Structure of Water and Hydrogen Bonding
Introduced| English | Meaning |
|---|---|
| polar molecule/ˈpəʊlə ˈmɒlɪkjuːl/ | polar molecule |
| electronegativity/ɪˌlektrəʊŋɡəˈtɪvɪti/ | The tendency of an atom in a chemical bond to attract the shared electrons towards itself. |
| partial charge/ˈpɑːʃl tʃɑːdʒ/ | partial charge |
| hydrogen bond/ˈhaɪdrədʒn bɒnd/ | An attraction between a hydrogen atom bonded to an electronegative atom and a suitable electronegative atom with a lone pair. |
| cohesion/kəʊˈhiːʒn/ | The attraction between molecules of the same substance, helping them remain together. |
| surface tension/ˈsɜːfɪs ˈtenʃn/ | The tendency of a liquid surface to resist stretching because of cohesive forces between its molecules. |
| adhesion/ədˈhiːʒn/ | Attraction between molecules of different substances. |
| specific heat/spəˈsɪfɪk hiːt/ | The energy required to raise the temperature of a unit mass of a substance by one degree. |
| solvent/ˈsɒlvənt/ | The substance in which a solute dissolves to form a solution. |
The strangest ordinary liquid
- Life started in water, and your cells are about 70% water.
- Water does things most small molecules cannot: it climbs up trees, beads on a leaf, and resists sudden temperature change.
- All of these "superpowers" come from one feature of the water molecule.
- That feature is its shape and the way it shares electrons.
Roughly what percentage of a typical cell is water?
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Cells are about 70% water — water is the medium in which the chemistry of life happens.
Water is a polar molecule
- Water is $\text{H}_2\text{O}$: one oxygen joined to two hydrogens in a bent shape.
- Oxygen has a stronger pull on electrons — a higher electronegativity 电负性 — than hydrogen.
- So the shared electrons sit closer to the oxygen.
- This gives oxygen a small negative partial charge 部分电荷 and each hydrogen a small positive one.
- A molecule with separated charge like this is a polar molecule 极性分子.
Why is a water molecule polar?
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Oxygen has a higher electronegativity, so the electrons sit closer to it — giving oxygen a partial negative and the hydrogens a partial positive charge.
Opposite charges pull: hydrogen bonds
- The positive H of one water molecule is attracted to the negative O of another.
- This weak attraction is a hydrogen bond 氢键.
- Each water molecule can hydrogen-bond to several neighbours at once.
Sticking together and to other things
- Cohesion 内聚力 is water sticking to itself — hydrogen bonds hold the molecules together.
- Cohesion creates surface tension 表面张力, letting insects walk on a pond.
- Adhesion 附着力 is water sticking to other surfaces.
- Together, cohesion and adhesion pull water up narrow tubes (capillary action), helping water rise in a plant.
Water sticking to itself is called ____.
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Cohesion is water molecules holding onto each other through hydrogen bonds; sticking to other surfaces is adhesion.
A single hydrogen bond is weak — far weaker than the bonds inside the molecule. Water's power comes from having a huge number of them, constantly breaking and re-forming.
A single hydrogen bond is stronger than the bond holding an O and H together inside a water molecule.
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A single hydrogen bond is weak. Water is powerful because it has an enormous number of them.
Select all the properties of water that come from hydrogen bonding.
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Cohesion, high specific heat, and solvent behaviour all come from hydrogen bonding. Being made of H and O is just its composition.
Water resists temperature change
- Water has a high specific heat 比热容: it takes a lot of energy to warm it up.
- The energy first goes into breaking hydrogen bonds, not into raising the temperature.
- This buffers cells and whole oceans against sudden temperature swings.
- Water is also an excellent solvent 溶剂 — its polar molecules surround and dissolve other polar and charged substances.
Will it dissolve in water?
Sort substances by whether water — a polar solvent — can dissolve them.
Why does sweating cool you down?
- To evaporate, water molecules must break their hydrogen bonds — which takes energy.
- They take that energy (as heat) from your skin.
- So as sweat evaporates, it carries heat away and your body cools.
Sweating cools you because evaporating water...
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Evaporation must break hydrogen bonds, which needs energy. That energy is taken as heat from your skin, cooling you.
Water is a polar molecule, so its molecules form hydrogen bonds. Those bonds explain cohesion, adhesion, surface tension, a high specific heat, and water's power as a solvent — the properties that make life possible.
Write from 4 hydrogen-bond neighbours
- One water molecule has two hydrogen atoms and two lone pairs on oxygen.
- The cell-water percentage in the lesson is about 70, which is not this count.
How many neighbouring molecules can one water molecule hydrogen-bond with at once?
Two donated and two accepted hydrogen bonds make 4.
How many hydrogen-bond neighbours can one water molecule have?
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Water can donate two and accept two. Other options — 70 (the rough percent of a cell that is water); 1 (as if only one bond were possible).