Question

water: a cool molecule
teen channel: science scene
the shape of water molecules helps water drops form. (image)
consider water’s surface tension and density when it freezes. what causes water to have these properties? what effects do these properties have? explain in your own words. cite evidence to support your explanations.
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type your response here.

Explanation:

To answer about water's surface tension, density when freezing, and their causes/effects:

1. Surface Tension: Caused by water molecules’ polarity (hydrogen bonding). Molecules at the surface are pulled inward by cohesive forces, creating a “skin” (e.g., insects walking on water, water forming drops).
2. Density When Freezing: Water expands (becomes less dense) as it freezes. Hydrogen bonds form a crystalline lattice, pushing molecules apart. This causes ice to float (e.g., icebergs, insulating ponds).
3. Causes: Polarity of \( \ce{H2O} \) (oxygen’s electronegativity creates partial charges) leads to hydrogen bonding, driving both properties.
4. Effects: Surface tension supports life (aquatic insects), and floating ice preserves underwater ecosystems.

Answer:

Surface Tension:

• Cause: Water molecules are polar (oxygen is electronegative, creating partial positive/negative charges). Hydrogen bonds between \( \ce{H2O} \) molecules create strong cohesive forces. At the surface, molecules lack upward/sideward bonds, so they’re pulled inward, forming a “tensioned” layer.
• Effect: Allows small objects (e.g., water striders) to rest on water; water forms spherical drops (minimizing surface area).

Density When Freezing:

• Cause: As water cools below 4°C, hydrogen bonds form a hexagonal lattice in ice, spacing molecules farther apart than in liquid water. This increases volume, decreasing density (\( \text{ice density} \approx 0.92 \, \text{g/cm}^3 \); liquid water \( \approx 1 \, \text{g/cm}^3 \)).
• Effect: Ice floats, insulating lakes/ponds (preserving aquatic life in winter); frozen pipes burst due to expansion.

Evidence/Support:

• Surface tension: Observe water beading on wax paper (cohesion > adhesion to wax).
• Freezing density: Ice cubes float in water; frozen lakes show ice layers on top.

(Note: Adjust wording to “your own words” as needed, citing these concepts.)