1.1 Matter
PRE-HEALTH PREP
Matter Explained
Elements, Compounds & Mixtures
General Chemistry 1.1
Matter is defined as anything possessing mass and volume, and it is composed of approximately 118 types of atoms known as elements. The classification of matter encompasses pure substances—elements and compounds—as well as mixtures, which are further divided into homogeneous and heterogeneous categories based on compositional uniformity. Properties of matter are examined through two fundamental distinctions: chemical versus physical properties, which differentiate changes that produce new substances from those that do not, and intensive versus extensive properties, which distinguish sample-size-independent characteristics from sample-size-dependent ones.
Table of Contents
What Is Matter?
Matter is anything that has mass and occupies space. Those two criteria — mass and volume — are the defining characteristics of matter, and chemistry, at its core, is the study of matter.
Matter is made up of incredibly tiny particles called atoms. The concept traces back to ancient Greek philosophers who spoke of atomos — indivisible building blocks of the physical world. While our modern understanding of the atom differs substantially from theirs, we have kept their word and built on their intuition for over two centuries.
Crucially, not all atoms are alike. There are approximately 118 different types of atoms, and each distinct type is called an element. Everything visible in the universe is composed of atoms drawn from this set of 118 elements. Learning to classify matter — as an element, compound, or mixture — is one of the first skills you will develop in general chemistry.
The 3 Phases of Matter
Solid, Liquid, and Gas
Matter exists in three principal phases, or states of matter: solid, liquid, and gas. (Yes, plasma is a fourth phase — but it falls outside the scope of a standard general chemistry course and will not appear on your exams.)
| Phase | Definite Shape? | Definite Volume? |
|---|---|---|
| Solid | Yes | Yes |
| Liquid | No | Yes |
| Gas | No | No |
A solid has both a definite shape and a definite volume. A liquid conforms to the shape of whatever container holds it, but its volume remains constant. A gas expands to fill any container and can be compressed into a smaller one — it has neither a definite shape nor a definite volume.
The Atomic View of Each Phase
The macroscopic behavior of each phase reflects what is happening at the atomic level.
In most solids — called crystalline solids — atoms (or molecules) are arranged in a precise, repeating pattern known as a crystal structure. These particles are essentially locked in place, with very little translational motion. Some solids are amorphous (glass is a classic example), meaning their atomic arrangement is disordered rather than periodic; these will be covered in a later chapter.
In a liquid, the particles are in contact with one another but are free to flow past each other. There is no long-range order — the arrangement is random and constantly changing.
In a gas, the particles are widely separated and move rapidly, with essentially no interaction between them. Gas-phase particles travel faster, on average, than their counterparts in the liquid phase.
Example: Visualizing the 3 States of Matter
Picture water in its three states. Ice (solid) has a rigid, crystalline structure — every water molecule holds its position. Liquid water has molecules packed closely together but tumbling freely past one another. Steam (gas) consists of widely dispersed, fast-moving molecules that fill whatever space they occupy.
Elements and Compounds: How Matter Is Classified
What Is an Element?
An element is a pure substance composed of only one type of atom. The periodic table organizes all 118 known elements, each represented by a chemical symbol — Na for sodium, C for carbon, and so on.
Be aware that the word "element" carries two related but distinct meanings in chemistry. It can refer to a specific type of atom (e.g., "the element sodium"), or it can refer to a pure substance made entirely of that one type of atom (e.g., sodium metal). Context will usually make the meaning clear.
What Is a Compound?
A compound is a substance composed of two or more different elements chemically bonded together in a fixed ratio. Compounds are a key category when you classify matter in general chemistry.
Example: Sodium Metal vs. Sodium Chloride
Sodium metal is an element: every atom present is sodium (Na). It is a solid at room temperature, and it is an example of a pure element.
Sodium chloride (table salt, NaCl) is a compound: it contains sodium and chlorine atoms bonded together in a one-to-one ratio. Even though it is a single, uniform substance, it is built from two different types of atoms — which makes it a compound, not an element.
The 7 Diatomic Elements
Most elements exist as individual atoms in their pure form, but seven elements naturally exist as diatomic molecules — pairs of two atoms bonded together. These seven diatomic elements are:
- Hydrogen — H₂
- Nitrogen — N₂
- Oxygen — O₂
- Fluorine — F₂
- Chlorine — Cl₂
- Bromine — Br₂
- Iodine — I₂
Mnemonic: Six of the seven diatomic elements — N, O, F, Cl, Br, I — trace the shape of the number 7 on the periodic table. Hydrogen is the seventh. Use the phrase "Never Have Fear Of Ice Cold Beer" (N, H, F, O, I, Cl, Br) to memorize all seven.
Keep in mind: this "two by two" pairing applies only to these elements in their pure, elemental form. When chlorine is part of a compound, it contributes a single chlorine atom — not a pair. Elemental chlorine is Cl₂; chlorine within a compound is just Cl.
Pure Substances vs. Mixtures
Pure Substances
A pure substance consists of a single type of matter — either a pure element or a pure compound. Both qualify.
- Pure element: pure oxygen gas (O₂), sodium metal (Na)
- Pure compound: pure water (H₂O) — composed of two elements, but only one compound is present
Mixtures
A mixture is a combination of two or more substances that are physically combined but not chemically bonded to one another. The key distinction from a compound is the presence or absence of chemical bonds between the components.
Example: Air vs. Water
Water (H₂O) is a compound: hydrogen and oxygen atoms are chemically bonded together. You cannot simply separate them by physical means.
Air is a mixture: it consists mostly of nitrogen (N₂) and oxygen (O₂) gases that are present together but are not bonded to each other. The individual gases can be separated by physical methods.
A mixture can combine multiple elements, multiple compounds, or any combination thereof. Salt water, for instance, is a mixture of two compounds — NaCl and H₂O — that coexist without forming chemical bonds between them.
Key Distinction — Bonded or Not? Ask yourself: are the components chemically bonded together? If yes, you have a compound. If no, you have a mixture. This single question resolves most element/compound/mixture classification problems.
2 Types of Mixtures: Homogeneous vs. Heterogeneous
Mixtures fall into two categories based on whether their composition is uniform throughout.
Homogeneous Mixtures (Solutions)
A homogeneous mixture has a uniform composition throughout — every sample drawn from any location in the mixture has exactly the same proportions of each component. Homogeneous mixtures are also called solutions. Most people picture liquid solutions, but solid solutions (like many metal alloys) and gaseous solutions (like air) exist as well.
Example: Air as a Homogeneous Mixture
Air is approximately 78% nitrogen and 21% oxygen, with small amounts of carbon dioxide, argon, and other gases. Whether you sample the air near the floor or near the ceiling of a room, you will find essentially the same composition. Air is therefore a homogeneous mixture.
Heterogeneous Mixtures
A heterogeneous mixture does not have a uniform composition. Different samples taken from different locations in the mixture will have different proportions of the components.
Example: A Salad (Non-Chemistry)
A salad is an intuitive heterogeneous mixture. Each forkful might contain tomato, cucumber, lettuce — or it might not, depending on where in the bowl you dig. The composition varies from sample to sample, which is the defining characteristic of a heterogeneous mixture.
Salt Water: A Case Study
Salt water is an instructive example because it can be either type of mixture, depending on how much salt is present.
When a small amount of salt is dissolved completely in water, the Na⁺ and Cl⁻ ions spread uniformly throughout the solution. The result is a homogeneous mixture.
However, water can only dissolve salt up to a certain concentration (its solubility limit). Add excess salt and the undissolved crystals sink to the bottom. Now a sample taken from the bottom of the container has a very different composition from one taken near the surface — making it a heterogeneous mixture.
Chemical vs. Physical Properties of Matter
Every substance has properties that chemists use to identify and describe it. These properties fall into two broad categories: chemical properties and physical properties.
The dividing line is simple: does the change convert one substance into a different substance?
- If yes → it is a chemical change, governed by a chemical property.
- If no → it is a physical change, governed by a physical property.
Physical Changes
A physical change alters the form or appearance of a substance without changing its chemical identity.
Example: Tearing Paper
Paper is made of cellulose. Tearing a sheet of paper in half does not convert cellulose into anything else — you still have two pieces of cellulose. The substance is unchanged; only its size and shape have changed. This is a physical change.
Example: Melting Ice
When ice melts, H₂O(s) becomes H₂O(l). The substance is identical before and after — only the phase changes. Melting is a physical change, and the characteristic temperature at which it occurs is called the melting point (0°C for water). Other phase changes — boiling, freezing, condensation, sublimation — are all physical changes as well.
Example: Dissolving Salt in Water
This is a classic trick question. When NaCl(s) dissolves in water, it becomes NaCl(aq) — the notation aq (aqueous) simply means "dissolved in water." Critically, the salt is still sodium chloride; no new substance has formed. If you evaporate the water, the salt reappears unchanged. Dissolving is therefore a physical change, even though the salt "disappears" visually.
Chemical Changes
A chemical change produces one or more new substances with different chemical identities from the starting materials.
Example: Burning Paper (Combustion)
Set that same sheet of paper on fire and the result is very different. Combustion converts the cellulose into carbon dioxide (CO₂) and water (H₂O) — entirely new substances. This is a chemical change. The chemical property that describes a substance's tendency to burn is called flammability.
Example: Rusting Iron (Oxidation)
An iron nail left in moist air slowly rusts. Iron (Fe) reacts with oxygen (O₂) to form iron oxide (Fe₂O₃) — a completely different compound. Because a new substance is produced, rusting is a chemical change. The associated chemical property is sometimes called oxidizability.
Common Mistake — Dissolving Is Not a Chemical Change: Students often assume that dissolving a solid in water must be a chemical change because the solid visibly disappears. It is not. No new substance is formed — the dissolved species is chemically identical to the original solid. Always ask: is a new substance produced?
Intensive vs. Extensive Properties of Matter
Properties can be further classified based on whether they depend on the size of the sample being measured.
The guiding question is: does the amount of matter present affect this property?
- If no → the property is intensive (independent of sample size).
- If yes → the property is extensive (dependent on sample size).
Intensive Properties (Independent of Sample Size)
Intensive properties are the same regardless of how much of a substance you have. They are characteristic of the substance itself, not the quantity measured.
| Intensive Property | Example |
|---|---|
| Melting point | Water always melts at 0°C, whether you have a drop or a lake |
| Boiling point | Water boils at 100°C at sea level regardless of quantity |
| Density | Water has a density of ~1 g/mL at room temperature, always |
| Temperature | A glass and a swimming pool left in the same environment reach the same temperature |
Density Is Intensive — Even Though Mass and Volume Are Not: Density = mass / volume. Both mass and volume are extensive properties (they scale with sample size), but their ratio stays constant for a given substance. That is why density is an intensive property and a reliable way to identify substances.
Extensive Properties (Dependent on Sample Size)
Extensive properties depend directly on the amount of matter present. The classic examples are mass and volume: a swimming pool of water has far greater mass and volume than a glass of water. Scale the sample up or down, and these values change proportionally.
Example: Glass of Water vs. Swimming Pool
Temperature: If both have equilibrated to the outdoor air temperature, they read the same temperature — intensive.
Density: Both measure ~1 g/mL — intensive.
Mass: The swimming pool is vastly heavier — extensive.
Volume: The swimming pool holds vastly more water — extensive.
Key Takeaways
- Matter is defined as anything with mass and volume; it is composed of atoms, of which approximately 118 distinct types (elements) exist.
- The 3 phases of matter — solid, liquid, and gas — differ in whether they possess a definite shape and/or definite volume, reflecting differences in atomic-level arrangement and motion.
- An element contains only one type of atom; a compound contains two or more elements chemically bonded in a fixed ratio. The 7 diatomic elements (H, N, O, F, Cl, Br, I) exist as diatomic molecules in their pure form.
- A pure substance is either a pure element or a pure compound. A mixture contains multiple substances that are physically combined but not chemically bonded.
- The 2 types of mixtures: homogeneous mixtures (solutions) have uniform composition throughout; heterogeneous mixtures do not.
- A chemical change produces a new substance; a physical change does not. Phase changes and dissolving are physical changes; combustion and rusting are chemical changes.
- Intensive properties (melting point, boiling point, density, temperature) are independent of sample size. Extensive properties (mass, volume) scale with the amount of matter present.