What is Chemistry?
Matter, Energy & the Science of Everything
Defining Chemistry
Chemistry is the branch of science that studies the composition, structure, properties, and transformations of matter. In practical terms, it asks: what is everything made of, and what happens when things interact?
Chemistry connects to physics at one end and flows into biology at the other, becoming the molecular language of life. This is why it is often called the central science.
Chemistry is the science of matter and the changes it undergoes. From the food you digest to the steel in a bridge, virtually every substance and process you encounter involves chemistry.
The scientific study of the composition, structure, properties, and reactions of matter — especially at the atomic and molecular scale.
Anything that has mass and takes up space. All physical substances — solids, liquids, gases — are matter.
Matter with a definite, uniform composition throughout. Substances are either pure elements or compounds.
A characteristic that describes matter. Can be physical (colour, density) or chemical (flammability, reactivity).
Matter
Matter is defined by two properties: it has mass (resists being moved) and it has volume (occupies space). Light and sound are not matter — they carry energy but have no mass.
Matter exists in different states, depending on particle arrangement and energy content.
Fixed shape and volume. Particles packed tightly in a regular arrangement, vibrating in place.
Fixed volume, variable shape. Particles close together but able to slide past each other.
No fixed shape or volume. Particles move freely at high speed, spreading to fill any container.
Super-heated ionised gas. Found in stars, lightning, and neon signs. Most common state in the universe.
A pure substance made of only one kind of atom. Cannot be broken down chemically. E.g. gold (Au), oxygen (O).
A pure substance formed when two or more elements chemically bond in a fixed ratio. E.g. water (H₂O), salt (NaCl).
Uniform composition throughout. Also called a solution. E.g. saltwater, air, steel.
Visibly different components, non-uniform composition. E.g. sand and water, granite, salad.
Physical properties can be observed without changing chemical identity: colour, melting point, density.
Chemical properties describe how a substance reacts: iron rusts in oxygen; wood burns in air. Observable only during a chemical change.
Energy
Every chemical reaction involves an exchange of energy. Reactions can release energy (burning wood) or absorb it (dissolving ammonium nitrate in water produces a cold sensation). Energy is the capacity to do work or produce heat.
Energy of motion. Atoms and molecules are always moving. Temperature is a measure of average particle kinetic energy.
Stored energy based on position or composition. Chemical potential energy is stored in bonds — released when fuels combust.
Total kinetic energy of all particles in a substance. Flows from hot objects to cold ones — governs many chemical reactions.
Energy stored in chemical bonds. Breaking and forming bonds exchanges energy with the surroundings — exothermic or endothermic.
Energy cannot be created or destroyed — only transformed. When petrol burns, chemical energy converts to heat and light. The total energy in the universe is unchanged.
Branches of Chemistry
Chemistry is a vast discipline. As you progress through this curriculum you will encounter each of these major branches.
- Organic
The chemistry of carbon-containing compounds. Includes molecules of life — proteins, DNA, carbohydrates — and pharmaceuticals, plastics, and fuels.
- Inorganic
Chemistry of elements and compounds not based on carbon. Covers metals, minerals, salts, and transition metals.
- Physical
Applies physics to chemical problems. Studies thermodynamics, kinetics, and quantum mechanics of matter.
- Analytical
Develops methods to identify and quantify substances using spectroscopy, chromatography, and titration.
- Biochemistry
Chemistry of living organisms — metabolic pathways, enzymes, genetic information, and the molecular basis of life.
- Nuclear
Concerned with atomic nuclei — radioactive decay, fission, fusion, and their applications in medicine and energy.
The Scientific Method
Chemistry is an experimental science. Knowledge is built through systematic observation and testing, not speculation. The scientific method is the framework that guides this process.
Notice a phenomenon. Gather data using senses or instruments — carefully and without bias.
Formulate a specific, testable question based on your observations.
Propose a possible explanation. Must be falsifiable — possible to prove wrong through testing.
Design and conduct a controlled test. Change one variable (independent), measure another (dependent), and hold all others constant (controlled).
Record and interpret results. Use tables, graphs, and calculations to find patterns in the data.
Does the data support or refute the hypothesis? Communicate findings. Science advances through repeated testing and peer review.
Describes an observed pattern in nature, usually mathematically. Describes what happens, not why. E.g. Law of Conservation of Mass.
A well-tested explanation for a broad set of observations, supported by overwhelming evidence. Explains why. E.g. Atomic Theory.
In everyday language, “theory” means a guess. In science, a theory is the highest standard of explanation — one that has survived rigorous testing across vast ranges of evidence. Scientific theories are never mere guesses.
Laboratory Safety
Before entering any laboratory, you must understand the rules that protect people from harm. Chemical reactions can be unpredictable, and many substances are corrosive, flammable, or toxic.
- Wear PPE at all times. Safety goggles, lab coat, and closed-toe shoes are non-negotiable when working with chemicals.
- Know the location of safety equipment. Fire extinguisher, eyewash station, emergency shower, and first aid kit — before you begin.
- Never smell chemicals directly. Use the wafting technique — fan vapours gently toward your nose from a distance.
- Read every label before use. Identify hazard symbols, handling instructions, and first aid measures.
- Handle glassware carefully. Never use cracked glassware. Use heat-resistant glass when heating.
- Dispose of chemicals properly. Never pour substances down a drain without checking the protocol.
- Tie back hair and loose clothing. Both can catch fire or dip into chemicals unexpectedly.
- Report all accidents immediately. Even minor incidents can escalate if ignored.
- Never work alone. Always have a partner or supervisor present during experiments.
Worked Examples
Approach: Is it a pure substance or a mixture? If pure — element or compound? If a mixture — homogeneous or heterogeneous?
Key test: Does the chemical identity of the substance change?
Practice Questions
Q1. Which of the following is a chemical property of iron?
Q2. Air is best described as a:
Q3. When ice melts into liquid water, this is best described as:
Q4. Matter is defined as anything that:
Q5. A scientific theory is best described as:
Key Takeaways
- Chemistry is the science of matter and the changes it undergoes — called “the central science” because it bridges physics and biology.
- Matter is anything with mass and volume. It exists in four states: solid, liquid, gas, and plasma.
- Matter is classified as pure substances (elements or compounds) or mixtures (homogeneous or heterogeneous).
- Physical properties are observed without changing chemical identity; chemical properties describe a substance’s reactivity.
- Energy exists as kinetic or potential. Chemical reactions always involve an exchange of energy with the surroundings.
- The scientific method — observe, question, hypothesise, experiment, analyse, conclude — is the foundation of all scientific knowledge.
- A scientific theory is a well-tested, evidence-supported explanation — far more than an everyday “guess.”
- Laboratory safety is non-negotiable: PPE, hazard awareness, and correct procedure at all times.