Water: a molecular cause for a biological property
הוצג| English | משמעות |
|---|---|
| cohesion/kəʊˈhiːʒn/ | דביקות |
| adhesion/ədˈhiːʒn/ | היצמדות |
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.
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.
What accounts for cohesion in liquid water?
רמת קושי 1/5 · 100 מטבעות + בונוס קטן בהשלמה הראשונה היום
Water molecules attract each other through hydrogen bonds between molecules, and that intermolecular attraction produces cohesion. The covalent O-H bonds sit INSIDE each molecule and are not what pulls molecules together.
Match each technical term to its precise meaning.
רמת קושי 2/5 · 140 מטבעות + בונוס קטן בהשלמה הראשונה היום
Use the definitions to distinguish related quantities and processes.
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.
In the lab: 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.
Which two habits make the investigation or model in this case more defensible?
רמת קושי 2/5 · 140 מטבעות + בונוס קטן בהשלמה הראשונה היום
Stating the conditions and assumptions lets others repeat the work exactly, and recording the supporting evidence lets them check the conclusion. Hiding disagreeing results removes the evidence that could challenge a wrong conclusion, and one measurement can never establish a wider claim.
Work from known quantities
- 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.
Your turn: Use c = 4,180 J/(kg K). Calculate Q for 0.200 kg water heated through 5.0 K.
Use c = 4,180 J/(kg K). Calculate Q for 0.200 kg water heated through 5.0 K.
רמת קושי 2/5 · 140 מטבעות + בונוס קטן בהשלמה הראשונה היום
The result is 4180 J. Known: m = 0.200 kg; c = 4,180 J/(kg K); temperature rise 5.0 K. Use Q = m c delta T. Q = 0.200 x 4,180 x 5.0 = 4,180 J. This excludes heating of the container and losses.
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.
Not true: Boiling water normally breaks its O–H covalent bonds. Boiling separates molecules from each other, overcoming intermolecular hydrogen bonds. The covalent O-H bonds inside each molecule survive - the steam is still H2O.
- Water molecules are polar.
- The same intermolecular interactions help explain surface tension and the energy needed to change temperature.
- Watch for: hydrogen bonds between molecules differ from covalent bonds within a water molecule.
Write from a heat transfer of 2508 J
- 0.200 kg of water is heated through 3.0 K. Use 4180 J per kilogram per kelvin.
- Find Q. Boiling separates molecules. It does not break the covalent O-H bonds.
Boiling separates molecules from each other, overcoming intermolecular hydrogen bonds.
רמת קושי 1/5 · 100 מטבעות + בונוס קטן בהשלמה הראשונה היום
True. Boiling separates molecules from each other, overcoming intermolecular hydrogen bonds. The covalent O-H bonds inside each molecule survive - the steam is still H2O.
Attraction between molecules of the same substance: write the technical term.
רמת קושי 1/5 · 100 מטבעות + בונוס קטן בהשלמה הראשונה היום
cohesion means Attraction between molecules of the same substance.
0.200 kg of water is heated through 3.0 K. Use 4180 J per kilogram per kelvin. What is Q, and what does boiling break?
0.200 times 4180 is 836, and 836 times 3 is 2508 J. Steam is still water.
What is Q for this heating?
רמת קושי 1/5 · 100 מטבעות + בונוס קטן בהשלמה הראשונה היום
0.200 times 4180 times 3.0 is 2508 J.