Chemistry Notes: Matter, Pure Substances, Mixtures, and Properties
States of Matter (solids, liquids, gases)
Solids
Definite shape and definite volume.
Example shown: a marker keeps its shape unless something changes it.
Molecules in a solid do not move much; they’re held in place.
Liquids
Definite volume, but take the shape of their container (e.g., the shape of my hand).
Molecules can slide past each other; they are more disorganized than in a solid but still held together.
Gases
No definite shape or definite volume.
Can be compressed; e.g., air in a tire can be pumped in until the tire cannot hold more.
Compare with a cup of water: water cannot be compressed to fit more without overflow.
Takeaway: States of matter are defined by shape and volume, and by how freely molecules can move.
Atoms, Elements, and Compounds
An element’s smallest unit is an atom.
Different elements have different atoms; we will study how atoms differ in Chapter 2.
A compound is formed when two or more different elements are chemically combined.
Water (H₂O) is a classic example: two hydrogen atoms and one oxygen atom form a compound.
Conditions for a compound: at least two different elements must be present.
What about single elements or molecules?
A single atom (e.g., helium) can exist by itself: that is an atom.
If you have two or more atoms of the same element, you get a molecule, not a compound.
Example: a diatomic molecule like helium? (Note: He is monatomic; a hypothetical diatomic He₂ would still be a molecule but not a compound since only one element is involved.)
If you have two or more atoms of different elements, you form a compound and it is also a molecule.
The statement in class: two of the same element → molecule (not a compound); two different elements → compound (and a molecule).
Examples discussed:
Carbon dioxide: CO₂ (two different elements → compound).
Ethylene glycol: contains carbon, oxygen, and hydrogen (a compound; a molecule with multiple elements).
A balloon filled with helium: helium as a single atom (an element; exists as a single-atom unit).
Summary:
Atom: smallest unit of an element.
Molecule: two or more atoms bound together (may be same element or different elements).
Compound: molecule that contains at least two different elements.
Pure Substances and Mixtures
Pure substances
Elements and compounds are pure substances.
Composition is fixed; e.g., water (H₂O) remains H₂O regardless of the container.
Example: a cup of water and a beaker of water both contain the substance H₂O; the composition does not change, though the amount may.
Mixtures
Mixtures consist of two or more substances mixed together; the composition can vary.
Two main types: homogeneous and heterogeneous.
Homogeneous mixtures (solutions)
Uniform composition and appearance; you can’t distinguish the components by sight.
Example: copper(II) sulfate dissolved in water; the blue liquid appears uniform.
Also called a solution; can be separated by processes like distillation.
Distillation example (salt water): heat to evaporate water; water vapor rises, condenses back to liquid in a condenser, leaving salt behind.
Distillation sequence: evaporate → condense → obtain pure water; salt remains in the original flask.
Heterogeneous mixtures
Non-uniform composition; components are distinguishable and often separable by sight.
Example: a salad with spinach, avocado, tomatoes, cheese, etc.
If you can see more than one phase, it is a heterogeneous mixture.
Quick identifications from transcript
Air: mixture (homogeneous or effectively uniform as a gas mixture) — homogeneous gas mixture.
Chicken noodle soup: heterogeneous mixture (visible components: noodles, chicken, carrots, etc.).
Juice with pulp: heterogeneous if pulp is visible.
General rule: if you can see more than one phase, it’s heterogeneous; if only one phase is seen, it’s homogeneous.
Separation methods mentioned
Filtration: for solid-liquid mixtures.
Distillation: used to separate a homogeneous mixture like salt water.
Questions posed in the transcript for practice
Is air an element, compound, or mixture? Answer: mixture (gases mix completely).
Is it homogeneous or heterogeneous? Air is homogeneous (a uniform gas mixture).
Is chicken noodle soup homogeneous or heterogeneous? Heterogeneous (visible components).
Physical and Chemical Properties; Changes
Physical properties
Observed without changing the substance’s identity.
Examples: chemical formula of water is H₂O; boiling water changes state (liquid to gas) but remains H₂O; density, mass, and volume measured without changing composition; color and odor are also physical properties.
Phase changes (solid ↔ liquid ↔ gas) are physical changes because the substance’s composition remains the same.
Boiling point is a physical property.
Chemical properties
Describe how a substance may react to become a different substance; composition changes chemically.
Examples: flammability; corrosiveness; reactivity with acids.
Important note from lecture: a color change can indicate a chemical change (color is a physical property, but a color change often accompanies chemical reaction).
Examples illustrating physical vs chemical changes
Water boiling: physical change (H₂O remains H₂O).
Copper penny reacting with nitric acid: chemical change; copper gas produced indicates new substances formed.
Note: color alone is a physical property; a color change during a reaction indicates a chemical change.
Intensive vs Extensive Properties
Intensive properties
Do not depend on the amount of substance present.
Examples: color, density, boiling point, refractive index.
A tiny sample of copper(II) sulfate is blue; a larger sample is blue as well.
Boiling point of water remains the same whether you have a small or large amount, though time to reach it may differ.
Density is independent of the amount.
Extensive properties
Do depend on the amount of substance present.
Examples: mass, volume, total energy, total amount of substance.
To weigh a marker, you need the entire object; you cannot weigh a partial cap and expect the true mass.
Volume measurement requires the entire sample for accuracy.
Summary: Intensive properties are intrinsic, whereas extensive properties scale with amount.
Preview of Next Topics (What’s Coming in the Course)
Units of measurement
Accuracy and precision
Significant figures and scientific notation
Prefixes and derived units
Density and temperature
Dimensional analysis (DA)
Instructor note: Dimensional analysis is challenging for many students; will be a major focus for the semester and used throughout.
Action item: read ahead on dimensional analysis and be ready to discuss on Wednesday; preparation for DA on Monday.
Recitation and Class Structure (as described in transcript)
A short break (~10 minutes) to simulate recitation
Recitation style will be practiced in the upcoming session
Connections to Foundational Principles and Real-World Relevance
Matter classification aligns with how substances are handled in labs (pure substances vs mixtures) and in industry (separation techniques like distillation and filtration).
Understanding physical vs chemical properties helps predict how materials will behave under different conditions (e.g., heating, reacting with acids, or undergoing combustion).
Distinguishing homogeneous and heterogeneous mixtures informs approaches to purification, separation processes, and quality control in manufacturing.
Intensive vs extensive properties underpins calibration, material testing, and the design of experiments where sample size or scale matters.
Notation and Key Formulas (LaTeX)
Water formula:
Density:
Pure substances retain composition: for water, remains the same regardless of container or amount.
Distillation concept (qualitative): Evaporation and subsequent condensation using a condenser to separate components based on volatility.
Quick Summary Takeaways
States of matter are defined by shape and volume and by molecular mobility: solid > definite shape/volume; liquid > definite volume, takes container shape; gas > no definite shape/volume, highly compressible.
Elements have atoms; compounds are formed from two or more different elements; molecules can be monatomic, diatomic, or polyatomic and can be either elements or compounds.
Pure substances vs mixtures; homogeneous vs heterogeneous; separation methods include filtration and distillation.
Physical properties can be observed without changing composition; chemical properties involve transformations of the substance.
Intensive vs extensive properties help distinguish intrinsic properties from those dependent on sample size.
Expect a deeper dive into measurement topics (accuracy, precision, significant figures, dimensional analysis) in upcoming classes.