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
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
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