Difficulty: Introductory | Prerequisites: None, though familiarity with the periodic table helps.
Chemistry begins with classifying what you are looking at. Before you can write a formula, balance an equation, or calculate a molar mass, you need to know whether you are dealing with a pure substance or a mixture, an element or a compound, and whether that compound is molecular or ionic. This classification system is the sorting framework for everything else in the course. If you have not seen the periodic table at all, spend ten minutes with it first.
Matter is either a pure substance (fixed composition, distinct properties) or a mixture (two or more substances combined physically). Pure substances are either elements (cannot be broken down further) or compounds (two or more elements chemically bonded). Compounds split into molecular (non-metals only) and ionic (at least one metal and one non-metal).
Substance
Any form of matter that has a constant composition and distinct properties. Water, oxygen, and sugar are all substances.
Think of it as: something "pure" in the chemistry sense, with a fixed recipe at the molecular level.
Mixture
Two or more substances combined physically (not chemically). They can be separated by physical means such as filtration, distillation, or evaporation.
Homogeneous mixture
A mixture with uniform composition throughout. You cannot see the individual components. Sugar dissolved in water is a classic example.
In simple terms, it looks the same no matter where you sample it.
Heterogeneous mixture
A mixture with non-uniform composition. You can often see the different parts. Sand in water is heterogeneous.
Element
A substance that cannot be decomposed into simpler substances by chemical means. There are currently 118 confirmed elements on the periodic table.
Compound
A substance made of two or more elements chemically bonded in a fixed ratio. Unlike a mixture, a compound can only be separated by chemical reactions, not physical methods.
Molecular compound
A compound made of two or more non-metals bonded together. These sit to the right of the staircase line on the periodic table. Examples: CO₂, H₂O, SF₆.
Ionic compound
A compound containing at least one metal and one non-metal. The metal loses electrons, the non-metal gains them, and the resulting ions are held together by electrostatic attraction. Example: NaCl.
Diatomic molecule
A molecule consisting of two atoms bonded together. Seven elements exist naturally as diatomic molecules: N₂, O₂, F₂, Cl₂, Br₂, I₂, H₂. The mnemonic is "Have No Fear Of Ice Cold Beer" (H, N, F, O, I, Cl, Br).
Note: P₄ (phosphorus) and S₈ (sulphur) also exist as multi-atom molecules in their standard states, but they are not diatomic.
Matter splits into pure substances and mixtures.
Mixtures are separated physically:
Homogeneous (uniform): sugar water, air, alloys.
Heterogeneous (non-uniform): sand in water, salad, granite.
Pure substances split into elements and compounds.
An element cannot be broken down further by chemical means.
A compound is two or more elements bonded in a fixed ratio. It can only be separated chemically.
118 confirmed elements on the periodic table.
Elements cannot be decomposed into anything simpler.
Seven elements are diatomic in their natural state: H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂.
P₄ and S₈ also exist as polyatomic molecules naturally.
Molecular compounds: two or more non-metals. Located to the right of the staircase (zigzag) line on the periodic table.
Examples: CO₂ (carbon dioxide), H₂O (water), SF₆ (sulphur hexafluoride).
Ionic compounds: at least one metal and at least one non-metal. One sits to the left of the staircase, one to the right.
Example: NaCl (sodium chloride). The metal (Na) donates electrons, the non-metal (Cl) accepts them.
Extensive property: depends on the amount of substance present. Examples: mass, volume.
Intensive property: does not depend on the amount. Examples: density, melting point, boiling point, colour, temperature.
Density is an intensive property. This is a commonly tested point.
Physical change: the identity of the substance stays the same. Examples: melting ice, dissolving sugar.
Chemical change: the identity of the substance changes. Examples: burning wood, rusting iron.
Classifying matter is how chemists decide which separation technique to use. A heterogeneous mixture can often be filtered; a homogeneous one might need distillation. In water treatment, knowing whether a contaminant is dissolved (homogeneous) or suspended (heterogeneous) determines the purification method.
The molecular vs ionic distinction matters for predicting properties: ionic compounds tend to have high melting points and conduct electricity when dissolved, while molecular compounds tend to have lower melting points and do not conduct.
Students often think all compounds are molecules. They are not. Ionic compounds (like NaCl) are not molecules; they are lattices of ions.
Students sometimes confuse "molecule" with "compound." H₂ is a molecule but not a compound (it contains only one element).
Students assume all elements are individual atoms. Several elements (H₂, O₂, N₂, etc.) exist naturally as molecules.
"Homogeneous" does not mean "pure." Sugar water is homogeneous (uniform) but is a mixture, not a pure substance.
⚠️ True/false questions testing the relationships between molecules, compounds, and elements are very common. From the lecture: "All compounds are molecules" (False, ionic compounds are ions). "All molecules are compounds" (False, e.g. H₂). "All elements are molecules" (False).
⚠️ Know the seven diatomic elements cold. They appear in balancing equations and in stoichiometry. If you write O instead of O₂ for elemental oxygen, your equation will be wrong.
⚠️ Be able to classify any substance as element or compound and as molecular or ionic just by looking at its formula and the periodic table.
⚠️ Density is intensive, mass and volume are extensive. This is a standard exam question.
True or false: salt water is a pure substance.
Fill in the blank: a compound made of two or more non-metals is called a ______ compound.
True or false: O₂ is a compound.
Name the seven diatomic elements.
Fill in the blank: a property that does not depend on the amount of substance present is called an ______ property.
Answers: 1. False (it is a homogeneous mixture). 2. Molecular. 3. False (it is a diatomic element, not a compound, because it contains only one element). 4. H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂. 5. Intensive.
Q: Classify NaCl as molecular or ionic, and explain why.
A: Ionic. Na is a metal (Group 1) and Cl is a non-metal (Group 17). The combination of a metal and a non-metal forms an ionic compound.
Q: Is sugar water homogeneous or heterogeneous? Is it a substance or a mixture?
A: Homogeneous mixture. It has uniform composition throughout (you cannot see the sugar once it dissolves), but it contains two substances combined physically.
Q: A student claims that because H₂ is made of two atoms, it must be a compound. What is wrong with this reasoning?
A: A compound requires two or more different elements. H₂ contains only hydrogen, so it is an element (a diatomic molecule), not a compound.
Q: Classify the following as extensive or intensive: (a) mass, (b) density, (c) temperature, (d) volume.
A: (a) Extensive. (b) Intensive. (c) Intensive. (d) Extensive.
Q: What is the key difference between a physical change and a chemical change?
A: A physical change does not alter the identity of the substance (e.g. ice melting to water). A chemical change produces a new substance with a different chemical identity (e.g. iron rusting to iron oxide).
The element vs compound distinction feeds directly into chemical nomenclature (naming compounds). Knowing whether a compound is molecular or ionic tells you which naming rules to apply. The concept of mixtures returns in solution chemistry, where molarity and dilution depend on understanding what is dissolved in what.
Matter classification, substance, mixture, homogeneous, heterogeneous, element, compound, molecular compound, ionic compound, diatomic molecules, extensive property, intensive property, physical change, chemical change, CHEM 101, general chemistry, Michigan State University