Introductory Chemistry – Lecture 1: Matter, Energy, and the Scientific Method
Chemistry as the Central Science
- Chemistry intersects with virtually every scientific and engineering discipline, providing a molecular‐level explanation for:
• Agriculture & environmental science (e.g.
– Nutrient cycles, soil chemistry, pollution remediation)
• Biology & medicine (biochemical pathways, drug design)
• Geology (mineral composition, geochemical dating)
• Physics (thermodynamics, quantum foundations)
• Engineering & materials science (polymers, alloys, semiconductors) - Practical implication: A solid foundation in chemical principles equips practitioners in diverse fields to solve real‐world problems (e.g. sustainable agriculture, renewable energy technologies).
Describing Matter: Composition & Structure
- Two complementary descriptors:
• Composition – the kinds and proportions of atoms present.
• Structure – the spatial arrangement of those atoms. - "What it’s made of" + "How it’s put together" → determines properties and reactivity.
Levels of Chemical Organization
- Atom
• Smallest unit that retains the chemical identity of an element. - Element
• Pure substance consisting of only one kind of atom.
• Cannot be decomposed into simpler substances by ordinary chemical means. - Compound
• Pure substance composed of two or more different elements chemically bonded in fixed ratios (law of definite proportions).
• Example reaction: \mathrm{Zn}+\mathrm{S}\;\longrightarrow\;\mathrm{ZnS} (formation of zinc sulfide through chemical change). - Molecule
• Discrete group of atoms held together by covalent bonds; smallest unit of a molecular compound capable of independent existence (e.g. \mathrm{H2O}, \mathrm{O2}).
Pure Substances vs. Mixtures
- Pure substances (elements OR compounds)
• Uniform, constant composition; single set of intrinsic properties. - Mixtures – physical combination of two or more substances.
• Homogeneous (solutions) – components are evenly distributed at the molecular level (e.g. salt water).
• Heterogeneous – components are unevenly distributed; phases or layers visible (e.g. sand in water, salad dressing). - Classification decision chart (implicit on slide):
- Only one type of atom? → Element.
- Multiple atom types but chemically bonded? → Compound.
- More than one substance? → Mixture.
– Evenly blended? → Homogeneous.
– Not evenly blended? → Heterogeneous.
Methods of Separating Mixtures (Physical, not Chemical)
- Filtration – separates solids from liquids (sand–water).
- Distillation – separates based on boiling‐point differences (salt water → fresh water).
- Chromatography – separates based on differential affinities between mobile & stationary phases.
- Magnetism, centrifugation, decanting, etc.
- Key principle: No chemical bonds are broken; identity of each component stays intact.
States of Matter & Particle View
- Solid – fixed shape & volume; particles closely packed, vibrate in place.
- Liquid – fixed volume but adopts container shape; particles closely packed but can flow.
- Gas – neither fixed shape nor volume; particles far apart, move freely.
- Temperature is a control knob: raising T generally promotes \text{solid} \to \text{liquid} \to \text{gas} transitions.
- Underlying axiom: "The behavior of any substance is determined by the arrangement of the particles that compose it."
Physical Properties & Physical Changes
- Physical properties – can be measured without changing composition (mass, color, density, melting point, conductivity, magnetism).
- Physical changes – alter state or appearance, not composition (phase changes, dissolution, cutting, grinding).
• All phase transitions (melting, freezing, vaporization, condensation, sublimation, deposition) are physical changes.
Chemical Properties & Chemical Changes
- Chemical properties – describe ability to undergo chemical change (flammability, acidity, oxidizing strength).
- Chemical changes (reactions) – transform substances into new substances with different compositions & properties.
• Example: Combination of Zn and S powders upon heating → formation of \mathrm{ZnS} (exothermic glow).
• Indicators: color change, gas/bubble formation, precipitate, energy change, new odor.
Energy Fundamentals in Chemistry
- Energy – capacity to do work or transfer heat.
• Measured in joules (J) or calories (1 cal = 4.184 J). - Two main forms:
- Potential Energy (PE) – energy due to position or composition.
– Stored in chemical bonds, gravitational height, electrostatic separation. - Kinetic Energy (KE) – energy of motion.
– E_k = \frac{1}{2} m v^2.
- Heat (q) – energy transfer due to temperature difference; flows from higher T to lower T.
Energy & Chemical/Physical Processes
- Systems naturally move from higher PE to lower PE when permitted – driving force for spontaneity.
- Exothermic change – releases energy (system loses heat, surroundings warm); q<0.
- Endothermic change – absorbs energy (system gains heat, surroundings cool); q>0.
- Photosynthesis vs. combustion:
• Photosynthesis stores solar energy in bonds of glucose (endothermic overall):
6\,\mathrm{CO2}+6\,\mathrm{H2O}+\text{light} \; \longrightarrow \; \mathrm{C6H{12}O6}+6\,\mathrm{O2}.
• Combustion of plant material releases that stored energy (exothermic). - "High energy" ≠ "stable"; lower‐energy arrangements are thermodynamically favored (e.g. products of exothermic reactions).
Stability, Potential Energy & Bonding
- Bond formation lowers potential energy (release of energy → stabilization).
- Bond breaking requires energy input (raises system energy).
- Visual metaphor (slide): stacked blocks vs. assembled structure; stable arrangement corresponds to lower PE.
The Scientific Method: Cycle of Inquiry
- Make observations – qualitative & quantitative data.
- Formulate hypotheses – tentative explanations (testable, falsifiable).
- Test with experiments – controlled procedures to gather evidence.
- Analyze / refine – support, modify, or reject hypotheses.
- Iterate → accumulation of knowledge.
- Key Terms
• Hypothesis – educated guess; narrow in scope.
• Theory – well‐substantiated, broad explanation; withstands repeated testing (e.g. atomic theory, kinetic‐molecular theory).
• Scientific law – concise statement that summarizes repeated observations (often mathematical; e.g. law of conservation of mass, PV = \text{constant} for Boyle’s law). - Philosophical note: Theories are not "upgraded" to laws; the two serve different roles (explanatory vs. descriptive).
Ethical, Practical & Interdisciplinary Considerations
- Accurate classification of substances crucial for environmental monitoring (e.g. distinguishing pollutants in water matrices).
- Understanding exothermic vs. endothermic events underpins safe industrial process design (preventing runaway reactions).
- Scientific method fosters evidence‐based policy (e.g. climate change mitigation relies on predictive chemical models of atmospheric CO$_2$).
Summary Checklist (Self‐Test)
- Can you differentiate element vs. compound vs. mixture?
- Can you name two techniques for separating mixtures and explain the underlying physical property exploited?
- Do you know all six fundamental phase changes and classify them as endo‐ or exothermic?
- Can you articulate the distinction between a theory and a law with an example of each?
- Can you determine whether a given process is exothermic or endothermic and predict entropy/enthalpy trends?