Tag Archives: density

OS11 Floating Hot Water on Cold Water

If you pour water into water, it mixes up and has more volume. That is what usually happens. But if you are really up to the challenge, you can make hot water sit on top of cold water.

jar of hot water, jar of cold water

Question: How can hot water be made to sit on cold water?


3 Jars



Food coloring

Microwave or stove

Eye dropper


Half fill a jar with water

Add ice until some ice remains floating in the water

Half fill another jar with water

Heat this water close to boiling in the microwave (use a pan on the stove)

Add a drop of food coloring

food coloring in hot water

Sometimes, when you heat water on the stove, you can look down into the pan and see the water moving. These currents are in the jar of hot water and carry the food coloring around.

How does the food coloring spread through this water?

Stir to finish mixing the coloring into the water

Half fill a jar with ice cold water with no ice in it

This is the hard part: You will use the eye dropper to add the hot water to the jar of cold water. Do this by sliding the water down the side of the jar. Continue adding water until the hot layer is 1.5 cm thick.

hot water on cold water

The hot water tries to stay up on top of the cold water. Right underneath the hot water is an area cooling off with a layer warming up so some of the food coloring goes down into these layers creating the lighter colored layer.

Observe the hot water layer every fifteen minutes for an hour


Describe how the food coloring moves through the hot water

Describe what the hot water layer does

At the start

In fifteen minutes

hot water mixing with cold water

Cold water has currents in it like the hot water does. That lowest layer of food coloring is getting pulled into these currents.

In thirty minutes

In forty-five minutes

In an hour


Note: Density is how much stuff is in a certain volume. Something with less density will float on top of something less dense.

Is hot water more or less dense than cold water? Why do you think so?

Is this difference very much? Why do you think so?

What happens to the temperature of the hot water over the hour?

What will happen to the density of the hot water over the hour?

What will happen to the food coloring over time? Why do you think so?

food coloring is mixed

The hot water cools off. The cold water warms up. The water currents carry the food coloring around until all of the water turns blue. Some of the currents are still visible in the center.

What I Found Out:

The drop of food coloring split into many long streamers in the hot water. They slowly moved around the jar as they sank toward the bottom. The streamers spread out through the water.
The drops of hot water went into the cold water a little bit then rose to the top. The layer of hot water spread across the cold water. The color was darker on the top of the hot water layer.
I stopped adding hot water when my layer was 1.5 cm thick. Gradually a layer of lighter blue water spread under the hot layer. It got almost as thick as the hot layer started out. Streamers sank toward the bottom. A center core of blue went from the hot layer to the bottom. Then the blue spread all through the water.
The hot water must be less dense than the cold water because it stays on top. The densities must be very similar because I had to be so careful not to mix them when I put the hot water in and a layer at the edge does mix a little.
As the jar sits on the table, the hot water cools off. The cold water warms up. As they get closer together in temperature, their densities get closer and the hot and cold water mix. Then the food coloring will spread throughout the water.

OS5 Float a Jar

Some things float. Some things sink. Some are in between. Why?

Often heavier things sink and lighter ones float. Can mass be the reason?

materials to do project

Question: Why do some things float and others sink?


Big bucket of water

Small jar with lid



Put the lid on the jar tightly

Measure the jar’s height and diameter in centimeters

massing the jar

Mass the empty jar in grams.

Mass the jar in grams

Float the jar in the bucket of water

float the empty jar

The empty jar floats high in the bucket of water.

Take the jar out of the water

Pour about 1.5 cm water into the jar

Put the lid on tightly

Mass the jar and water

mass the jar and water

Each addition of water increases the mass of the jar and increases its density.

Float the jar in the water

Describe how well the jar floats

Add another 1.5 cm water to the jar, mass it and float it

Continue to do this until the jar sinks


Jar measurements:


Diameter ( biggest distance across)


Empty jar:

How well it floated

Jar with 1.5 cm water:

How well it floated

float the jar and water

The water in the jar tends to push the bottom into the water. It definitely doesn’t float as well as the empty jar.

Jar with 3 cm water:

How well it floated

Jar with 4.5 cm water

How well it floated

Jar with 6 cm water:

How well it floated

Jar with 7.5 cm water:

How well it floated

Jar with 9 cm water:

How well it floated

Calculate the volume of the jar (volume = πrrh where π is 3.14, r is half the diameter and squared, h is height)
Density is how much stuff is in a certain space. Calculate the density of the empty jar by dividing its mass by its volume. The units will be grams/cc or cubic centimeter.
Calculate the density of the jar with each addition of water.
There is another way you can use if your jar is not very square like mine with the top and bottom tapering. Fill your jar to the very brim with water and put the lid on tightly. Mass it. Subtract the mass of the empty jar. Water is supposed to have 1 gram equal to 1 cc so the answer should be close to the volume of the jar.
Water has a density of 1 g/1 cc.
Compare the density of each water level of jar to the density of water to how well the jar floated.
Does density show when something will float or sink? Why do you think so?
A ship is made of iron, a very heavy metal but floats. Why does the ship float?

What I Found Out:
My jar had a diameter of 6.8 cm so the radius was 3.4 cm and a height of 12 cm. This gave it a volume of 435.6 cc.
When I filled it with water and massed it, the mass was 736.5 g. The empty jar was 251.7 g so the volume by that method was 484.8 g or cc.
Since my jar had a mass of 251.7 g, the empty jar had a density of 0.6 g/cc. The jar floated on top of the water.
I did not measure the water very carefully but the mass was the important thing.
After adding the first amount of water, the mass went up to 332.9 g and the density went to 0.8 g/cc. The jar still floated sideways and high.

Each jar does float but lower in the water and upright

The jar floats upright so the water in the bucket is a little over the water level inside the jar.

The second amount of water increased the mass to 406.1 g raising the density to 0.9 g/cc. The jar still floated but with the bottom angled down into the water.
The third amount of water increased the mass to 443.6 g raising the density to 1.0 g/cc. The jar now floated upright in the water with most of the jar under water.
The fourth amount of water increased the mass to 482.4 g raising the density to 1.1 g/cc. The jar still floated but was much lower in the water.
The fifth amount of water increased the mass to 546.6 g raising the density to 1.3 g/cc.. The jar floated but only the lid was still above water.
The sixth amount of water increased the mass to 595.4 g raising the density to 1.4 g/cc. Now only the very top of the lid was above the water.
The seventh amount of water had a mass of 631.7 g raising the density to 1.5 g/cc. The jar sank immediately to the bottom of the bucket.
If I used the volume from filling the jar with water, the densities were less. They were: empty 0.5 g/cc; 1st 0.7 g/cc; 2nd 0.8 g/cc; 3rd 0.9 g/cc; 4th 1.0 g/cc; 5th 1.1 g/cc; 6th 1.2 g/cc; and 7th 1.3 g/cc.
When the jar floated each time, the water level outside was a little higher than on the inside. This difference increased a little each time.

almost full jar does not float

When the density is a little more than the density of water, the jar sinks.

The jar barely floated once the density of the jar was the same as the density of water. The density didn’t need to be much more than that of water before the jar sank.
For the jar density does seem to match to how well the jar floated. This might have been more striking if I had a more precise volume for my jar. I think density determines whether the jar sinks or floats because the jar floated with the air part above water and the full part below until the density got too much and it sank.
Ships work the same way. They have big rooms filled with air to make the ship’s density less than the density of water. This makes them work just like the jar.