Chemistry: Core Concepts & Measurements (Last-Minute)

Chemistry and its role

• Chemistry: the study of matter, its chemical and physical properties and changes; energy changes accompany matter changes.
• Matter: anything that has mass and occupies space. Energy: the ability to do work.
• Mass ≠ Weight; weight = gravitational attraction. A correlation between mass and volume will be covered.
• Role of chemistry: public health, pharmaceutical industry, food science, medical practitioners, forensic sciences.

The Scientific Method

• Scientific Method: systematic approach to discovery
• Steps: Observation → Formulation of a question → Pattern recognition → Theory development
• Hypothesis: tentative explanation for observations
• Theory: hypothesis supported by extensive testing
• Experimentation → Data (individual measurements) → Results (outcome of experiments) → Information summarization
• Scientific law: summary of a large amount of information

Dalton’s Atomic Theory (What is matter?)

• Matter is made of tiny particles: atoms
• Elements have atoms that are identical
• Compounds are combinations of atoms of two or more elements
• The ratio of elements in 1 molecule of a compound is fixed
• Atoms cannot be created or destroyed
• In chemical reactions, atoms are rearranged, separated, or recombined

Classification of Matter

• By State: solids, liquids, gases
• By Composition: pure substances (elements/compounds) and mixtures (homogeneous/heterogeneous)

Three States of Matter

• Gas: particles widely separated; no definite shape or volume
• Liquid: particles closer; definite volume; no definite shape
• Solid: particles very close; definite shape and volume

Solids, Liquids, Gases: Quick Comparison

• Particle arrangement: Close / Somewhat close / Far apart
• Attraction: Strong / Reasonably strong / Weak
• Movement: Very slow / Glide / Fast
• Shape: Fixed / Takes container shape / Takes container shape
• Volume: Fixed / Fixed / Fills container
• Thermal expansion: Minimal / Minimal / High
• Compressibility: Small / Small / Large
• Examples: Solids – table salt, brass, copper penny; Liquids – water, milk; Gases – oxygen, air

Matter: Definitions

• Matter = anything that occupies space and has mass
• Space = volume
• Mass = amount of matter
• Weight = gravitational attraction; Mass ≠ Weight
• We will relate mass and volume later

Pure Substances

• Element: pure substance that cannot be changed into a simpler form by any chemical reaction
• Compound: pure substance resulting from the combination of two or more elements in a fixed ratio
• Examples: Element – carbon, copper, hydrogen; Compound – water, rust, sugar, table salt

Mixtures

• Mixture: two or more pure substances where each retains its identity
• Homogeneous: uniform composition; parts not visible (e.g., brass = Cu + Zn, soft drinks)
• Heterogeneous: non-uniform composition; parts visible (e.g., oil and water)

Particulate View (Conceptual Organization)

• Pure substance vs Mixture
• Element vs Compound
• Homogeneous vs Heterogeneous

Measurements and Notation

• Two representations of numbers: standard notation vs scientific notation
• Significant figures (digits): uncertainty is reflected by the number of significant digits
• Measurements and tools determine significant figures
• Purpose: quantify and communicate precision

Scientific Notation vs Standard Notation

• Scientific notation is used for very large or very small numbers for easier handling and correct significant figures.
• Example conversions:
• For numbers > 1: move decimal to the left; N = m × 10^x with x > 0. $N = m \times 10^{x}$ where 1 ≤ m < 10.
• For numbers < 1: move decimal to the right; x is negative. $N = m \times 10^{x}$ with x < 0.
• Example: 0.0000860 → $8.60 \times 10^{-5}$

Scientific Notation Rules (Summary)

• To express >1: move decimal left by x places; exponent x is positive.
• To express <1: move decimal right by x places; exponent x is negative.
• Coefficient m should be between 1 and 10.

Scientific Notation Practice (Selected Examples)

• 25306 → $2.5306 \times 10^{4}$
• 0.290 → $2.90 \times 10^{-1}$
• 100.086 → $1.00086 \times 10^{2}$
• 0.0000860 → $8.60 \times 10^{-5}$

Significant Figures (SF)

• Definition: Information-bearing digits; digits known with certainty plus one uncertain digit.
• The measuring device determines the number of SF in a measurement; the uncertainty is tied to SF used.

Rules to Recognize Significant Figures

• All nonzero digits are significant: e.g., 7.314 has 4 SF; 73.14 has 4 SF.
• Zeros between nonzero digits are significant: 60.052 has 5 SF.
• Zeros at the end of a number (trailing zeros):
  • Significant if the number contains a decimal point (e.g., 4.70 has 3 SF).
  • Not significant if the number does not contain a decimal point (e.g., 100 has 1 SF; 100. has 3 SF).
• Leading zeros are not significant (e.g., 0.0032 has 2 SF).

How Many SF in Examples (Conceptual)

• Examples exist; apply the rules above to count SF.

Significance in Calculations: Rounding Rules

• Exact (counted) numbers have infinite SF; inexact numbers carry uncertainty.
• Rounding: when dropping digits, if the dropped digit is < 5, leave the last kept digit; if ≥ 5, increase the last kept digit by 1.
• Rounding example: 33496.6 rounded to 3 SF → 33500 (illustrative).

Rounding and Precision in Calculations

• Addition/Subtraction: result cannot have more decimal places than the least precise term.
  • Example form: 37.68 L, 6.71862 L, 108.428 L, 152.82662 L → 152.83 L.
• Multiplication/Division: result has as many SF as the factor with the fewest SF.
  • Example rule: 4.2 (2 SF) × 10^3 (1 SF in exponent not counting) → result limited by the factor with 2 SF.

Rounding in Scientific Notation

• Addition and subtraction in sci notation can be done by:
  • Converting to standard form and adding, or
  • Adjusting exponents so powers of 10 match and then adding.
• Example outcome (conceptual): $1.02 \times 10^{-4}$ after proper alignment.

Exact vs Inexact Numbers

• Exact: counted items; infinite SF; no uncertainty.
• Inexact: measured quantities; contain uncertainty in the last digit.

Quick Reference Rules

• Addition/Subtraction: least number of decimal places governs the result.
• Multiplication/Division: least number of significant figures governs the result.
• Rounding: drop <5 do not change; drop ≥5 increase previous digit by 1.
• Scientific notation: use $N = m \times 10^{x}$ with 1 ≤ m < 10; x integer (positive for >1, negative for <1).