Matter and Energy · 6 min read
Classification of Matter
What Is Matter
Matter is anything that has mass and takes up space. Every physical object in the universe consists of matter. Air, water, rocks, living organisms, and manufactured products all qualify as matter. The three fundamental states of matter encountered in everyday life are solid, liquid, and gas. A fourth state called plasma exists at extremely high temperatures.
Pure Substances
A pure substance has a definite and constant composition throughout. Pure substances fall into two categories: elements and compounds.
Elements are substances that cannot be broken down into simpler substances by ordinary chemical means. Each element consists of only one type of atom. The periodic table organizes all known elements. Examples include oxygen, carbon, gold, iron, and helium. Elements serve as the basic building blocks for all other substances.
Compounds form when two or more elements chemically combine in fixed ratios. Water is a compound containing hydrogen and oxygen in a 2:1 ratio. Table salt is a compound of sodium and chlorine. Compounds have properties different from the elements that form them. Sodium is a reactive metal and chlorine is a poisonous gas, but sodium chloride is safe table salt.
Mixtures
A mixture contains two or more substances physically combined. The components in a mixture retain their individual properties and can be separated by physical means. Mixtures do not have fixed compositions.
Homogeneous mixtures look the same throughout, so any sample has the same makeup. Salt water, air, and brass are homogeneous. A solution is a homogeneous mixture in which a solute (such as salt) dissolves in a solvent (such as water).
Heterogeneous mixtures are not uniform; you can often see the different parts. Salad, granite, and oil floating on water are heterogeneous.
| Type | What it is | Examples |
|---|---|---|
| Element | Pure substance made of one kind of atom | Oxygen (O₂), gold (Au), iron (Fe) |
| Compound | Pure substance of two or more elements chemically combined in a fixed ratio | Water (H₂O), table salt (NaCl), carbon dioxide (CO₂) |
| Homogeneous mixture | Physically combined; same makeup throughout | Salt water, air, brass |
| Heterogeneous mixture | Physically combined; parts are not evenly spread | Salad, granite, sand and water |
Separation Techniques
Mixtures can be separated by physical means; separating a compound into its elements takes a chemical change.
| Method | Property used | Example |
|---|---|---|
| Filtration | Particle size | Sand trapped by filter paper while water passes through |
| Evaporation | Boiling point | Water evaporates and leaves salt behind |
| Magnetism | Attraction to a magnet | Iron filings pulled out of sand |
| Distillation | Different boiling points | Pure water collected from salt water |
Physical and Chemical Properties
Physical properties can be observed or measured without changing what the substance is. Mass, volume, density, color, odor, melting point, boiling point, hardness, magnetism, and electrical conductivity are physical properties. Density is explained in the next chapter.
Chemical properties describe how a substance interacts with other substances or changes into new substances. Flammability, reactivity with acids, tendency to rust, and ability to support combustion are chemical properties. Chemical properties can only be observed when a substance undergoes a chemical change.
| Property | Type | Example |
|---|---|---|
| Melting and boiling point | Physical | Water boils at 100 °C at sea level |
| Density | Physical | Vegetable oil is less dense than water |
| Flammability | Chemical | Paper burns; glass does not |
| Reacts with oxygen (rusts) | Chemical | Iron rusts in moist air |
| Reacts with acid | Chemical | Baking soda fizzes in vinegar |
Physical and Chemical Changes
A physical change alters the size, shape, or state of matter but does not make a new substance. The particles stay the same; they are only moved apart, closer together, or rearranged in space. Melting ice, boiling water, dissolving sugar, cutting paper, and breaking glass are physical changes. All changes of state are physical changes: liquid water and ice are both H₂O.
A chemical change (chemical reaction) produces one or more new substances with new properties. The atoms of the starting substances break apart and join in new combinations. Burning wood, rusting iron, cooking an egg, and digesting food are chemical changes. Many are hard to reverse.
| Question to ask | Physical change | Chemical change |
|---|---|---|
| Is a new substance formed? | No; same substance | Yes; new substance with new properties |
| What happens to particles? | Same molecules, moved or spread out | Atoms rearrange into new molecules |
| Can it be undone easily? | Often (refreeze, evaporate) | Usually not by simple means |
| Examples | Melting, freezing, dissolving, tearing, crushing | Burning, rusting, baking, rotting, digesting |
Is It Physical or Chemical?
| Change | Type | Clue |
|---|---|---|
| Ice cube melts | Physical | Still water, only the state changed |
| Sugar dissolves in tea | Physical | Sugar is still there; evaporate the tea and it remains |
| Water boils into steam | Physical | Bubbles are water vapor, not a new gas |
| Salt water evaporates and leaves salt | Physical | Salt and water were only mixed |
| Iron nail rusts | Chemical | Rust (iron oxide) is a new, flaky, reddish substance |
| Wood burns | Chemical | Ash, smoke, and gases form; light and thermal energy released |
| Cake batter is baked | Chemical | New texture and taste; cannot turn back into batter |
| Apple slice turns brown | Chemical | Color change from a reaction with oxygen |
States of Matter and Changes of State
The state of matter depends on how close together the particles are and how freely they move. Most substances become less dense as they change from solid to liquid to gas, but water is an exception: ice is less dense than liquid water, so ice floats.
| State | Shape | Volume | Particles |
|---|---|---|---|
| Solid | Definite | Definite | Tightly packed; vibrate in place |
| Liquid | Takes the shape of its container | Definite | Close together; slide past each other |
| Gas | Fills its container | Fills its container | Far apart; move quickly in all directions |
| Change | From → To | Thermal energy | Example |
|---|---|---|---|
| Melting | Solid → liquid | Added | Ice melts in a drink |
| Freezing | Liquid → solid | Removed | Water freezes in a tray |
| Evaporation / boiling | Liquid → gas | Added | Puddle dries; pot boils |
| Condensation | Gas → liquid | Removed | Droplets form on a cold glass |
| Sublimation | Solid → gas | Added | Dry ice turns to gas |
| Deposition | Gas → solid | Removed | Frost forms on a window |
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