Floating and Sinking

Most kids love playing with water, and in hot weather water is a good thing to do science outdoors with. (Not to mention that the ocean is the theme in Primaria this year!) Discovering what sinks and what floats is a natural entry point for science because it is so simple that the youngest kids can appreciate it, yet it can lead to quite sophisticated concepts for the older ones who are ready to handle those.  Furthermore, I designed this activity to lead naturally up to a submarine-building activity I want to do next time.

I started with just a simple glass of water visible.  I asked each
child if they thought a wood chip would float or sink.  For me, this
next step is really important.  If the vote is not unanimous, I ask if
we can settle the issue just by counting the votes.  Science is not a
democracy!  We have to do the experiment and pay attention to the
results if we want to make any progress!  And if the vote is unanimous, I
ask them if maybe we don't need to do the experiment.  We agree
(sometimes with some nudging from me) that even if we all think it's
going to float, we should still do the experiment because sometimes we
could all be wrong in our predictions.  I really want to emphasize
these aspects of the scientific method as early as possible, and this
activity is a good place to do it.

Then I repeat with several objects, such as a stone, a marble, a piece
of plastic, a bolt, a paper clip, etc.  The kids have some idea that
lighter things are more likely to float, so the paper clip gives some
pause.  I try not to use the word "density" because this means nothing
to the pre-K/K kids, but I do try to summarize that floating/sinking
is expected for something that is light/heavy for its size, not just
light/heavy in some absolute sense. 

Then we get to the more interesting demo.  (Some of them desperately
want to play with this stuff already, but I promise they can play if
they pay attention for just a bit longer.)  I pull out a hard-boiled
egg and we see that it sinks.  But if I add plenty of salt to the
water, the egg begins to float.  This shows that the salt is mixing
with the water in a way which makes the water heavier.  (By the way,
floating an egg is apparently how people used to determine they had
added enough salt to their pickling solution when making pickles.)
Then we repeat the whole thing with sand.  Try as we might, the egg
does not float and the sand just collects at the bottom rather than
dissolving in the water.  Here we have the observational basis for
some chemistry: salt in water forms a solution, but sand in water does
not.  (I didn't state it this technically, but we did talk about how
ocean water behaves at the beach...the salt is an integral part of it,
as we can tell by its taste, but the sand is not.)  We also have the
idea of different kinds of mixtures, which ties in nicely with the
previous pre-K/K science activity.

Finally we get to the play time.  But this is serious play.  I bring
out one tub of water in which I place some aluminum-foil boats.
Although they are metal, they do not sink.  I challenge them to figure
out how to sink the boats. In parallel, a second tub contains empty
8-oz plastic soda bottles which I also challenge the children to sink.
The challenge aspect is really important.  They come up with the ideas
and try them out. It seems like play time, but it has a purpose.  This
particular challenge has the extra purpose that it builds up to the
future submarine activity.

With the foil, I have extra challenges ready for those who quickly
figure out how to sink the boats with stones.  I challenge them to sink
the foil just by crumpling it up into a ball.  It is surprisingly
difficult to do this; small air bubbles trapped in the foil are
surprisingly effective at floating it even after squeezing as hard as
possible.  Some of them easily recognize that air bubbles must be the
problem, while others need some hints.  The persistent ones finally
succeed in hammering out the air bubbles using anything vaguely
hammer-like.  Meanwhile, others have gone in a slightly different
direction, crumpling the foil around a stone so that it forms a ball
with high average density.

With the plastic bottles, students take one of two initial strategies:
filling the bottles with water, or with stones/sand.  Those who try
water see that water is not heavier than water, so that a waterlogged
plastic bottle still does not sink. Then they tend to start over with
stones/sand.  However, the stone/sand strategy is surprisingly
ineffective.  You can fill a bottle 1/4 full or even 1/2 or even 2/3 full of
stones/sand and it still doesn't sink.  There's just too much air in
the bottle.  However, few students have the patience (or the time left
in the activity) to fill the small-necked bottle completely with stones/sand.
They figure out (possibly with some hints) that they can
replace the bothersome air with water and finally get it to sink.
This is really good background for the submarine activity!

I think we spent 20 minutes with each group of about 5 kids, and that
was the perfect amount of time and the perfect size group.  Larger groups could be
accommodated with more tubs of water; more than 3 kids per tub would not be good.