# What is Chemistry?

> Matter, mass and weight, how science builds knowledge, the branches of chemistry and where it all came from.

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

### Story time: The kitchen laboratory
Amira's mum is baking bread. She mixes flour, water, salt and a spoonful of yeast. An hour later the dough has doubled in size. In the oven it turns golden brown and the whole house smells amazing.

"How did it grow?" asks Amira.

"The yeast eats sugar and gives off a gas," says her mum. "The gas makes bubbles in the dough. Then the heat changes the outside of the bread so it turns brown."

Without knowing it, Amira's mum has just described three chemical reactions. Chemistry is not only something that happens in labs with test tubes. It happens in your kitchen, inside your body and in every car engine. This chapter explains what chemistry studies and how chemists find things out.

**In short:** Chemistry is the study of matter and how it changes. Matter is anything that has mass and takes up space, so heat, light and sound are not matter, and chemists study it through experiments that give qualitative (word) and quantitative (number) data.

## Chemistry is the study of matter

**Chemistry** is the study of **matter**: what it is made of, how it behaves, and how it changes.

**Matter** is anything that **has mass and takes up space** (has volume). Air, water, your desk and your body are all matter.

Some things are **not** matter, because they have no mass and take up no space: **heat, light, sound** and **thoughts**. These are forms of **energy** or ideas.

Everything made of matter is built from tiny particles called **atoms**. A **chemical** is any substance with a fixed make-up, such as water, salt or oxygen. Everything you can touch is made of chemicals.

## Mass and weight

These two words are often mixed up in everyday life, but in science they mean different things.

| | Mass | Weight |
|---|---|---|
| What it is | The **amount of matter** in an object | The **pull of gravity** on that matter |
| Unit | kilogram (kg) or gram (g) | newton (N) |
| Measured with | A balance | A spring scale |
| Changes with location? | **No** | **Yes** |

An astronaut with a mass of 70 kg has a mass of 70 kg on Earth, on the Moon and in space. On the Moon, gravity is weaker, so her **weight** is only about one sixth of her weight on Earth.

## What chemists look at

Chemists ask four kinds of questions about matter.

| Area | Question | Example |
|---|---|---|
| **Composition** | What is it made of, and in what amounts? | Water is 2 hydrogen atoms to 1 oxygen atom |
| **Structure** | How are the atoms arranged in space? | Diamond and graphite are both carbon, arranged differently |
| **Properties** | What is it like? | Copper is shiny, bendy and conducts electricity |
| **Reactions** | What happens when it meets other chemicals? | Iron rusts in wet air |

## How science builds knowledge

Chemists collect two kinds of data.

- **Qualitative data** describes qualities using words: "the solution turned blue", "it smells like rotten eggs".
- **Quantitative data** uses numbers and units: "the mass is 4.2 g", "the temperature rose by 12 °C".

Over time, observations grow into bigger ideas.

| Word | Meaning | Example |
|---|---|---|
| **Hypothesis** | A testable prediction | "If I heat the water, more sugar will dissolve" |
| **Model** | A picture, diagram or idea that explains something we cannot see directly | The Bohr model of the atom |
| **Theory** | A well-tested explanation of **why** things happen, supported by lots of evidence. It can change if new evidence appears | Atomic theory, kinetic particle theory |
| **Law** | A statement of **what** always happens, often as an equation. It does not explain why | The law of conservation of mass |

### Watch out: "It's just a theory"
In everyday speech, "theory" means a guess. In science, a theory is the opposite: an explanation backed by a mountain of evidence. It is still open to change if new evidence disagrees with it.

## Research and technology

- **Pure research** seeks knowledge for its own sake. "What elements are in this new orange?"
- **Applied research** solves a specific problem. "Can a chemical from this orange shorten a cold?"
- **Technology** is the practical use of scientific knowledge to make products or tools. A drought-proof orange tree made by changing its genes is technology.

## The branches of chemistry

| Branch | Studies | Example |
|---|---|---|
| **Organic** | Compounds that contain carbon (the chemistry of living things, fuels and plastics) | Making a new medicine |
| **Inorganic** | Substances that mostly do not contain carbon | Metals, minerals, salts |
| **Physical** | Energy, rates of reaction and how matter behaves | How fast a battery discharges |
| **Analytical** | What substances are present and how much | Testing water for lead |
| **Biochemistry** | Chemistry inside living organisms | How enzymes digest food |

## A very short history

- **Ancient times**: people used fire, made pottery, smelted metals and made dyes and medicines, without understanding why they worked.
- **About 2500 years ago, Greece**: philosophers such as Aristotle believed everything was made from four "elements": **earth, water, air and fire**. Democritus suggested matter was made of tiny particles he called *atomos*, meaning "cannot be cut". They used reasoning, not experiments.
- **The Middle Ages**: **alchemists** tried to turn cheap metals such as lead into gold and searched for an "elixir of life". Their work was secret and not tested properly, so it was a **pseudoscience** (false science). But they did invent useful lab equipment and methods.
- **From the 1600s**: scientists such as Robert Boyle began using the **scientific method**: careful experiments, measurements, and sharing results so others could check them. Modern chemistry was born.

**Mind map**

```text
What is chemistry?
  Matter
    Has mass and volume
    Made of atoms
    Not matter: heat, light, sound
  Mass vs weight
    Mass: amount of matter
    Weight: pull of gravity
  Science words
    Qualitative and quantitative
    Model, theory, law
    Pure vs applied research
  Branches
    Organic, inorganic
    Physical, analytical
    Biochemistry
  History
    Greek four elements
    Alchemy
    Scientific method
```

### Year 11: Laws, theories and the atom
The **law of conservation of mass** (Lavoisier, 1789) says what happens: mass is never gained or lost in a reaction. **Atomic theory** (Dalton, 1803) explains why: reactions only rearrange atoms, and atoms are not created or destroyed.

This is the normal pattern in science. A law is found from measurements first. A theory comes later to explain it. A theory is never "promoted" to a law, because they answer different questions: **what** versus **why**.

### Year 12: Models have limits
Every model leaves something out. The particle model treats atoms as hard balls, which works for gases but cannot explain bonding. The Bohr model explains hydrogen's spectrum but fails for bigger atoms.

Good scientists know **when a model is useful** and **where it breaks down**. In exams, when asked to "evaluate a model", give one thing it explains well, one thing it cannot explain, and the evidence that led to a better model.

## Quick summary

- **Chemistry** is the study of matter and how it changes.
- **Matter** has mass and takes up space. Heat, light and sound are not matter.
- **Mass** is the amount of matter (kg). **Weight** is the pull of gravity on it (N) and changes from place to place.
- **Qualitative** data uses words. **Quantitative** data uses numbers.
- A **theory** explains why. A **law** describes what always happens.
- Pure research seeks knowledge. Applied research solves problems. Technology uses science to make things.
- Five branches: organic, inorganic, physical, analytical, biochemistry.

## Quiz

Try these questions from the chapter quiz.

1. Chemistry is the study of:
   - A. Living things only
   - B. Matter and the changes it undergoes
   - C. Stars and planets
   - D. Forces and motion only
   - Answer: B. Matter and the changes it undergoes. Chemistry studies what matter is made of and how it changes.

2. Matter is anything that:
   - A. Can be seen
   - B. Has mass and takes up space
   - C. Gives off light
   - D. Is a solid
   - Answer: B. Has mass and takes up space. Air cannot be seen, but it is still matter because it has mass and volume.

3. Which of these is NOT matter?
   - A. Smoke
   - B. Air
   - C. Heat
   - D. Water
   - Answer: C. Heat. Heat is a form of energy. It has no mass.

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