Introduction to General Chemistry

Classification of Matter

Chemistry is the study of matter and the changes it undergoes, with the atom as its basic functional unit. Matter is anything that occupies space and has mass. Matter can be classified into two main types: pure substances and mixtures.

• Pure Substance: A type of matter composed of only one kind of atom or molecule. Examples include elements (e.g., gold) and compounds (e.g., water).

• Mixture: Matter composed of two or more different elements or compounds that are physically mixed together but not chemically bonded. Mixtures can be homogeneous (uniform composition, e.g., saltwater) or heterogeneous (non-uniform composition, e.g., salad).

Example: Crystalline sugar is a pure substance; salsa is a heterogeneous mixture.

Physical and Chemical Changes

Changes in matter can be classified as physical or chemical:

• Physical Change: A change in the physical state or appearance of a substance without altering its chemical composition (e.g., melting ice, dissolving sugar in water).

• Chemical Change: A change that results in the formation of one or more new substances with different properties (e.g., burning wood, rusting iron).

Example: Dissolving sugar in water is a physical change; burning wood is a chemical change.

Reversible and Irreversible Changes

Physical and chemical changes can be further classified as reversible or irreversible:

• Reversible Change: The original structure can be restored (e.g., freezing and melting water).

• Irreversible Change: The original structure cannot be restored (e.g., burning paper).

Example: Dissolving sugar in water is reversible; burning wood is irreversible.

Chemical and Physical Properties

• Chemical Property: A property observed during a chemical reaction, indicating the ability of a substance to change into a new substance (e.g., flammability, reactivity with acids).

• Physical Property: A measurable property that describes the state of a chemical compound without changing its chemical structure (e.g., color, melting point, density).

Example: Mercury is a silvery liquid at 25°C (physical property); sodium reacts violently with water (chemical property).

Intensive vs. Extensive Properties

• Intensive Properties: Do not depend on the amount of substance present (e.g., density, boiling point, color).

• Extensive Properties: Depend on the amount of substance present (e.g., mass, volume, energy).

Example: Density is intensive; mass is extensive.

Measurement in Chemistry

Temperature and Thermal Energy

• Thermal Energy: The sum of kinetic and potential energies of all atoms in an object.

• Temperature: The average kinetic energy of the particles in a substance.

Temperature is measured in units of Celsius (°C), Kelvin (K), or Fahrenheit (°F).

Temperature Conversion Formulas:

Scientific Notation

• Used to express very large or very small numbers in a compact form.

• General format: , where and is an integer.

Example:

SI Units and Measurements

• The International System of Units (SI) is based on seven base units (meter, kilogram, second, ampere, kelvin, mole, candela).

Metric Prefixes

• Metric prefixes are modifiers that are multiples of ten (e.g., kilo-, centi-, milli-).

Significant Figures

• Significant figures are the digits in a measurement that are known with certainty plus one estimated digit.

• Rules for counting significant figures depend on the presence of zeros and decimal points.

Example: 0.00364 has 3 significant figures.

Precision in Measurements

• When recording measurements, include all certain digits plus one uncertain digit.

Significant Figures in Calculations

• For multiplication/division: The result should have the same number of significant figures as the value with the fewest significant figures.

• For addition/subtraction: The result should have the same number of decimal places as the value with the fewest decimal places.

Conversion Factors and Dimensional Analysis

• Conversion Factor: A ratio used to express a quantity in different units (e.g., ).

• Dimensional Analysis: A method to convert one unit to another using conversion factors.

Example: To convert 115 minutes to hours:

Density

• Density is the amount of mass per unit volume.

Density Formula:

For solids and liquids, units are typically g/cm3 or kg/m3; for gases, units are g/L.

Density of Geometric and Non-Geometric Objects

• For regular shapes, use geometric formulas for volume (e.g., for a cube).

• For irregular shapes, use water displacement to determine volume.

Example: If a cube has sides of 3.0 cm and a density of 10.5 g/cm3, its mass is g.

Summary Table: Classification of Matter

Additional info: This guide covers foundational concepts in general chemistry, including matter classification, properties, measurement, and basic mathematical operations essential for laboratory and theoretical work.