# Classifying Matter

> Pure substances and mixtures, elements and compounds, solutions, and physical and chemical properties.

From HSC Chemistry on WhizzNotes, free study notes for NSW students: https://whizznotes.com/chemistry/classifying-matter/. Updated 3 November 2026.
Year 11 and Year 12 boxes go deeper than the Year 10 text around them.

### Story time: Sorting the pantry
Lucas was told to tidy the pantry. He made three shelves.

On the first shelf he put things that were one single substance all the way through: sugar, salt, baking soda. On the second shelf went things that were obviously a mix of bits he could see: trail mix, muesli, a jar of chunky soup. On the third shelf were things that looked the same all through but were clearly made of more than one thing: cordial, vinegar, salty water.

His sister, a chemistry student, laughed. "You've just invented the chemist's way of sorting matter. Pure substances, heterogeneous mixtures and homogeneous mixtures."

**In short:** All matter is either a pure substance or a mixture. Pure substances are elements (one kind of atom) or compounds (elements chemically joined in a fixed ratio), while mixtures are physically combined in any ratio and can be separated by physical methods.

## The big picture

All matter can be sorted with a few questions.

*Diagram: Classification tree: matter splits into pure substances and mixtures. Pure substances split into elements and compounds. Mixtures split into homogeneous and heterogeneous.*
*Every sample of matter fits somewhere on this tree.*

## Pure substances

A **pure substance** has a **fixed composition**. Every part of it is exactly the same, and it has a fixed melting and boiling point.

- An **element** is a pure substance made of **only one type of atom**. It cannot be broken down into simpler substances by physical or chemical means. Examples: gold (Au), oxygen ($\mathrm{O_2}$), carbon (C).
- A **compound** is a pure substance made of **two or more different elements chemically joined** in a **fixed ratio**. Examples: water ($\mathrm{H_2O}$), table salt (NaCl), sugar ($\mathrm{C_{12}H_{22}O_{11}}$).

A compound can only be split into its elements by a **chemical reaction**. Passing electricity through water splits it into hydrogen and oxygen gas.

A compound behaves **nothing like** the elements it is made from. Sodium is an explosive metal and chlorine is a poisonous gas, but sodium chloride is the salt on your chips.

## Mixtures

A **mixture** contains two or more substances that are **physically combined**, not chemically joined. The amounts can vary, and each substance keeps its own properties. Mixtures can be separated by **physical** methods.

| Type | Meaning | Examples |
|---|---|---|
| **Homogeneous** mixture | The **same all the way through**. You cannot see the different parts. Also called a **solution** | Salt water, air, cordial, vinegar, brass and steel (solid solutions) |
| **Heterogeneous** mixture | **Not the same** all through. You can see the different parts | Soil, sand and water, trail mix, salad dressing, beef stew |

## Solutions

In a **solution**, one substance (the **solute**) is dissolved in another (the **solvent**).

$$\text{solute} + \text{solvent} \rightarrow \text{solution}$$

In salt water, salt is the solute and water is the solvent. Water dissolves so many things that it is called the **universal solvent**.

Solutions are not only liquids:

| Solute in solvent | Example |
|---|---|
| Gas in gas | Air (oxygen in nitrogen) |
| Gas in liquid | Fizzy drink (carbon dioxide in water) |
| Liquid in liquid | Vinegar (acetic acid in water) |
| Solid in liquid | Sugar in tea |
| Solid in solid | Brass (zinc in copper), an **alloy** |

### Watch out: Mixture or compound?
| Mixture | Compound |
|---|---|
| Physically combined | Chemically joined |
| Any ratio | Fixed ratio |
| Parts keep their properties | New properties |
| Separated by physical methods | Separated only by chemical reactions |

Air is a mixture of nitrogen, oxygen and other gases. Water is a compound.

## Properties of matter

A **property** is a feature that describes a substance.

- A **physical property** can be observed or measured **without changing what the substance is**: colour, smell, state, density, melting point, boiling point, hardness, conductivity, solubility.
- A **chemical property** describes how a substance **reacts** to become a new substance: flammability, reacting with acid, rusting, reacting with water.

"Water boils at 100 °C" is a physical property. "Sodium reacts violently with water" is a chemical property.

### Intensive and extensive properties

- An **intensive** property does **not** depend on how much you have: density, colour, melting point, boiling point. A gold ring and a gold bar have the same density.
- An **extensive** property **does** depend on the amount: mass, volume, length, total energy.

Intensive properties are useful for **identifying** a substance, because they are the same for any size sample.

### Exam tip: A quick test for purity
If a substance melts at one sharp, exact temperature, it is probably pure. If it melts over a range of temperatures, it is probably a mixture.

**Mind map**

```text
Classifying matter
  Pure substances
    Fixed composition
    Element: one type of atom
    Compound: elements joined
  Mixtures
    Physically combined
    Homogeneous: solution
    Heterogeneous: see parts
  Solutions
    Solute dissolves
    Solvent does the dissolving
    Water: universal solvent
  Properties
    Physical: no new substance
    Chemical: how it reacts
    Intensive: not amount
    Extensive: depends on amount
```

### Year 11: Colloids and suspensions
Between solutions and obvious mixtures there are two in-between types.

- A **suspension** has particles big enough to settle out if left to stand, such as muddy water or some medicines ("shake well before use"). It can be separated by filtering.
- A **colloid** has particles too small to settle and too small to filter out, but big enough to scatter light. Milk, fog, jelly and paint are colloids.

The **Tyndall effect** tells them apart from solutions: shine a torch through. A beam is visible in a colloid (like headlights in fog) but not in a true solution.

### Year 12: Law of definite proportions
A compound always contains the same elements in the same **proportions by mass**, no matter where it comes from. This is the **law of definite proportions** (Proust, 1799).

Pure water, whether it comes from rain, a lab or melted ice on Mars, is always about 11% hydrogen and 89% oxygen by mass:

$$\%\,\mathrm{H} = \frac{2 \times 1.0}{18.0} \times 100\% = 11.1\%$$
$$\%\,\mathrm{O} = \frac{16.0}{18.0} \times 100\% = 88.9\%$$

A mixture has no such rule: salt water can be 1% salt or 20% salt.

## Quick summary

- Matter is either a **pure substance** or a **mixture**.
- Pure substances are **elements** (one type of atom) or **compounds** (elements chemically joined in a fixed ratio).
- Mixtures are physically combined, can be in any ratio and can be separated physically.
- **Homogeneous** mixtures (solutions) look the same throughout. **Heterogeneous** mixtures do not.
- A solution is a **solute** dissolved in a **solvent**.
- **Physical** properties are observed without changing the substance. **Chemical** properties describe how it reacts.
- **Intensive** properties do not depend on amount. **Extensive** properties do.

## Quiz

Try these questions from the chapter quiz.

1. A pure substance made of only one type of atom is:
   - A. An element
   - B. A compound
   - C. A mixture
   - D. A solution
   - Answer: A. An element. Elements contain one kind of atom.

2. Which is a compound?
   - A. Oxygen gas
   - B. Air
   - C. Carbon dioxide
   - D. Salt water
   - Answer: C. Carbon dioxide. Carbon and oxygen are chemically joined in a fixed ratio.

3. Which is a heterogeneous mixture?
   - A. Salt water
   - B. Air
   - C. Soil
   - D. Sugar
   - Answer: C. Soil. You can see different parts in soil.

The whole chapter quiz is on the page, for signed-in students: https://whizznotes.com/chemistry/classifying-matter/#quiz