Classify pure substances, mixtures, elements, compounds, homogeneous mixtures, and heterogeneous mixtures. Distinguish physical and chemical properties and changes.
A lesson of Chemistry, in Matter, Measurement, and Chemical Thinking, about 35 minutes.
By the end of this lesson, you will be able to place any sample on the classification tree, tell a compound from a mixture by how it separates, and distinguish physical from chemical properties and changes.
One Question Splits Everything
$$\textbf{Can it be separated by physical means?}$$
That single question is the top of the tree, and every other distinction follows
from it.
No, it cannot → it is a pure substance, with fixed composition throughout.
Yes, it can → it is a mixture, made of substances that kept their own
identities.
Pure substances then split again:
Element: one kind of atom. Iron, oxygen, helium. Cannot be broken down
chemically at all
Compound: two or more elements chemically BONDED in a fixed ratio. Water is
always $\mathrm{H_2O}$, never $\mathrm{H_3O}$
And mixtures split by how uniform they are:
Homogeneous: the same throughout, with no visible boundaries. Salt water,
air, steel. Also called a solution
Heterogeneous: visibly different regions. Sand in water, granite, salad
Compound Against Mixture: the Real Distinction
This is the pair people confuse, and three differences separate them completely.
$$\text{water} \quad \mathrm{H_2O} \quad \text{vs} \quad \text{hydrogen gas mixed with oxygen gas}$$
Fixed ratio. A compound has one, always. A mixture can be any proportion: salt
water is salt water whether it is slightly salty or nearly saturated.
New properties. A compound behaves like neither ingredient. Sodium is a metal
that explodes in water; chlorine is a poisonous green gas; sodium chloride is
table salt. A mixture keeps the properties of its parts, which is why you can
still taste the salt in salt water.
How it separates. A mixture separates physically: filter it, evaporate it,
use a magnet. A compound needs a chemical reaction, because breaking it means
breaking bonds. Electricity splits water into hydrogen and oxygen; no filter ever
will.
That last one is the practical test. If a physical process separates it, it
was a mixture the whole time.
A clear liquid. Boiling it away leaves a white solid behind.
Something was left behind, so was it a pure substance or a mixture? Type one word mixture
Boiling is a physical process. Did separating it require a chemical reaction? Type yes or no no
Before boiling it looked the same throughout. Was it homogeneous or heterogeneous? Type one word homogeneous
How many separate substances were in it, at least? 2
Separation Techniques and What They Exploit
Each method exploits one physical difference, and choosing correctly means asking
which property differs.
Method
Exploits
Example
filtration
particle size
sand from water
evaporation
boiling point, keeps solid
salt from water
distillation
boiling point, keeps liquid
alcohol from water
magnetism
magnetic response
iron from sand
chromatography
attraction to a surface
dyes in ink
decanting
density
oil from water
Evaporation keeps the solid and distillation keeps the liquid, which is the
difference students most often lose. Boiling salt water in an open dish gives you
the salt; condensing the steam gives you the water.
Nothing here is a chemical process. Every one of these leaves the substances
themselves unchanged, which is precisely why they only work on mixtures.
Sort It: Which Separation Method
Physical and Chemical Properties
A physical property can be measured without changing what the substance is:
color, density, melting point, boiling point, hardness, conductivity, solubility.
A chemical property describes how it reacts, and testing it necessarily
changes the substance: flammability, reactivity with acid, corrosion, toxicity.
Testing a chemical property destroys the sample, and that is the cleanest way
to tell the two apart. Measuring iron's density leaves you with iron; testing
whether iron rusts leaves you with rust.
Intensive properties do not depend on how much you have: density, color,
melting point. A gram of gold and a kilogram of gold have the same density.
Extensive properties do: mass, volume, length.
Intensive properties are what identify a substance, which is why density and
melting point appear in every reference table and mass never does.
Physical and Chemical Changes
A physical change alters form but not identity. Ice melting is still
$\mathrm{H_2O}$; dissolving sugar leaves sugar molecules intact; cutting paper
gives smaller paper.
A chemical change produces a new substance with new properties, because bonds
were broken and new ones formed.
Four signs to look for:
Color change with no dye added
A gas produced, when nothing was boiling
A precipitate, a solid forming out of two clear solutions
Heat or light released or absorbed unexpectedly
None of these is conclusive on its own, which is worth being honest about.
Boiling water makes bubbles and is physical. A dissolving salt can absorb heat
and is physical. The decisive question is the same one as before: is it
reversible by physical means? Frozen water melts back; burned paper does not
unburn.
Sort It: Physical or Chemical Change
Worked Example 1: Compound or Mixture
A gray powder. A magnet pulls some of it out, leaving yellow powder.
Worked Example 2: Property or Change
Worked Example 3: Reading the Signs
Two clear liquids are mixed. A white solid appears and the beaker feels warm.
Common Mistakes
Mistake 1: Calling a homogeneous mixture a compound
Salt water looks uniform and separates by evaporation. Mixture.
Mistake 2: Confusing evaporation with distillation
Evaporation keeps the solid; distillation keeps the liquid.
Mistake 3: Treating bubbles as proof of a chemical change
Boiling makes bubbles and is physical.
Mistake 4: Thinking dissolving is chemical
The sugar molecules are unchanged and recoverable.
Mistake 5: Calling mass an identifying property
Mass is extensive. Density and melting point identify a substance.
Where This Goes Next
The physical-chemical line is genuinely useful and genuinely blurry, and chemists
are comfortable with that.
Dissolving salt is called physical, because evaporation recovers it. But the
ionic bonds in $\mathrm{NaCl}$ really do break, and $\mathrm{Na^+}$ and
$\mathrm{Cl^-}$ ions really do become surrounded by water molecules. Bonds broke
and new attractions formed, which is the definition of a chemical change. It is
classified as physical only because it reverses.
Alloys are stranger still. Steel is called a mixture, but the carbon atoms sit
in specific positions in the iron lattice and change its properties completely.
It is not a compound, because the ratio varies, and it is not really a simple
mixture either.
Plasma breaks the states-of-matter scheme, and it is the most common state in
the universe by a wide margin.
The lesson underneath is worth carrying: scientific categories are useful tools
rather than facts about nature. They work well in the middle of their range and
get argued over at the edges, and the edges are usually where the interesting
chemistry is. Knowing a boundary case is a boundary case is a more advanced
understanding than being able to sort the easy ones.
Check Your Understanding
Five of them, no instructions this time.
Summary
One question splits everything: can physical means separate it? No means pure
substance, yes means mixture.
Elements have one kind of atom; compounds are bonded in a fixed ratio with new
properties.
Homogeneous means uniform throughout, and looking uniform is not evidence of
purity.
Each separation method exploits one physical difference. Evaporation keeps the
solid, distillation keeps the liquid.
A chemical change makes a new substance and is not reversible physically. The
four signs are suggestive, never conclusive.