A.1.1 · 水

What would explain this observation?

• A water droplet holds together on a leaf. Its behaviour comes from interactions between molecules, not a skin made of solid water.
• Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.

Build the model

• Water molecules are polar. Attraction between the partially positive hydrogen of one molecule and a partially negative oxygen of another gives hydrogen bonding. Cohesion 内聚力 is attraction between water molecules; adhesion 黏附力 is attraction to another surface.
• cohesion: Attraction between molecules of the same substance; adhesion: Attraction between different substances.

Choose evidence that can test it

• The same intermolecular interactions help explain surface tension and the energy needed to change temperature. Heating changes molecular motion and interactions; it does not normally break the covalent O–H bonds.
• Compare equal drops on clean surfaces under fixed temperature. Measure contact angle or spreading consistently. A detergent changes the system, so record concentration rather than treating all liquids as equivalent.

Work from known quantities

• State the known values and their units. Choose the relation because its assumptions fit this case, then rearrange before substitution.
• Known: 0.100 kg of water warms by 5.0 K; specific heat capacity is 4,180 J/(kg K). Q = mcΔT = 0.100 × 4,180 × 5.0 = 2,090 J. This calculation excludes energy absorbed by the container and lost to the surroundings.

Example:

Use c = 4,180 J/(kg K). Calculate Q for 0.200 kg water heated through 5.0 K. Use the same sequence: known quantities → model → relation → substitution → unit and interpretation.

Check the conclusion and its limits

• Hydrogen bonds between molecules differ from covalent bonds within a water molecule. Polarity alone does not make every substance dissolve well in water.
• Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.

Warn:

Boiling water normally breaks its O–H covalent bonds. This claim is false: Hydrogen bonds between molecules differ from covalent bonds within a water molecule. Polarity alone does not make every substance dissolve well in water.

Key:

Water: a molecular cause for a biological property: The same intermolecular interactions help explain surface tension and the energy needed to change temperature. Heating changes molecular motion and interactions; it does not normally break the covalent O–H bonds.