Even though the water appears completely still, its particles are continuously moving in random directions, bumping into food coloring molecules and scattering them evenly (diffusion).
Model Key Diffusion Vectors
Time = 0 (Drop)
Dye
Concentrated
Time = 5 min
•→ ←• ↑• •↓ →• •↗
Dispersed Everywhere
Look for: Motion arrows pointing randomly in all directions; dye spreading outwards.
Page 2 Key
Suggested Scoring: 50 Total Points (10 pts per postulate & synthesis)
POSTULATE 4
Suggested Demo: Food Coloring in Cold vs. Hot Water
1. Expected Observations:
In hot water, food coloring disperses completely in 10–15 seconds. In cold ice water, the food coloring sinks to the bottom and diffuses very sluggishly over several minutes.
2. Scientific Reasoning (Grading Standard):
Thermal energy increases the kinetic energy (speed) of particles. High-energy particles collide with greater force and frequency, spreading out further and dispersing the dye rapidly.
Model Key Kinetic Energy Model
Cold (Low KE)
Short arrows (•→) Close spacing
Hot (High KE)
Long arrows (•⟶) Wide spacing
Look for: Longer/more numerous velocity indicators and greater spacing between hot particles.
POSTULATE 5
Suggested Demo: Drops on a Penny (Water vs. Alcohol)
1. Expected Observations:
Water forms a huge dome on the penny holding 30–40+ drops before bursting. Rubbing alcohol or soapy water flattens quickly and spills off after only 12–18 drops.
2. Scientific Reasoning (Grading Standard):
Strong intermolecular cohesive forces between polar water molecules pull them together, creating surface tension that resists gravity until the mass overcomes attractive forces.
Model Key Attraction Bonds
Water (Strong)
•••
Strong cohesion bonds (bulging bead)
Alcohol (Weak)
––
Weak attraction (flattens out)
Look for: Lines, springs, or arrows representing attractive forces pulling inward.
Full Credit Criteria
1. Tire Pressure in Winter:
Key Answer: Colder temperatures decrease particle kinetic energy (speed) (Postulate 4). Slower-moving air particles collide with the inner tire walls less frequently and with less force. As they slow down, particles move closer together (Postulate 1), occupying less volume and lowering internal air pressure without any air leaking.
2. Aroma Across the Room:
Key Answer: Scent particles evaporate and move in constant random motion (Postulate 3), diffusing through empty spaces between air particles (Postulate 1). The heat from the oven increases the kinetic energy and speed of scent particles (Postulate 4), allowing them to travel rapidly across the room.
3. Solid vs. Liquid vs. Gas:
Key Answer: In solids, very strong attractive forces hold particles tightly together in fixed positions with minimal spacing (vibrating in place), preserving fixed shape. In gases, attractive forces are negligible, and huge empty spaces exist between particles, allowing them to fly freely and expand to fill any container.
