Matter and Its Forms

Matter is anything that has mass and occupies space. On Earth, it exists primarily in three states:

• Solids have a definite shape and a definite volume. Their particles are packed tightly in fixed positions and vibrate in place. Examples: ice, iron, table salt.
• Liquids have a definite volume but take the shape of their container (with a flat or slightly curved upper surface due to gravity). Their particles are close together but can slide past one another. Examples: water, mercury, ethanol.
• Gases have neither definite shape nor definite volume — they expand to fill their container completely. Their particles are far apart and move freely. Examples: oxygen, nitrogen, carbon dioxide.

A fourth state, plasma, exists at extremely high temperatures (such as in stars and lightning). Plasma is a gas-like state containing electrically charged particles, but it rarely appears in introductory chemistry courses.

Important distinction: mass measures the amount of matter in an object and does not change with location, while weight measures the gravitational force on that mass and varies (e.g., less on the Moon).

All matter can be classified using a simple decision tree:

1. Is it a pure substance (constant, uniform composition) or a mixture (variable composition)?
2. If pure: is it an element (cannot be broken down further by chemical means) or a compound (two or more elements chemically combined)?
3. If a mixture: is it homogeneous (uniform throughout) or heterogeneous (visibly different regions)?

Elements are the simplest form of pure substance — iron, gold, oxygen, carbon. About 90 occur naturally. Each element is made of only one type of atom.

Compounds are pure substances made of two or more elements in a fixed ratio, joined by chemical bonds. Water (H₂O) is always 2 hydrogen : 1 oxygen. Compounds can be broken into simpler substances by chemical changes, but not by physical ones. Crucially, a compound’s properties differ entirely from those of its constituent elements — sodium (reactive metal) + chlorine (toxic gas) = table salt (harmless solid).

Unlike pure substances, mixtures contain two or more substances that are physically (not chemically) combined. Their composition can vary, and their components can be separated by physical means (filtering, evaporation, distillation).

Heterogeneous mixtures have visibly different regions — you can see the separate components. Examples:

• Italian dressing (oil and vinegar layers)
• Granite (visible grains of quartz, mica, feldspar)
• Chocolate chip cookies (distinct chips, dough, nuts)

Homogeneous mixtures (also called solutions) look uniform throughout — every sample has the same composition. Examples:

• Salt water (salt dissolved uniformly in water)
• Air (uniform blend of N₂, O₂, Ar, and trace gases)
• Maple syrup, gasoline, sports drinks

The key test: if you take a sample from any part of the mixture and it looks and behaves identically, it’s homogeneous. If different regions look or behave differently, it’s heterogeneous.

Every substance has characteristic properties that help identify it. These fall into two categories:

Physical properties can be observed or measured without changing the substance’s chemical identity:

• Color, odor, density, melting point, boiling point, hardness, electrical conductivity
• Some require a state change to observe (melting point of iron), but the substance’s identity is unchanged

Chemical properties describe a substance’s ability to undergo chemical changes — transformations into different substances:

• Flammability (can it burn?), toxicity, acidity, reactivity with oxygen (does it rust?)
• Iron rusts in moist air (chemical property); chromium does not (also a chemical property — its resistance to oxidation)

The distinction matters: physical properties help you identify a substance; chemical properties tell you what it can become. You can observe a physical property by inspection, but you can only observe a chemical property by attempting (or observing) a chemical reaction.

Changes in matter also fall into two categories:

Physical changes alter a substance’s form or state but not its chemical composition. The same substance is present before and after:

• Ice melting to water (H₂O remains H₂O)
• Sugar dissolving in coffee
• Grinding a solid into powder
• Magnetizing or demagnetizing a metal

Chemical changes (chemical reactions) produce one or more new substances with different properties:

• Wood burning (cellulose → CO₂ + H₂O + ash)
• Iron rusting (Fe + O₂ + H₂O → iron oxide)
• Food digesting, a banana browning, milk souring

Clues that a chemical change has occurred: color change, gas production (bubbles), formation of a precipitate (solid from solution), energy release or absorption (heat, light). However, these are clues, not proof — some physical changes also produce bubbles (boiling) or color changes (dissolving a dye).

The law of conservation of matter applies to all changes: the total mass of matter before a change equals the total mass after. Matter is neither created nor destroyed.

Properties can also be classified by whether they depend on the amount of substance present:

Extensive properties change when the amount of matter changes:

• Mass (more matter = more mass)
• Volume (more matter = more volume)
• Heat content / energy

Intensive properties are independent of the amount of matter:

• Temperature (a cup and a gallon of milk at 20 °C are both at 20 °C — combining them doesn’t change the temperature)
• Density (a gram of gold and a kilogram of gold both have the same density, 19.3 g/cm³)
• Color, melting point, boiling point

Intensive properties are especially useful for identifying substances because they don’t depend on sample size. If you measure an unknown metal’s density and get 19.3 g/cm³, that strongly suggests gold — regardless of whether you have a gram or a kilogram of it.

A practical way to tell them apart: mentally double the sample. If the property doubles, it’s extensive. If it stays the same, it’s intensive.

When asked to classify a sample of matter, work through these questions in order:

1. Can it be separated by physical means (filtering, distilling, sorting)? If yes, it is a mixture. If no, it is a pure substance.
2. If a mixture: is the composition uniform throughout? Uniform → homogeneous (e.g., saltwater). Non-uniform → heterogeneous (e.g., granite).
3. If a pure substance: can it be broken down into simpler substances by chemical means? Yes → compound (e.g., H₂O decomposes into H₂ and O₂ by electrolysis). No → element (e.g., gold cannot be simplified further).

This hierarchy is definitive: every sample of matter fits into exactly one category. A common mistake is confusing phase with classification — ice, liquid water, and steam are all the same compound (H₂O) in different states, not different types of matter.