Matter and Substances - Quick Reference

Matter

• Definition: matter is anything that has mass and occupies space.
• Mass vs weight: mass is independent of gravity; weight depends on gravity; in practice, these terms are often used interchangeably.

Physical vs Chemical Properties and Changes

• Physical property/change: identity of the matter does not change.
• Example: ice (solid water) melts to liquid water; still $ext{H}_2 ext{O}$; this is a physical change.
• Chemical change: atoms/molecules change identity; a new substance forms.
• Example: combustion of $ext{H}<em>2$ and $ext{O}</em>2\rightarrow\text{H}_2 ext{O}$ (hydrogen burning) is a chemical change.
• In reactions, matter is conserved overall (conservation of mass).

Intensive vs Extensive Properties

• Intensive properties: do not depend on the amount of substance (e.g., density $\rho$, boiling point $T_b$).
• Extensive properties: depend on amount (e.g., mass $m$, volume $V$).

Phases of Matter and Molecular Behavior

• Solids: rigid; atoms do not pass each other; very limited movement.
• Liquids: atoms can move past each other but remain in contact.
• Gases: particles move freely and are far apart.
• Phase boundaries are set by melting/boiling points and pressure; these define phase changes.

Phase Diagrams and Normal Conditions

• Phase diagrams plot temperature vs. pressure; regions correspond to solid, liquid, or gas.
• Normal conditions used in many data sets: $T = 25^ ext{\circ}\mathrm{C}$ and $P = 1\ \mathrm{atm}$.

Matter, Elements, Atoms, and Molecules

• Elements: pure substances, simplest form of matter; cannot be decomposed by chemical means (in this context).
• Atoms: smallest unit that retains the properties of an element.
• Molecule: two or more atoms bonded together (can be the same or different elements).
• Examples:
  • $\mathrm{H<em>2}$, $\mathrm{O</em>2}$, $\mathrm{H_2O}$.
• Ångström: unit for atomic-scale distances, symbol $\text{Å}$.
• There are $118\,$known elements.

Pure Substances vs Mixtures

• Pure substance: has a constant composition; can be an element or a compound.
• Mixture: combination of two or more substances that can be separated physically.
• Separation: mixtures can be separated by physical methods; pure substances cannot.
• Homogeneous vs Heterogeneous:
  • Homogeneous: uniform composition (e.g., coffee with sugar fully dissolved; aqueous solution $\mathrm{(aq)}$).
  • Heterogeneous: nonuniform composition; parts can be separated (e.g., a coffee with undissolved milk).
• Examples:
  • Water: $\mathrm{H_2O}$ is a pure substance (a compound).
  • Hydrogen and oxygen gases mixed together form a mixture (can be separated physically).
  • Water formation from $\mathrm{H<em>2}$ and $\mathrm{O</em>2}$ is a chemical reaction yielding a new pure substance.

Aqueous Notation

• $\mathrm{aq}$ (often written as $\mathrm{(aq)}$) indicates dissolved in water.

Conservation of Matter

• Matter cannot be created or destroyed; atoms are conserved and merely rearranged during physical or chemical changes.

Quick Contrast: Physical vs Chemical Change (Recap cues)

• If the identity of the substance changes, it is a chemical change.
• If the identity remains the same but the form changes (e.g., state changes, dissolution), it is a physical change.

Quick Reference Symbols

• States: $\text{S},\;\text{L},\;\text{G}$
• Aqueous: $\mathrm{aq}$
• Molecules/compounds: use chemical formulas, e.g., $\mathrm{H<em>2O},\;\mathrm{H</em>2},\;\mathrm{O_2}$