Background boosters for elementary teachers
Q: What Is the Difference Between
Solids and Liquids?
By Bill Robertson
72 Science and Children
Brian Diskin
A:
This might seem like
a silly question. Don’t
we know the difference?
Liquids don’t have a definite
shape and take the shape of their
containers, and solids have a definite
shape and don’t take the shape of
their containers, right? Well, let’s
consider a few things. Place a drop
of water in a short glass. Does this
water take the shape of the glass?
Nope. It just sits there on the bottom
of the glass, rounded on the top. Do
all solids have a definite shape? Not
long ago, I backed into another car,
and that car’s door changed shape
rather easily. And if you want a really
confusing example, place a small
amount of cornstarch in the palm
of your hand and slowly add drops
of water, mixing the cornstarch and
water as you go. At some point,
you’ll get a substance that flows like
a liquid until you smack it really
hard, at which point it seems solid.
Clearly we have to be more careful in our definition of liquids and
solids than liquids taking the shape
of their containers and solids having a definite shape. First, though,
we can dispense with a myth about
glass actually being a liquid because
old churches and other buildings
have window glass that is thicker
at the bottom than at the top. I always heard that this was due to
the glass flowing downward, very
“Huh…The snow feels soft to me!”
slowly, over time. It turns out that
this variation in thickness is due to
how glass used to be manufactured.
It used to be quite difficult to get
sections of glass to be of uniform
thickness—they invariably were
thicker in some areas. When using
glass for windows, builders simply
placed the thicker parts of the glass
at the bottom for stability. So, glass
isn’t a liquid, but it is a special kind
of solid. There are two main categories of solids—crystalline and
amorphous. Crystalline solids have
a uniform structure throughout and
fit our initial definition that solids
have a definite shape. If you deform
crystalline solids slightly, they will
return to their original shape. If you
deform them too much, they’ll simply break apart. Diamonds, sodium
chloride (table salt) and other crystals, including metals, are good examples of crystalline solids. Amorphous solids don’t have a uniform
structure throughout, and most of
them can deform quite a bit without breaking. Rubber and plastic
are examples of amorphous solids,
and glass is also an amorphous solid.
Amorphous solids are the solids that
blur the distinction between solids
and liquids. These solids don’t have
a definite melting point. Instead, as
you heat them up, they gradually
soften and eventually act enough
like a liquid to be considered a liquid. Think Play Doh.
Because amorphous solids can
deform a lot without breaking apart,
they are often difficult to distinguish
from liquids. A great example of
this is the layer of the Earth, called
the asthenosphere, which is just underneath the crust. You might know
that the Earth’s crust is separated into
large plates that move slowly across
the Earth’s surface. Well, these plates
essentially “float” on the asthenosphere, and the contents of the asthenosphere undergo convection, which
is a circulation we normally associate
with liquids. So, is the asthenosphere
liquid? No, it’s considered an amorphous solid. Think Play Doh again!
Just for kicks, let’s confuse things
further. You can mix together solids
and liquids to create colloids, which
are just one state of matter suspended
in another. Foams are colloids consisting of a gas suspended in a liquid.
And your cornstarch and water is a
colloid that’s a solid (the cornstarch)
suspended in a liquid (the water). As
you know by playing with cornstarch
and water, colloids can have interesting properties. Cornstarch and water
acts like a liquid until you smack it
really hard. You can even walk on a
large tub filled with the stuff as long
as you don’t stop, in which case you’ll
sink into it (see NSTA Connection).
So, what’s going on with cornstarch
and water? Is it solid or liquid? It’s
both. If you move it slowly or let it
flow on its own, it’s a liquid—water containing cornstarch particles
throughout that are able to slide
across one another. If you hit it hard,
though, the cornstarch particles pile
up next to each other, and the water
between them is squeezed out, not
unlike packing snow
Figure 1.
together, piling up
the ice and squeezing out the water.
Key
With the particles of
cornstarch stacked
Water
up next to each othCornstarch
er, they can’t slide
across one another,
and the substance is
essentially a solid (see
Figure 1). Releasing
the pressure allows
the water back in between the cornstarch
particles, and it flows
like a liquid again.
As with many
With water interspersed in the corncategories created
starch, the cornstarch molecules can
by scientists, the
slip by one another.
distinction between
solids and liquids
is useful, but not
always clear-cut. I
think of it as being
somewhat like the
distinction between
living and nonliving. The distinction
is pretty clear until
When you squeeze the water out, the
you analyze it a bit
compacted cornstarch molecules act
and realize that there
are cases that aren’t
like a solid.
even close to simple,
such as whether or
not a virus is alive. And of course,
what about zombies? Guess science
NSTA Connection
doesn’t quite have all the answers. n
Visit the NSTA YouTube page at
bit.ly/YGHDAB to see a video of
Bill Robertson ([email protected]
people running on a pool of a
com.com) is the author of the
cornstarch and water mixture
NSTA Press book series, Stop Fak(oobleck).
ing It! Finally Understanding Science So You Can Teach It.
Summer 2013 73