Classifying Pure Substances and Mixtures

Welcome. This course begins with a review of matter and chemical change, then moves into measurement, motion, forces, and energy. In this first lesson, you will build a reliable way to decide whether a sample is a pure substance or a mixture, using either a written description of its composition or a particle diagram.

The central idea is simple but important: a sample can contain more than one type of atom and still be pure. What matters is whether the sample contains one chemical substance only or two or more substances combined.

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The two big categories of matter

A pure substance has a fixed, definite composition. Every part of a pure sample is made of the same chemical substance.

A pure substance can be either:

• an element, made of one type of atom, such as pure copper;
• a compound, made of different types of atoms chemically bonded in a fixed arrangement, such as pure water.

A mixture contains two or more different substances together. Its composition can vary: one sample of saltwater may contain much more salt than another, but both are still saltwater mixtures.

Classification	What is present?	Can the composition vary?	Example
Pure element	One kind of atom	No	Copper
Pure compound	One kind of compound particle or repeating structure	No	Pure water
Mixture	Two or more substances together	Yes	Saltwater, air, soil

The word pure does not mean “made from one kind of atom.” Pure water contains hydrogen and oxygen atoms, but every water particle has the same composition. It is therefore a pure substance.

By contrast, a container holding water and dissolved salt contains two substances: water and sodium chloride. Even if it looks completely clear, it is a mixture.

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A quick visual introduction

Particle diagrams are models, not photographs. In a particular diagram:

• Different colours, sizes, or patterns usually represent different elements.
• Spheres drawn together usually represent atoms that are chemically bonded.
• A group of bonded atoms is one particle, often a molecule.
• Empty space between groups means the groups are separate particles; it does not automatically mean the sample is a mixture.

The key task is to take an inventory: How many distinct kinds of particles are present?

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Reading a particle diagram as an inventory

A particle diagram represents a pure substance when it shows only one kind of particle repeated throughout.

That repeated particle could be:

1. Single identical atoms
   This represents a pure element.

2. Identical groups made from the same kind of atom
   This can still represent a pure element. For example, some elements occur naturally as pairs of identical bonded atoms.

3. Identical bonded groups containing two or more atom types
   This represents a pure compound. Although there are different atoms within each group, all the groups are the same compound particle.

A particle diagram represents a mixture when it contains more than one kind of particle. The different particles may be:

• two different elements;
• an element and a compound;
• two different compounds.

A useful rule is:

One repeated particle type means a pure substance. More than one particle type means a mixture.

Notice what this rule does not say. It does not say that a diagram with two colours must be a mixture. If every particle is the same bonded red-and-white group, it is a pure compound. Nor does it say that a sample is pure merely because its particles are evenly spread out; some mixtures, such as air or saltwater, are evenly distributed too.

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Applying the rule to four diagrams

Let’s classify each box in the Four particle diagrams image carefully.

Diagram i: pure substance

Every particle is a single teal sphere. There is only one type of atom and therefore only one type of particle.

Classification: pure substance, specifically an element.

The particles are spread apart, which may suggest a gas in this model, but spacing is not what determines whether it is pure. The deciding evidence is that all particles are identical.

Diagram ii: pure substance

Each particle has one large green sphere bonded to several smaller orange spheres. There are two types of atom, but every bonded cluster is identical to every other cluster.

Classification: pure substance, specifically a compound.

This is the most important case to recognize. Different atom types do not automatically make a mixture. Because the atoms are chemically joined in one repeated arrangement, the entire sample is one compound.

Diagram iii: mixture

There are teal particles in one region and orange particles in another. They are separate particle types, not bonded into identical combined groups.

Classification: mixture.

The substances are unevenly distributed. The later lesson on homogeneous and heterogeneous mixtures will give this pattern a more specific name, but for now the main classification is simply mixture.

Diagram iv: mixture

There are teal particles and pink particles. They are spread throughout the same container, but they remain separate particles rather than becoming identical bonded units.

Classification: mixture.

This diagram looks uniform, unlike diagram iii, yet it is still a mixture because it contains two different substances. Uniform appearance is not enough to make a sample pure.

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Composition gives the same answer

In a written problem, you may not receive a particle diagram. Instead, look for statements about what the sample contains.

A sample is likely a pure substance when its description says it contains only one element or one compound:

• “The sample contains only copper atoms.”
  Pure substance: an element.

• “The sample is distilled water containing only water particles.”
  Pure substance: a compound.

• “The solid consists only of sodium chloride.”
  Pure substance: a compound.

A sample is a mixture when its description identifies two or more substances:

• “A solution contains water and salt.”
  Mixture.

• “The gas contains nitrogen, oxygen, and carbon dioxide.”
  Mixture.

• “The label says 70% isopropyl alcohol and 30% water.”
  Mixture.

Percentages are especially strong evidence for a mixture: they show that the relative amounts can be changed. A bottle that is 70% alcohol has a different composition from one that is 90% alcohol, even though both contain alcohol and water.

Physical separation is another helpful clue. Components of a mixture can often be separated without changing them chemically. For example, water can evaporate from saltwater, leaving salt behind. In a pure compound, the elements are chemically combined, so separating them requires a chemical reaction rather than an ordinary physical method.

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A dependable classification routine

When you see a particle diagram, use this sequence:

1. Identify the particles.
   Decide which spheres or bonded clusters count as one particle.

2. Compare the particles.
   Check whether every particle has the same structure, including the same atom types and arrangement.

3. Classify the sample.

   • One repeated particle type: pure substance.
   • More than one particle type: mixture.

4. Add supporting detail when possible.
   A pure substance made of one atom type is an element. A pure substance made of different bonded atom types is a compound.

For a written description, use the same logic:

1. List the substances named in the description.
2. If only one chemical substance is present, classify it as pure.
3. If two or more substances are present, classify it as a mixture.
4. Use evidence from the wording, such as percentages, ingredients, variable composition, or physical separation.

A strong scientific justification names the classification and the evidence:

“This sample is a mixture because the diagram shows two distinct kinds of unbonded particles.”

or

“This sample is a pure substance because every particle shown is the same bonded group of atoms.”

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Key takeaways

• A pure substance contains only one chemical substance and has a fixed composition.
• Pure substances include both elements and compounds.
• A mixture contains two or more substances that are not chemically bonded into one repeated particle type.
• In particle diagrams, focus on the number of different particle types, not simply the number of colours or how evenly particles are distributed.
• Identical bonded groups containing different atoms represent a pure compound, not a mixture.

Next, you will sharpen the distinction inside the pure-substance category: telling elements apart from compounds using particle arrangements and chemical formulas.