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There are so many ways to showcase density out there, but did you know that you can also show density differences with temperature?
In this experiment, we are looking at how hot and cold water interact due to their different densities.
This experiment will have you saying, “You are my density, I mean, my destiny.” (Any Back to the Future fans out there?)
How to make the Hot and Cold Water Density STEM experiment
Supplies you will need
For this experiment, you will need the following:
- Hot water
- Cold water
- Glass jars (2)
- Towel
- Food coloring
- Thin sheet of plastic
Before you start
You may need a set of pot holders to flip the hot water jar on top of the cold water jar.
Instructions
Here is how to do this experiment with your child:
Step 1: Fill jars with water
Lay down a towel and fill one jar with cold water. Add a few drops of food coloring (we used blue) and give it a good stir to mix.
Be sure to fill the jar very close to full!
Next, pour some hot water into the other jar. If your jar is cold or room temperature, be sure to pour a little bit of hot water at a time, give it a swirl around the jar, and slowly add more.
Add some food coloring to the jar of hot water (preferably a different color, like red) and give it a stir.
Step 2: Cover the jar of hot water with the plastic sheet and flip
This step is TRICKY, but you can do it!
Place the plastic sheet over the jar of hot water. You may need some pot holders to handle the jar, especially if you used really hot water like we did.
Secure the plastic sheet over the jar of hot water with your fingers. Use your other hand to grip the glass jar and quickly flip the hot water jar upside down onto the cold water jar.
The plastic sheet will still be between the two jars at this point. Before you remove it, line up the edges of the two glass jars so they are stacked directly on top of one another. Then, carefully remove the plastic sheet.
Step 3: Observe!
There might be a little mixing when you first remove the plastic sheet, but, for the most part, the two different colors of water will stay separate!
This is because hot water is less dense than cold water, so the cold water stays put down below the jar of hot water.
Step 4: Try the experiment the opposite way
Now, just to drive home the concept of density with respect to temperature, try the experiment again with the cold water on top of the hot water jar.
Once you remove the plastic sheet, you will see that the colors immediately start to mix, turning both jars of water into a deep purple color!
That’s because the cold water on top is more dense than the hot water below, so they want to switch places.
The STEM behind the Hot and Cold Water Density Experiment
This experiment teaches:
- Density
- Convection
- Stability and instability
How it works
In this experiment, we are looking at how hot and cold water interact due to their different densities.
When we quickly turn the hot water jar upside down over the cold water jar and remove the barrier between them, there is very little mixing between the hot and cold water (there may be initially, but that is just from disturbing the water).
Since hot water is less dense than cold water, the hot water remains layered above the cold water and does not mix with it.
If we switch it up and place the cold water jar on top of the hot water jar, we will see that they mix rapidly due to convection currents. The denser cold water sinks quickly while the hot water tries to rise.
Density
Density is a property of matter that essentially tells you how “packed together” the matter is in a given substance.
In this experiment, density is one of the big lessons. As water is heated, the molecules in it speed up and spread out, occupying more space. Since density is mass over volume and we are increasing the volume by the molecules spreading out, that means the density is lower in heated water.
The exact opposite is true as well! When we have cooler water, the water molecules slow down and get slightly closer together, resulting in less volume. Since the mass stays the same, the resulting density is more than that of hot water.
Convection
Convection would happen if we decided to switch things up and place the cold water on top of the hot water.
As we talked about above in the Density section, we know that cold water is denser than hot water.
What happens when something is more dense than the liquid below it? It sinks!
If we were to place the cold water on top of the hot water, the higher density of the cold water would cause it to sink rapidly as the hot water rises. That circular motion of heat rising and cold sinking is convection.
Here is a great experiment with a clear visual of convection happening in real time. It really helps you understand that circular motion of convection!
The convection occurring when the hot and cold water mix makes the two mix rapidly, bringing the two to equilibrium.
Stability and instability
This experiment is also a great opportunity to talk about stability and instability in a fluid system.
When we place the less dense (hot water) on top of the cold water (more dense), then the fluid is stable. In a stable system, when a small disturbance is introduced, everything is able to go back to where it was before.
When the denser liquid is placed on top, the fluid system becomes unstable. In that case, a small disturbance leads the system to get even further from its equilibrium. That’s why convection starts and the two liquids mix.
This is what happens in our atmosphere too!
When we have cooler air down below and warmer air above, the atmosphere is stable. However, when there is warmer air below (like by surface heating, a front, or topography) and cooler air above, that results in convection and an unstable atmosphere.
More experiments about density to try out with your child
- Discover Density: Build an hourglass to show density differences
- Sink or Float? Teaching density to children
- The Secret of the Floating Mandarin: Can a peel make a difference?
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