Chemistry is all around you
When you brush your teeth, the fluoride in toothpaste protects your enamel. When you cook, burning natural gas releases heat. The lithium battery in your phone runs on a chemical reaction. Medicines, plastics, fertilizers, the fibers in your clothes: almost everything you can touch has chemistry behind it.
What does chemistry study?
Chemistry化学 is the science of the composition, structure, properties, and changes of matter, studied at the level of molecules and atoms.
| Focus | The question it asks | Example |
|---|---|---|
| Composition | What is a substance made of? | Water is made of hydrogen and oxygen |
| Structure | How are the particles arranged and joined? | Diamond and graphite are both carbon, arranged differently |
| Properties | What is the substance like? What changes can it undergo? | Alcohol catches fire easily and evaporates quickly |
| Changes | How does one substance become another, and what happens to energy? | Iron rusting; natural gas burning and giving off heat |
Chemistry also creates new substances. Many of today’s medicines, plastics, and synthetic fibers were made from scratch by chemists.
Three levels for looking at matter
Chemists often think about the same event on three levels at once:
- Macroscopic: what you can see or measure, such as bubbles, color changes, or heat.
- Particulate: what the invisible molecules and atoms are doing.
- Symbolic: how it’s written down with formulas and equations.
These three levels run through this whole site. Learning to switch between them is one of the most important habits in chemistry. Pick an example below:
Macroscopic: what you can see
Heated to 100 °C, water bubbles vigorously and turns into steam.
Particulate: what the particles do
Symbolic: how chemists write it
The water molecules themselves don't change; they just spread farther apart. This is a physical change.
Macroscopic: what you can see
Hydrogen burns quietly in air with a pale blue flame and forms water.
Particulate: what the particles do
Symbolic: how chemists write it
The molecules change: atoms rearrange into new molecules and a new substance forms. This is a chemical change.
Macroscopic: what you can see
Pass electricity through water and bubbles appear at both electrodes: hydrogen at one, oxygen at the other.
Particulate: what the particles do
Symbolic: how chemists write it
Water molecules break apart, and their atoms recombine into hydrogen and oxygen molecules.
Chemistry and the other sciences
Chemistry is often called “the central science.” It connects physics (energy and atomic structure), biology (the reactions of life), medicine (drugs), materials, the environment, and earth science. A solid grasp of chemistry helps with all the other sciences.
How to do well in chemistry
- Observe and experiment: chemistry is built on experiments.
- Keep asking “why?”: go from what you see to the particles you can’t.
- Learn the language as you go: element symbols, formulas, and equations.
- Use your math: unit conversions, ratios, and calculations come up constantly, and the Math Tool Cards on this site will help.
- Which elements is water made of?
It asks what the substance is made of.
- Diamond and graphite are both made of carbon atoms. Why is one so hard and the other so soft?
Same atoms, different arrangement: that's structure.
- Does alcohol catch fire easily? What is its boiling point?
These are features of the substance itself: properties.
- What new substance forms when iron rusts?
It asks how one substance turns into another: a change.
- Which elements does table salt contain?
It asks what the substance is made of.
- How are the carbon and oxygen atoms connected in a carbon dioxide molecule?
How atoms are joined is a question of structure.
- Can oxygen support burning?
It's a feature of the substance itself: a property.
- How can carbon dioxide be turned into fuel?
It's about transforming substances: a change, and creating something new.
Common mistakes
Mistake 1: “Chemicals are harmful.” Everything is made of chemical elements, including water, oxygen, and food. Whether something is harmful depends on what it is and how much of it there is.
Mistake 2: “Natural is always better than synthetic.” Vitamin C made in a factory and vitamin C in fruit are the same substance, with exactly the same properties.
Mistake 3: “Chemistry only studies what you can see.” Chemistry cares most about molecules and atoms you can’t see, because they determine the properties you can.
Quick check
Which of these is NOT something chemistry studies?
Earth's orbit belongs to astronomy and physics. The other three all involve the composition, properties, or changes of substances, or making new ones.
Which describes boiling water at the particle level?
Boiling is a physical change. The water molecules stay the same; only the spaces between them grow.
Summary
- Chemistry studies the composition, structure, properties, and changes of matter, and creates new substances.
- Chemistry is everywhere: food, clothing, housing, transport, medicine, energy, and materials all depend on it.
- Get used to looking at chemistry on three levels: macroscopic, particulate, and symbolic.
What is “On to AP”? AP (Advanced Placement) is a program of college-level courses and exams run by the College Board in the United States; high school students who score well can earn college credit. Stage 3 of this site covers AP Chemistry. Every lesson ends with an “On to AP” box about how that lesson’s ideas show up in AP Chemistry. It’s written for later: skip it for now and come back when you reach Stage 3.
On to AP (for later)
AP Chemistry puts a lot of weight on moving between representations: describing what you observe, drawing a particle diagram, and writing the equation for the same process. That’s AP Science Practice 1, Models and Representations.