Foundations of chemistry

1. Elements

• Definition: Pure substances consisting of only one type of atom.

• Examples: Hydrogen (H), Oxygen (O), Carbon (C).

• Key Characteristics:

  • Cannot be broken down into simpler substances.

  • Represented by chemical symbols (e.g., Na for sodium).

2. Molecules

• Definition: Two or more atoms bonded together, which can be the same or different elements.

• Examples:

  • Oxygen gas (O₂) – a molecule made of two oxygen atoms.

  • Water (H₂O) – a molecule made of hydrogen and oxygen atoms.

• Key Characteristics:

  • Smallest unit of a compound or an element that retains its properties.

3. Compounds

• Definition: Pure substances composed of two or more different elements chemically bonded.

• Examples:

  • Carbon dioxide (CO₂)

  • Sodium chloride (NaCl)

• Key Characteristics:

  • Have fixed ratios of elements (e.g., water always has 2 hydrogen atoms and 1 oxygen atom).

  • Can only be separated into elements through chemical reactions.

4. Mixtures

• Definition: Physical combinations of two or more substances where each retains its properties.

• Types:

  • Homogeneous Mixtures: Uniform composition (e.g., saltwater).

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

• Key Characteristics:

  • Components can be separated physically (e.g., filtration, evaporation). Filtration involves passing a mixture through a filter to separate solids from liquids, while evaporation removes a liquid by heating, leaving the dissolved solids behind.

5. Physical and Chemical Changes

• Physical Changes:

  • Do not change the substance’s chemical identity.

  • Examples: Melting, freezing, dissolving, breaking.

• Chemical Changes:

  • Result in the formation of new substances with new properties.

  • Examples: Rusting, burning, decomposition.

  • Evidence of chemical changes includes color change, temperature change, formation of a gas, and formation of a precipitate. These signs are elaborated further in the 'Evidence of a Chemical Reaction' section.

6. Evidence of a Chemical Reaction

• Signs of a chemical reaction:

  1. Color Change – A new color forms that wasn’t present before.

  2. Formation of Gas – Bubbles appear in the mixture.

  3. Precipitate Formation – A solid forms and separates from a liquid.

  4. Energy Change – Heat, light, or sound is released or absorbed.

  5. Odor Change – A new smell is noticeable.

7. Balancing Chemical Equations

• Steps to Balance Equations:

  • Write the unbalanced equation.

  • Count the atoms of each element on both sides of the equation.

  • Add coefficients to balance the atoms (start with the most complex molecule).

  • Check your work to ensure the same number of atoms for each element on both sides. (Maintain the Law of Conservation of Matter)

• Example:

  • Unbalanced: H₂ + O₂ → H₂O

  • Balanced: 2H₂ + O₂ → 2H₂O

8. Scientific Notation

• Definition: A way to express very large or very small numbers using powers of 10.

• Examples:

  • 5,000 = 5 × 10^3

  • 0.00042 = 4.2 × 10^-4

  • 1,230,000 = 1.23 × 10^6

  • 0.00000089 = 8.9 × 10^-7

• Key Operations:

  • Multiplication: Multiply the base numbers, then add the exponents.

  • Division: Divide the base numbers, then subtract the exponents.