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What is Chemistry?

Chemistry is the branch of science that studies:

  • The composition of matter (what things are made of)
  • The structure of substances (how atoms are arranged)
  • The properties of matter (such as melting point, color, density)
  • The changes matter undergoes during chemical reactions

Chemistry is often called the “central science” because it connects physics, biology, geology, environmental science, and medicine.

Basic Concepts of Chemistry

a. Matter

  • Definition: Anything that has mass and occupies space.
  • States of Matter: Solid, Liquid, Gas, and Plasma
  • Classification:
    • Element: Pure substance made of one kind of atom (e.g., O₂, Fe)
    • Compound: Two or more elements chemically combined (e.g., H₂O)
    • Mixture: Two or more substances physically mixed (e.g., salt water)

b. Atom

The smallest unit of matter.

Made up of:

  • Protons (positive charge)
  • Neutrons (no charge)
  • Electrons (negative charge)

Atomic number = number of protons

c. Molecule

Two or more atoms bonded together (e.g., H₂, CO₂)

Branches of Chemistry

BranchFocus Area
Organic ChemistryStudy of carbon-containing compounds
Inorganic ChemistryStudy of non-carbon compounds
Physical ChemistryStudy of physical properties and energy changes
Analytical ChemistryTechniques to analyze substances
BiochemistryChemistry of living organisms
Environmental ChemistryStudy of chemicals in the environment
Industrial ChemistryChemical processes in industry

4. Chemical Reactions

a. What is a chemical reaction?

A process where substances (reactants) are transformed into new substances (products).

b. Types of Reactions

  • Combination/Synthesis: A + B → AB
  • Decomposition: AB → A + B
  • Single Replacement: A + BC → AC + B
  • Double Replacement: AB + CD → AD + CB
  • Combustion: Hydrocarbon + O₂ → CO₂ + H₂O

c. Signs of Chemical Change

  • Color change
  • Gas formation
  • Temperature change
  • Precipitate formation

5.The Periodic Table

A table organizing elements by increasing atomic number.

Arranged into:

  • Groups (columns): Elements with similar properties
  • Periods (rows): Elements with the same number of electron shells

Key Groups:

  • Group 1: Alkali metals
  • Group 2: Alkaline earth metals
  • Group 17: Halogens
  • Group 18: Noble gases

6. Chemical Bonding

a. Ionic Bonding

  • Between metals and non-metals
  • Transfer of electrons
  • Example: NaCl
b. Covalent Bonding
Between non-metals

Sharing of electrons

Example: H₂O

c. Metallic Bonding

  • Between metal atoms
  • Electrons flow freely (“sea of electrons”)

7. Chemical Energy and Thermodynamics

Endothermic: Absorbs heat (e.g., melting ice)

Exothermic: Releases heat (e.g., burning wood)

Laws of Thermodynamics: Describe energy changes and conservation

8. Chemical Equations and Stoichiometry

a. Balancing Equations

  • Mass is conserved: same number of atoms on both sides

b. Stoichiometry

  • Calculating the amount of reactants/products using mole ratios

9. Acids, Bases, and pH

a. Acids

  • Sour taste, pH < 7
  • Donate H⁺ ions
  • Example: HCl

b. Bases

  • Bitter taste, pH > 7
  • Donate OH⁻ ions
  • Example: NaOH

c. pH Scale

  • Measures acidity/basicity (0–14 scale)

10. Real-World Applications of Chemistry

Medicine: Drug design, diagnostics

Agriculture: Fertilizers, pesticides

Environment: Pollution control, recycling

Food: Preservatives, flavors

Energy: Batteries, fuel cells, nuclear power

Molecular Structure