Chapter 1 (Lesson 2)

1.4 Physical and Chemical Changes and Properties of Matter

• Changes describe how substances transform, while properties define the characteristics that determine these transformations.

• Properties help predict how substances change, and observing changes provides insights into these properties.

• Both properties and changes are classified as physical and chemical.

• Physical Change: a change that alters only the state or appearance of a substance; no new substance is formed; does not alter the composition of the substance; the atoms or molecules that compose a substance do not change their identity during a physical change.

• Chemical Change: a change in the chemical identity or composition of a substance; a new substance with a completely new nature is formed.

• Physical Change example: Water boils, changing from a liquid to a gas; the gas remains composed of water molecules; no new substance is formed, so this is a physical change.

• Chemical Change example: Rusting of iron; when nail is exposed to air or moisture, iron reacts with oxygen and forms iron oxide (Fe₂O₃); a new substance is formed, so this is a chemical change.

• Physical and Chemical Changes (2 of 2)

1.4 Physical and Chemical Changes and Properties of Matter (continued)

• Physical Properties: a property displayed by a substance without changing its composition or identity.

  • Examples: the smell of gasoline; odor, taste, color, appearance, melting point, boiling point, and density are all physical properties.
  • Note: d istily, density is a physical property.

• Chemical Properties: a property displayed only by changing the composition or identity of a substance.

  • Examples: the flammability of gasoline; corrosiveness, acidity, reactivity, and toxicity are chemical properties.

• Conceptual Connection 1.3: The diagram on the left represents liquid water molecules in a pan. Which of the three diagrams (a, b, or c) best represents the water molecules after they have been vaporized by boiling?

• Self-Assessment Quiz (1) Q3: Which change is a physical change?

  • A. Wood burning
  • B. Gasoline evaporating
  • C. Dynamite exploding
  • D. Iron rusting
  • Answer: B (Gasoline evaporating)

• Self-Assessment Quiz (2) Q4: Which property of rubbing alcohol (isopropyl alcohol) is a chemical property?

  • Density = 0.786\;\mathrm{g/cm^3}
  • Flammability
  • Boiling point = 82.5^{\circ}\mathrm{C}
  • Melting point = -89^{\circ}\mathrm{C}
  • Correct: Flammability

1.5 Energy: A Fundamental Part of Physical and Chemical Change

• Energy is the capacity to do work, such as pushing a box across the floor, pedaling a bicycle, or moving a car.

• Energy is involved in both physical and chemical changes of matter.

• Examples:

  • Energy release: Combustion of gasoline releases energy, which moves a car.
  • Energy absorption: Photosynthesis in plants absorbs energy.

• Nature and Forms of Energy

  • Nature of Energy:
  • Energy is always conserved in a physical or chemical change; it is neither created nor destroyed (law of conservation of energy).
  • Energy can be transformed from one form to another.
  • Forms of Energy:
  • Kinetic Energy: associated with an object's motion.
  • Potential Energy: associated with an object's position.
  • Mechanical Energy: associated with an object's movement or position.
  • Thermal Energy: associated with an object's temperature/heat.
  • Chemical Energy: associated with chemical reactions.

• Example: Energy in gasoline

  • Gasoline contains chemical potential energy. As it undergoes combustion, energy is released.
  • Some energy is used to move the car (mechanical; kinetic energy).
  • Some energy is released as heat (thermal energy).
  • The molecules that compose gasoline have a relatively high potential energy (high potential change) (more appropriately, they have high chemical potential energy); the molecules formed after combustion have lower potential energy (more stable).
  • Note: Systems with high potential energy tend to change in a direction that lowers their potential energy, releasing energy into the surroundings.

• Conceptual connections to 1.3 and real-world relevance:

  • Energy transformations underpin everyday phenomena (driving, heating, cooling, fueling).
  • Understanding the balance of energy forms helps predict outcomes of physical and chemical changes.

• Additional notes and cross-links:

  • For chemical changes, the identity of substances changes, often accompanied by energy transfer (exothermic vs endothermic processes).
  • For physical changes, energy transfer may occur (e.g., phase transitions involve heat exchange) but the substance's identity remains unchanged.

• Key formulas (conceptual):

  • Energy conservation in a process: E{ ext{initial}} = E{ ext{final}}
  • General energy decomposition: E_{ ext{total}} = K + U
  • Gasoline combustion (illustrative, not balanced):
    ext{Fuel} + \mathrm{O2} \rightarrow \mathrm{CO2} + \mathrm{H_2O} + Q
  • Chemical energy in fuels: E_{ ext{chemical}}

• Summary connections:

  • Physical vs chemical changes determine whether new substances form and how energy is involved.
  • Physical properties help predict how a substance will respond under different conditions, while chemical properties indicate how it may react.
  • Energy underpins both the mechanisms of change and their practical outcomes (motion, heat, work).