Introduction to Chemistry

What is Chemistry?

Chemistry is the scientific study of matter, its properties, and the changes it undergoes. It is foundational to understanding the natural world and is central to many science-related fields, including biology, physics, medicine, and environmental science.

• Matter: Anything that has mass and occupies space.

• Properties of Matter: Characteristics that allow us to distinguish different types of matter.

• Changes in Matter: Can be physical (no new substances formed) or chemical (new substances formed).

Classification of Matter

States of Matter

Matter exists in three primary states: solid, liquid, and gas. Each state is defined by the arrangement and movement of its particles.

• 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 by Composition

Matter can be classified based on its composition as either a pure substance or a mixture.

• Pure Substance: Has a fixed composition and distinct properties. Includes elements and compounds.

• Mixture: Contains two or more substances physically combined. Can be homogeneous (uniform composition, also called a solution) or heterogeneous (non-uniform composition).

Elements and Compounds

Elements are substances that cannot be broken down into simpler substances by chemical means. Compounds are substances composed of two or more elements chemically combined in fixed proportions.

• Atoms: The basic building blocks of matter; each element consists of one type of atom.

• Molecules: Groups of atoms bonded together; can represent elements (e.g., O2) or compounds (e.g., H2O).

Law of Constant Composition

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

Mixtures

Mixtures can be separated into their components by physical means. They are classified as:

• Homogeneous Mixtures (Solutions): Uniform composition throughout (e.g., salt water).

• Heterogeneous Mixtures: Non-uniform composition (e.g., granite).

Classification Flowchart

A decision scheme can be used to classify matter as an element, compound, homogeneous mixture, or heterogeneous mixture.

Properties of Matter

Physical and Chemical Properties

Properties of matter are divided into two main categories:

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

• Chemical Properties: Can only be observed when a substance is changed 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

• Physical Change: Alters the form or appearance but not the composition (e.g., melting, boiling).

• Chemical Change: Results in the formation of new substances (e.g., burning, rusting).

Separation of Mixtures

Methods of Separation

Mixtures can be separated by exploiting differences in physical properties:

• Filtration: Separates solids from liquids.

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

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

Energy in Chemistry

Forms of Energy

Energy is the capacity to do work or transfer heat. It exists in two fundamental forms:

• Kinetic Energy: Energy of motion, given by

• Potential Energy: Stored energy due to position or composition.

Units of Measurement

SI Units and Metric System

Chemistry relies on quantitative measurements, which require standardized units. The International System of Units (SI) is used globally.

Metric Prefixes

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

Mass, Length, and Volume

• Mass: SI base unit is kilogram (kg); metric base unit is gram (g).

• Length: SI base unit is meter (m).

• Volume: Derived unit; commonly measured in liters (L) or milliliters (mL).

Temperature Scales

Temperature is measured in Kelvin (K), Celsius (°C), or Fahrenheit (°F). The Kelvin scale is the SI standard.

• Kelvin to Celsius:

• Celsius to Fahrenheit:

• Fahrenheit to Celsius:

Density

Density is a physical property defined as mass per unit volume:

• Formula:

• Common units: g/cm3 (solids, liquids), g/L (gases)

Energy Units

• Joule (J): SI unit of energy.

• Calorie (cal):

• Nutritional Calorie (Cal):

Measurement and Uncertainty

Accuracy and Precision

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

• Precision: How closely repeated measurements agree with each other.

Significant Figures

Significant figures reflect the precision of a measured quantity. Rules for determining significant figures:

• All nonzero digits are significant.

• Zeroes between nonzero digits are significant.

• Leading zeroes are not significant.

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

When performing calculations:

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

• Multiplication/Division: Round to the same number of significant figures as the measurement with the fewest significant figures.

Dimensional Analysis

Dimensional analysis is a method for converting between units using conversion factors. Always set up conversion factors so that units cancel appropriately.

• Example: To convert inches to centimeters, use .

• Multiple conversions can be chained together as long as each is an equality.