Study Notes

All matter is classified in a hierarchy:

Matter

└── Pure Substance (fixed composition)

├── Element: one type of atom (e.g., gold, oxygen gas)

└── Compound: two or more elements chemically combined in a fixed ratio (e.g., water H₂O)

Matter

└── Mixture (variable composition)

├── Homogeneous mixture: same composition throughout, appears uniform (e.g., saltwater, air)

└── Heterogeneous mixture: different composition in different parts, visible distinction (e.g., sand + water, trail mix)

Key distinction: A pure substance has a definite, fixed composition. A mixture can be separated by physical means.

Physical properties: can be observed or measured WITHOUT changing the chemical composition.

Examples: color, density, melting point, boiling point, mass, volume, state (solid/liquid/gas)

Chemical properties: describe how a substance reacts or changes into a different substance.

Examples: flammability, reactivity with acid, ability to rust, ability to combust

Physical change: changes form or appearance but NOT chemical composition.

Examples: cutting paper, melting ice, dissolving salt

Chemical change: creates a new substance with different chemical properties.

Examples: burning wood, rust forming, cooking an egg

Evidence of chemical change: color change, gas produced, precipitate forms, temperature change, light produced

Intensive vs. Extensive properties:

• Intensive: do NOT depend on amount of substance (density, temperature, color, melting point)

• Extensive: DO depend on amount (mass, volume, length)

Matter exists in four states:

• Solid: definite shape and volume; particles tightly packed, vibrate in place

• Liquid: definite volume, no definite shape; particles slide past each other

• Gas: no definite shape or volume; particles move rapidly and are far apart

• Plasma: high-energy ionized gas; rare on Earth but common in stars

Changes between states involve energy transfer without changes in chemical composition (physical changes).

Physical separation techniques exploit differences in physical properties:

• Distillation: separates liquids with different boiling points by heating and collecting vapors separately. Example: separating alcohol from water. CAN recover specific components.

• Evaporation: removes a liquid by heating, leaving behind dissolved solids. Example: evaporating saltwater to recover salt. Recovers the SOLID, not the liquid.

• Filtration: separates insoluble solids from liquids using a filter. Example: removing sand from water. Recovers the solid on the filter and the liquid in the filtrate.

Each technique works best for specific types of mixtures. You must know which technique to use for a given separation goal.

Two fundamental laws govern how elements combine in compounds:

Law of Definite Proportions (Proust, 1799): A pure compound always contains the same elements in the same mass ratio, regardless of the source or sample size.

Example: Water is always 8:1 oxygen to hydrogen by mass. A 18g sample or 180g sample of water — always 8:1.

Law of Multiple Proportions (Dalton, 1803): When two elements form MORE THAN ONE compound, the masses of one element that combine with a fixed mass of the other are in small whole-number ratios.

Example: Carbon and oxygen form CO and CO₂. For a fixed amount of carbon:

- In CO: 16g of oxygen

- In CO₂: 32g of oxygen

- Ratio: 16:32 = 1:2 (whole number ratio)

Historical context: Ancient alchemists sought to transform metals into gold (phlogiston theory). Proust and Dalton's careful quantitative measurements helped establish modern chemistry by showing that matter follows precise, reproducible laws.