Mass, Weight, and Identifying Chemicals - Quick Notes

Mass and Weight

  • Mass: amount of matter in a sample; a measure of inertia.
  • Weight: force of gravity on an object; depends on location.
  • Relationship: Weight depends on gravity; Mass stays constant across locations.
  • Example: Astronaut’s mass is the same on the Moon, but weight changes because gravity is weaker there.
  • In everyday chemistry on Earth, mass and weight are often treated similarly.
  • Key idea: W = m g, where g is the local gravitational acceleration; mass m is constant across locations.

The Same and Not the Same

  • All matter is made of chemicals; there are nearly infinite chemicals.
  • Chemistry starts with telling chemicals apart (the same vs not the same).
  • Some samples share many properties; others are easy to distinguish.
  • Starting point: the ability to tell chemicals apart is fundamental to chemistry.

Intensive vs Extensive Differences

  • Intensive difference: does not depend on how much of the chemical you have.
  • Extensive difference: depends on the amount of substance.
  • Example: Mass is extensive; it depends on how much material you have and cannot distinguish salt from sugar.
  • Important: Only intensive differences are useful for telling chemicals apart (cannot rely on amount).
  • Mnemonic: opposite of intensive is extensive.

Methods of Identifying Chemicals by Physical Differences

  • Physical differences do not depend on amount (intensive):
    • Color
    • Texture of small pieces
  • Additional intensive physical differences used at home/lab:
    • Whether it dissolves easily in water
    • How thick (viscosity) a liquid is when poured
    • The temperature needed to start changing to gas (boiling/flash point)

Methods of Identifying Chemicals by Chemical Differences

  • Chemical differences do not depend on amount (intensive):
    • Smell
    • Taste
  • Additional intensive chemical differences used at home/lab:
    • Whether gas bubbles form when mixed with an acid (e.g., vinegar with baking soda yes; vinegar with salt no)
    • Whether it rusts or not (e.g., iron or silver rust; stainless steel resists rust; plastics usually do not rust)
  • Note: Smell and taste can be hard to describe; when using senses to detect differences, describe them as smelling or tasting like something.
  • Describing data:
    • Touch, sight, and hearing relate to physical differences and can be described precisely.
    • Smell and taste relate to chemical differences and are described by how they smell or taste (often less precise).