Floating and sinking are both things that happen because of density! Density is a measurement of how heavy a certain volume is.

Let's say you have two balloons: one balloon is filled with the air from your breath, the other is filled with water, and they are both the same size. Which balloon do you think weighs more? The one filled with air or the one filled with water?

I'm sure you can tell that the one filled with water is heavier. Because of the difference in weight, we can say that the water-filled balloon is more dense.

But we need to look at what density depends on, that is, what traits change density? Does color affect density? What about temperature or pressure? (We'll come back to this later)

Density is equal to the mass of an object divided by the volume of the object. For example, if you know that a nickel (5 cent piece) weighs 5 grams, and has the dimensions: 21.21 millimeters in diameter and 1.95 millimeters in height (Wikipedia), we can calculate the density using basic geometry (if you need help, search volume of a cylinder). We obtain the density of a nickel: 7.257 grams per milliliter. For reference, pure water (near the freezing temperature) has a density of 1.00 grams per milliliter. Nickels have a higher density, meaning a nickel weighs more than the same volume of water, and we know that when we drop a nickel into a glass of water, it sinks.

So it all comes down to comparing densities. As we saw with the nickel and water example, since the nickel is more dense than water, it will sink. If we place a cork in water, however, it will float. This is because the density of a cork is about 0.25 grams per milliliter, which is less than the density of water.

Now back to your original question: can water float on water? Yes! This has to do with the way density changes based on outside conditions. Say for instance you have two glasses of water: one at 200 degrees Fahrenheit (hot hot hot!) and one at 35 degrees Fahrenheit (cold cold cold!). If you gently combine the two glasses into a bigger glass, you will find that the warmer water will rise to the top, and the cooler water will sink to the bottom. The same thing happens in a room with tall ceilings! Hotter air rises to the top and cooler air sinks to the bottom.

Earlier we asked: "What things affect density?" In the case above, we are looking at how temperature affects density. For most substances (air, water, metals, etc.) if temperature increases, density will decrease. So, if we have water at 200˚F and water at 35˚F, the water at 200˚F will be less dense than the water at 35˚F, and it will float on the top!

There are other things that affect density, like pressure and dissolved things (like salt and sugar in water!) Density can be complex, and knowing how it changes with changing conditions is very important for scientists to know. Thankfully we can look up the density of most things, and that allows us to know what will float on what.

For practice, look up the density of the following substances (at room temperature). Then determine which sink and float:

1. Lead in pure water
2. Lead in mercury
3. Butter in pure water
4. Cork in butter
5. Sugar water in pure water
6. Dead Sea water in pure water