan on using two liquids that are the same color, you’ll also want food coloring and something to mix with (I used a bamboo straw).
I chose to use seven substances in my layered tower – hand soap, honey, 70% isopropyl alcohol, milk, pancake syrup, vegetable oil, and water. I colored the water green, the alcohol blue, and the milk purple (the hand soap I chose was also white). Here are the liquids listed with their densities in descending order:
milk (dyed purple)
water (dyed green)
rubbing alcohol (dyed blue)
When assembling your density column, add the liquids in descending order. This way, the heaviest liquids will be on the bottom first.
When initially pouring liquids into the container, you should aim for the middle of the container and not touch its edges so that none of the thick substance gets stuck on its sides. However, after adding hand soap, you should use the dropper or baster to slowly and carefully add the remaining liquids.
This is the part where I rushed. As a consequence, a few of my layers mixed. There are supposed to be seven layers but the milk, water, and rubbing alcohol seem to have started commingling into one layer.
Also, the oil ended up on top of my column, not the rubbing alcohol.
The end result did look interesting, though! I liked the color palette.
For the next part of the experiment, collect a few objects that will fit comfortably in the container, like bottle caps, buttons, coins, or small toys. Predict which will float on what layer and which, if any, will sink to the bottom.
I used a cork, paper clip, quarter, screw, and toothpick. I thought the toothpick and cork would both float on the same layer but that the paper clip would be somewhere beneath them. I also did not think the quarter was going to sink to the bottom, but guessed that the screw would sink.
In the end, the cork, paper clip, and toothpick all floated on top of the oil (the top layer) and the quarter and screw both sank to the bottom of the container.
It seems I didn’t choose items with very different densities!
This was fun to do. If I were to do it again I would definitely be more deliberate in my adding of the top layers, to ensure that they separate entirely.
What liquids did you use when doing this experiment? Comment and share below.