Introduction to Chemistry

What is Chemistry?

Chemistry is the scientific study of the properties, composition, and behavior of matter. It is a central science that connects and enhances our understanding of various fields such as biology, physics, medicine, and environmental science.

• Matter is anything that has mass and occupies space.

• Chemistry explores how substances interact, combine, and change to form new substances.

• Applications of chemistry are found in energy, biochemistry, technology, and medicine.

Matter and Its Classification

Atoms, Elements, Compounds, and Molecules

All matter is composed of atoms, which are the fundamental building blocks. Elements consist of only one type of atom, while compounds are made from two or more different elements chemically bonded together. Molecules are groups of atoms bonded together, representing the smallest unit of a compound that retains its chemical properties.

• Element: Substance made of only one kind of atom.

• Compound: Substance made of two or more different kinds of elements in fixed proportions.

• Molecule: Two or more atoms bonded together.

States of Matter

Matter exists in three primary states: solid, liquid, and gas. Each state is characterized by distinct physical properties and particle arrangements.

• Solid: Definite shape and volume; particles are closely packed in a fixed arrangement.

• Liquid: Definite volume but no definite shape; particles are close but can move past one another.

• Gas: No definite shape or volume; particles are far apart and move freely.

Classification of Matter by Composition

Matter can be classified based on its uniformity and composition into mixtures and pure substances. Mixtures can be homogeneous (uniform throughout) or heterogeneous (not uniform). Pure substances are either elements or compounds.

• Homogeneous mixture (solution): Uniform composition throughout (e.g., saltwater).

• Heterogeneous mixture: Non-uniform composition (e.g., salad, sand in water).

• Element: Pure substance that cannot be decomposed into simpler substances.

• Compound: Pure substance that can be decomposed into simpler substances.

The Law of Constant Composition

Compounds always have the same relative number of atoms of each element, regardless of the sample size. This is known as the Law of Constant Composition (Law of Definite Proportions).

• Example: Water (H2O) always contains two hydrogen atoms for every one oxygen atom.

Properties and Changes of Matter

Physical and Chemical Properties

Properties of matter are classified as physical or chemical:

• Physical properties: Can be observed without changing the substance (e.g., boiling point, density, color).

• Chemical properties: Can only be observed when a substance is transformed into another substance (e.g., flammability, reactivity).

Intensive and Extensive Properties

• Intensive properties: Independent of the amount of substance (e.g., density, boiling point).

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

Physical and Chemical Changes

Changes in matter are classified as physical or chemical:

• Physical change: Does not alter the composition (e.g., melting, boiling, dissolving).

• Chemical change: Alters the composition, producing new substances (e.g., combustion, oxidation).

Separation of Mixtures

Physical Separation Methods

Mixtures can be separated into their components by exploiting differences in physical properties:

• Filtration: Separates solids from liquids using a porous barrier.

• Distillation: Separates substances based on differences in boiling points.

• Chromatography: Separates substances based on their ability to adhere to a solid surface.

Measurement in Chemistry

SI Units and Metric System

The International System of Units (SI) is used for scientific measurements. Each physical quantity has a standard unit.

Metric System Prefixes

Prefixes are used to indicate multiples or fractions of base units.

Volume

Volume is a derived unit, calculated as length cubed. Common units include the liter (L) and milliliter (mL).

• 1 L = 1 dm3

• 1 mL = 1 cm3

Temperature Scales

Temperature is measured using the Celsius, Kelvin, and Fahrenheit scales. The Kelvin scale is the SI unit and starts at absolute zero.

• Celsius (°C): 0°C is the freezing point, 100°C is the boiling point of water.

• Kelvin (K): 0 K is absolute zero; K = °C + 273.15.

• Fahrenheit (°F): Used in the U.S.; °F = (9/5)°C + 32.

Density

Density is a physical property defined as mass per unit volume. It is commonly expressed in g/mL or g/cm3.

• Formula:

• Example: If a substance has a mass of 10 g and a volume of 2 mL, its density is 5 g/mL.

Numbers in Chemistry: Accuracy, Precision, and Significant Figures

Exact and Inexact Numbers

• Exact numbers: Defined values or counted quantities (e.g., 12 eggs in a dozen).

• Inexact numbers: Measured values, subject to uncertainty.

Uncertainty, Accuracy, and Precision

All measurements have some degree of uncertainty due to limitations of measuring instruments.

• Accuracy: How close a measurement is to the true value.

• Precision: How close repeated measurements are to each other.

Significant Figures

Significant figures reflect the precision of a measured or calculated quantity.

• All nonzero digits are significant.

• Zeros between significant digits are significant.

• Leading zeros are not significant.

• Trailing zeros are significant if a decimal point is present.

• Addition/Subtraction: Round to the least significant decimal place.

• Multiplication/Division: Round to the least number of significant figures in any number used.

Dimensional Analysis

Unit Conversions

Dimensional analysis is a method for converting one unit to another using conversion factors. The conversion factor is set up so that units cancel, leaving the desired unit.

• Example: To convert meters to inches, use the relationships 1 m = 100 cm and 1 in = 2.54 cm.