CHEMISTRY 101: Classifying matter by state and composition
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CHEMISTRY 101: Classifying matter by state and composition

October 24, 2019

We’re going to study chemistry from the
perspective of an Atoms-First approach Using this perspective we have two fundamental postulates that we’ll
come back to again and again. The first is that
matter is particulate, or made of particles. Matter is anything that has a mass and takes up
space. The second postulate is that the structure of
those particles determines the properties of the matter as a
whole. Particles that are the building blocks of matter
we’ll call atoms. Two or more atoms can bond
together to form a molecule. And water is an example. Water is a particle
made up of two hydrogen atoms and one
oxygen atom. And our postulate would say that the three
dimensional arrangement of these three atoms which we will call the molecular geometry–that’s
something we’ll learn about when we discuss
VSEPR theory– helps determine the properties of water. The geometry of water is called bent making it a
polar molecule–again will learn more about this
a later– but it gives it particular properties. If water had a linear geometry like carbon
dioxide, which is dry ice, it might have a much lower boiling point and it
could possibly even be a gas at room
temperature. So the structure of water affects the boiling
point, which is a property of water. So in chemistry, we try to understand the properties of matter by
studying the particles that compose it and their
structure. When we classify matter we can talk about its
states, which is solid, liquid, or gas and we can also talk about its composition or
the type of particles that compose it. The state of matter depends on the relative
positions of the particles that compose it. So for solids the particles are close together in
fixed positions with respect one another and they simply vibrate while maintaining those
fixed positions. For liquid the particles can move with respect to
one another, but they stay relatively close. But for gases the particles are far away from
each other so there’s a great deal of empty
space between particles. So the state of matter depends on the relative
positions of particles that compose it and also the forces between the particles
relative to temperature. We can begin to explain some of the properties
of each state given the relative positions of the
particles. For example solids have a definite volume and
a definite shape because the particles are in fixed positions with
respect to one another. Liquids on the other hand have an indefinite
shape because the particles move with respect
to one another, but they still have a definite volume because the
particles remain relatively close to one another. And gases have an indefinite volume and
indefinite shape because they not only move
with respect to one another but they’re very far away from one another. They can take the shape of the container they’re
in. This is also why solids are not compressible, but gases are compressible–those molecules
have lots of space between them in gases. We can also classify matter with respect to
composition. Matter can be classified as either
a pure substance or mixture. And then we can further classify pure
substances as either element of compound and mixtures as either heterogeneous are
homogeneous. Pure substances are composed of only one
type of particle. For example, if the Goodyear blimp is filled with
helium gas, helium exists a single atom. One type of particle–it’s a Noble Gas. So
helium gas is just composed of these helium
atoms. Water is also a pure substance. Water is made up of water molecules, which are
all the same type of particle. Pure substances can be further classified as
either element or compound. One difference between helium and water is that
helium is an element. Elements are the simplest building block
chemically for matter. Whereas water is considered a compound, and
compounds contain two or more different
elements. Mixtures contain two or more different particles. Examples would be mixtures of sand and water
or a cup of hot tea. Homogeneous mixtures are uniform throughout. Our cup of hot tea is a homogeneous mixture. If I took just a teaspoon out of my cup of hot tea
it would have the same composition as the rest
of the cup. Not so with sand water. Sand water does not have a uniform
composition, so it’s a heterogeneous mixture. There are clear areas in my mixture of sand and
water that have more sand or more water. Another example of a heterogeneous mixture
would be beef stew. A spoonful of beef stew might have a chunk of
beef and the next spoonful might have
vegetables. So these are some ways we can classify matter
by state and composition.

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