□ STARTER — Phase Changes, Density & Energy

Read the article below. Then click Begin STARTER.

Dry Ice & Disappearing Solids: What’s Really Happening?

Your teacher drops a chunk of dry ice into warm water. A swirl of fog spills over the beaker and crawls across the table. It looks like magic, but it’s chemistry. Dry ice is solid carbon dioxide (CO₂). At room pressure it doesn’t melt into liquid—it sublimes: it goes solid → gas directly. The fog you see is tiny water droplets formed when cold CO₂ cools the air and water vapor condenses.

States of Matter & Physical Changes

Matter can be a solid, liquid, or gas. Changing between these is a physical change—the particles are the same substance, just arranged or moving differently.

  • Melting: solid → liquid (at the melting point) — absorbs energy (endothermic).
  • Freezing: liquid → solid (at the freezing point) — releases energy (exothermic).
  • Vaporization: liquid → gas. It happens two ways:
    • Evaporation at the surface, at many temperatures.
    • Boiling with bubbles throughout the liquid, at the boiling point.
  • Condensation: gas → liquid — releases energy (exothermic).
  • Sublimation: solid → gas — absorbs energy (endothermic).
  • Deposition: gas → solid — releases energy (exothermic).

Energy Changes: Endothermic vs. Exothermic

During a phase change, temperature holds steady while energy flows in or out to rearrange particles. Endothermic changes absorb energy (melting, evaporation/boiling, sublimation). Exothermic changes release energy (freezing, condensation, deposition). This idea—how heat flows and how energy changes—belongs to thermochemistry.

Temperature, Particles, and Motion

Temperature measures the average kinetic energy of the particles. Particles are always in motion; the faster they move, the higher the temperature. Heat flows from hotter to cooler objects until their temperatures become the same.

Energy comes in more than one form. Kinetic energy is energy of motion. Potential energy is stored energy. Stretch a rubber band—you store elastic potential energy. In chemistry, chemical potential energy is stored in the arrangement of atoms and bonds.

Density: Why Fog Crawls & Ice Floats

Density is how much mass is packed into a certain volume (mass ÷ volume). If a substance is denser than the fluid around it, it tends to sink; if it’s less dense, it tends to rise. CO₂ is denser than air, and the cool, tiny liquid droplets in “dry-ice fog” are dense too—so the cloud sinks and spills over the table. Ice floats because solid water is less dense than liquid water.

To measure density later, you’ll need accurate mass and volume. That’s where careful measurements and the periodic table come in.

Pure Substances & Mixtures

An element is a pure substance made of identical atoms (listed on the periodic table). A mixture combines two or more pure substances with variable amounts. A homogeneous mixture (like salt water) is uniform throughout; a heterogeneous mixture (like salad dressing) is not uniform—you can see different parts.

Real-World Connections

  • Cooking: Boiling pasta vs. simmering sauce uses vaporization and heat flow.
  • Weather: Fog, clouds, and dew form by condensation; snow can form by deposition.
  • Safety: Alcohols evaporate quickly (useful) but vapors can ignite—respect thermochemistry.
  • Engineering: Hot-air balloons rise because heated air has lower density.

Key Vocabulary

Sublimation: Solid → gas (endothermic).
Condensation: Gas → liquid (exothermic).
Evaporation: Liquid → gas at the surface.
Boiling: Liquid → gas with bubbles throughout at the boiling point.
Melting/Freezing point: Temperature where solid↔liquid change occurs.
Endothermic/Exothermic: Energy absorbed vs. released.
Temperature: Measure of average particle kinetic energy.
Density: Mass per unit volume (m÷V).
Element: Pure substance of identical atoms.
Mixture: Variable combination of substances (homogeneous vs heterogeneous).
Thermochemistry: Study of heat and energy changes in processes.

Works offline in modern browsers.