Pure Substances and Mixtures

Is it really “pure”?

The air you breathe, the spring water you drink, and the stainless steel spoon you eat with each look like one single thing. Through a chemist’s eyes, though, each of them is several substances mixed together.

A word about “molecules”: This lesson uses the word molecule. For now, think of it this way: many substances, such as water, oxygen, and carbon dioxide, are made of extremely small molecules, and each molecule is made of even smaller atoms joined together. All the molecules of one substance are identical, and different substances have different molecules. A formula such as $\ce{H2O}$ shows which atoms make up one molecule: $\ce{H2O}$ is 2 hydrogen atoms and 1 oxygen atom, and $\ce{O2}$ is 2 oxygen atoms. Lesson 1.3.1 covers molecules and atoms in detail, and lesson 1.5.1 covers formulas.

A pure substance纯净物 is made of one substance only. Examples: oxygen, water, carbon dioxide, iron.
A mixture混合物 is made of two or more substances mixed together. Examples: air, seawater, spring water, milk, stainless steel.

	Pure substance	Mixture
What it contains	One substance	Two or more substances
Fixed composition?	Yes, and it can be written as one formula, such as $\ce{H2O}$	No: sugar water can be very sweet or barely sweet
Properties	Fixed properties, such as melting and boiling points	Each part keeps its own properties

What does “each part keeps its own properties” mean? The oxygen in air still supports burning, and the nitrogen is still unreactive. They’re simply mixed together; no chemical change has happened.

At the particle level: one substance, one kind of particle

In a substance made of molecules, all the molecules of that substance are identical, and different substances have different molecules. So:

A pure substance contains only one kind of molecule.
A mixture contains two or more kinds of molecules.

Count the kinds of molecules, not the kinds of atoms. A container holding both oxygen molecules, $\ce{O2}$ , and ozone molecules, $\ce{O3}$ , contains only oxygen atoms, but it holds two kinds of molecules and therefore two substances. It’s a mixture.

Each box below shows the particles in a sample. The small circles are atoms, and atoms joined together form one molecule. Count how many kinds of molecules each box holds, then decide whether it’s a pure substance or a mixture:

Practice: is each box a pure substance or a mixture?

HHOHHOHHOHHOHHOHHO
NNNNOOOONNNN
OOOOOOOOOO
HHOOHHOOOOHH
OOCOOCOOCOOCOOC
OOOOOOOOOOOO
HHOHHOHOOHHOOHHHO
HeHeHeHeHeHe

Homogeneous and heterogeneous mixtures

Mixtures come in two kinds:

A homogeneous mixture均匀混合物 has the same composition throughout, with no visible boundaries: salt water, sugar water, air. A homogeneous mixture is also called a solution (lesson 1.7.4).
A heterogeneous mixture不均匀混合物 varies from place to place, and you can see different parts: muddy water, sand mixed with iron filings, oil and water.

Example 1: Classifying everyday materials

Is each one a pure substance or a mixture? (1) Ice floating in water; (2) clean, fresh air; (3) liquid oxygen; (4) spring water.

Step-by-step solution

(1) Ice in water. Ice and liquid water are two states of the same substance, $\ce{H2O}$ . There’s only one substance: pure substance.
(2) Clean, fresh air. “Clean” only means there’s no pollution. Air still contains nitrogen, oxygen, argon, carbon dioxide, and more: mixture.
(3) Liquid oxygen. It’s oxygen in its liquid state, one substance only: pure substance.
(4) Spring water. Besides water, it contains various dissolved minerals: mixture.

Example 2: One kind of atom means pure, right?

A container holds a mixture of oxygen gas and ozone ( $\ce{O3}$ ). It contains only oxygen atoms. Is it a pure substance?

Step-by-step solution

Look at the kinds of molecules. The container holds $\ce{O2}$ molecules and $\ce{O3}$ molecules: two kinds.
Match them to substances. Two kinds of molecules means two substances, oxygen and ozone, and they have different properties (ozone, for example, has a sharp smell).
Conclusion. It’s a mixture. To classify a sample, count the kinds of substances (molecules), not just the kinds of atoms.

Practice: pure substance or mixture?

Ice cubes in water
Clean, fresh air
Liquid oxygen
Seawater
Distilled water
Stainless steel
Dry ice
Vinegar
Carbon dioxide

Common mistakes

Mistake 1: Judging by the name or the label. “Ice water” sounds like a mix, but it’s one substance. “Clean, fresh air” sounds pure, but it’s a mixture. Count the substances inside.

Mistake 2: “The parts of a mixture react with each other.” Mixing just puts substances together. None of them turns into a new substance, and each keeps its own properties.

Mistake 3: “Anything made of one kind of atom is a pure substance.” A mix of oxygen and ozone contains only oxygen atoms, but it’s a mixture. Count the kinds of molecules (substances).

Quick check

Which of these is a pure substance?

Which statement about mixtures is correct?

Summary

A pure substance contains one substance, has a fixed composition and fixed properties, and can be written as one formula.
A mixture contains two or more substances, its composition can vary, and each part keeps its own properties.
For substances made of molecules: one kind of molecule means a pure substance; more than one kind means a mixture.
Classify by the kinds of substances (molecules), not by the name, and not just by the kinds of atoms.

On to AP (for later)

AP topic 1.4, Composition of Mixtures, uses particle diagrams and mass percent to describe mixtures and stresses that a pure substance has a fixed composition while a mixture’s composition can vary. Topics 3.7 and 3.8 take a closer look at the most important homogeneous mixtures, solutions, and how to draw particle diagrams of them.

Pure Substances and Mixtures纯净物与混合物

Learning goals

Before you start

Is it really “pure”?