The hydrogen bonding in water creates __________, a cohesive force that enables one to slightly overfill a glass or a bug to walk on water.

Question

BlackTom AI · Accepted Answer

Hydrogen bonding between water molecules produces a cohesive force at the liquid’s surface called surface tension, which explains why a glass can be slightly overfilled and why some insects can walk on water.

Relevant knowledge and concepts:
- Hydrogen bonds: each water molecule can form H-bonds with neighbors, creating strong intermolecular cohesion.
- Cohesion vs. adhesion: cohesion is attraction between like molecules (water–water); adhesion is attraction between unlike materials (water–glass).
- Surface tension (γ): the energy or force per unit length at a liquid’s surface caused by unbalanced cohesive forces on surface molecules. Typical γ for water ≈ 0.0728 N/m at 20°C.
- Mechanical balance at the surface: the surface acts like a stretched membrane; an object is supported if surface tension forces around its contact perimeter counteract its weight.

Step-by-step reasoning:
1. Water molecules at the surface have fewer neighboring molecules above them, so net cohesive forces pull surface molecules inward, creating a minimized-area surface with tension.
2. This tension manifests as an apparent “skin” that resists deformation; hence water can bulge above a glass rim without spilling until gravity overcomes the cohesive force.
3. For a small insect or a needle, the upward force from surface tension equals γ times the contact length L around the object (F_up ≈ γ · L). The object stays afloat if F_up ≥ weight (mg).
4. Capillary action is related but distinct: it requires adhesion to a surface plus cohesion to pull liquid up a narrow tube. Surface tension alone (cohesion at a free surface) is the primary cause of the overfilled glass and insect support.

Why other options are incorrect:
- Phase change: describes transitions (liquid↔gas↔solid), not the cohesive surface force.
- Heat exchange: concerns energy transfer, unrelated to the mechanical “skin” effect.
- Capillarity: involves adhesion and confined geometry (rise in narrow tubes), not the free-surface phenomena described.
- Adhesion: attraction to other materials (e.g., water–glass), not the water–water cohesion causing the surface “skin.”

Conclusion: The correct answer is surface tension, matching the provided answer.

The hydrogen bonding in water creates __________, a cohesive force that enables one to slightly overfill a glass or a bug to walk on water.单项选择题

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