Introduction to Matter

Basic Definitions

Chemistry is the study of matter, its properties, and the changes it undergoes. Understanding the fundamental concepts of matter is essential for all further study in chemistry.

• Matter: Anything that has mass and occupies space (volume).

• Mass: A measure of the amount of matter in an object. Common units: grams (g), kilograms (kg).

• Volume: The amount of space an object occupies. Common units: liters (L), cubic centimeters (cm3), milliliters (mL).

• State of Matter: The physical form in which matter exists: solid, liquid, or gas.

• Composition: The types of particles (atoms, molecules) that make up a sample of matter. Example: Copper (element), Water (compound).

• Atom: The basic building block of matter.

Classification of Matter

Elements and Compounds

Matter can be classified as either a pure substance or a mixture. Pure substances include elements and compounds.

• Element: A substance that contains only one type of atom.

  • Monatomic elements: Consist of unbonded, "like" atoms (e.g., He, Ne).

  • Polyatomic elements: Consist of several "like" atoms bonded together (e.g., O2, S8).

  • Diatomic elements: Elements that exist as molecules of two atoms (e.g., O2, N2).

  • Allotropes: Different forms of the same element in the same state of matter (e.g., O2 and O3 for oxygen; diamond and graphite for carbon).

• Compound: A substance that contains two or more different types of atoms chemically bonded together. Compounds have properties different from their constituent elements.

  • Example: Na (sodium) and Cl2 (chlorine) combine to form NaCl (table salt).

  • Atoms can only be altered by nuclear means; molecules can be altered by chemical means.

  • Example reactions:

    • Dehydration of sugar: $$C_{12}H_{22}O_{11}(s) \rightarrow 12C(s) + 11H_2O(g)$$

    • Electrolysis of water: $$2H_2O(l) \rightarrow 2H_2(g) + O_2(g)$$

Mixtures

Mixtures are combinations of two or more substances that are not chemically bonded. They can be separated by physical means.

• Homogeneous mixture (solution): Has uniform composition and properties throughout (e.g., salt water, air).

• Heterogeneous mixture: Has different composition and properties in different parts of the sample (e.g., salad, sand in water).

• Alloy: A homogeneous mixture of metals (e.g., brass, bronze).

• Suspension: A heterogeneous mixture that settles over time (e.g., muddy water).

Contrast: 24K gold (pure substance) vs. 14K gold (mixture/alloy).

Chart for Classifying Matter

Properties of Matter

Chemical and Physical Properties

• Chemical properties: Describe how a substance reacts with other substances (e.g., reactivity with water, flammability).

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

Extensive and Intensive Properties

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

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

Examples of Properties

• Electrical conductivity

• Reactivity with water

• Heat content (total energy)

• Ductile: Can be drawn into wire

• Malleable: Can be hammered into shape

• Brittle: Breaks easily

• Magnetism

States of Matter and Changes of State

States of Matter

• 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 each other.

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

Changes of State

• Melting: Solid to liquid

• Freezing: Liquid to solid

• Vaporization (boiling/evaporation): Liquid to gas

• Condensation: Gas to liquid

• Sublimation: Solid to gas

• Deposition: Gas to solid

Energy in Chemistry

Kinetic Energy and Conservation of Energy

• Kinetic energy: The energy of motion.

• Law of Conservation of Energy: Energy cannot be created or destroyed, only transformed from one form to another.

Example: Combustion of acetylene: $$2H_2 + O_2 \rightarrow 2H_2O$$

Energy Changes

• Endothermic change: System absorbs heat (e.g., water boiling, ice melting).

• Exothermic change: System releases heat (e.g., water freezing, combustion).

Quantitative Concepts: The Mole and Counting Atoms

The Mole

• Atoms and molecules are extremely small; chemists use the mole to count them.

• 1 mole = $$6.02 \times 10^{23}$$ particles (Avogadro's number).

• For any element, 1 mole has a mass in grams equal to its atomic mass (from the periodic table).

Island Diagram

Sample Problems

• How many moles is $$3.79 \times 10^{25}$$ atoms of zinc?

• How many atoms is 0.68 moles of zinc?

• How many grams is 5.69 moles of uranium?

• How many grams is $$2.65 \times 10^{23}$$ atoms of neon?

• How many atoms is 421 g of promethium?

Density and Density Calculations

Definition and Units

• Density (d): The mass per unit volume of a substance.

• Formula: $$d = \frac{m}{V}$$ where m = mass, V = volume.

• Typical units: g/cm3 for solids, g/mL for liquids.

• The density of a liquid or solid is nearly constant, regardless of sample size.

• Density of water at 4°C: 1.00 g/mL.

Sample Density Calculations

• A sample of lead (Pb) has mass 22.7 g and volume 2.0 cm3. Find the density.

• Another sample of lead occupies 16.2 cm3 of space. Find the mass.

• A solid cylinder has radius 1.8 cm and height 1.5 cm. Find the density if the mass is 119.5 g.

• A rectangular solid has edge lengths 8.2 cm, 5.1 cm, and 4.7 cm. Will a 153 g sample sink in water?

Separating Mixtures

Mixtures can be separated by physical means, which do not involve changing the chemical identity of the substances.

• Sorting: Separating based on physical characteristics.

• Filtration: Separating solids from liquids using a filter.

• Magnet: Using a magnet to separate magnetic materials.

• Chromatography: Separating substances based on their movement through a medium.

• Density: Separating substances based on differences in density.

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

Summary Table: Classification of Matter

Key Point: Pure substances have fixed composition; mixtures can vary in composition.

Additional Info

• For all calculations involving moles, always use Avogadro's number: $$6.02 \times 10^{23}$$.

• When classifying matter, always consider both composition and uniformity of properties.