1: Matter and measurement

1: Matter and Measurement: The Cornerstones of Chemistry

Chemistry begins with understanding matter – anything that occupies space and has mass – and how we quantify its properties through measurement. This foundational knowledge underpins all chemical investigation.

Classifying Matter:
Matter exists in various forms:

• States: Solid (fixed shape/volume), Liquid (fixed volume, variable shape), Gas (variable shape/volume), Plasma (ionized gas). Changes between states are physical changes.
• Composition: Matter is classified as:
  • Pure Substances: Fixed composition and properties. Either elements (one type of atom, e.g., gold, oxygen) or compounds (two or more elements chemically combined in fixed ratios, e.g., water - $H_2O$, sodium chloride - $NaCl$).
  • Mixtures: Combinations of two or more pure substances physically blended. Can be homogeneous (uniform composition, e.g., salt water, air) or heterogeneous (non-uniform composition, e.g., granite, oil and water).

Properties of Matter:
We characterize matter by its properties:

• Physical Properties: Observable/measurable without changing composition (e.g., color, density, melting point, solubility, conductivity).
• Chemical Properties: Describe how matter reacts or changes composition to form new substances (e.g., flammability, reactivity with acid).
• Intensive Properties: Independent of sample amount (e.g., density, temperature, melting point - useful for identification).
• Extensive Properties: Depend on sample amount (e.g., mass, volume, energy).

Measurement and Units:
Quantifying matter requires precise measurement using the International System of Units (SI):

• Mass: Kilogram (kg), gram (g) - measure of quantity of matter (distinct from weight).
• Length: Meter (m).
• Time: Second (s).
• Temperature: Kelvin (K) - absolute scale where $0$ K is absolute zero; Celsius (°C) is also common ($K = ^\circ\text{C} + 273.15$).
• Amount of Substance: Mole (mol) - fundamental for counting atoms/molecules.

Accuracy, Precision, and Significant Figures:

• Accuracy: How close a measurement is to the true value.
• Precision: How close repeated measurements are to each other (reproducibility). Precise measurements can lack accuracy if instruments are not calibrated.
• Significant Figures: The meaningful digits in a measured value, indicating its precision. Rules govern reporting and calculations to reflect measurement uncertainty.

Dimensional Analysis (Unit Conversions):
This essential technique uses conversion factors (ratios equal to 1, based on equivalence statements like $1 \text{ m} = 100 \text{ cm}$) to convert between units systematically. It ensures units cancel correctly and answers have the proper dimensions (e.g., converting $\mathrm{cm}^3$ to $\mathrm{L}$, or $\mathrm{g}$ to $\mathrm{mol}$). Mastery is crucial for stoichiometry.