Water
Introduced| English | Meaning |
|---|---|
| polar/ˈpəʊlə/ | Describing a molecule or bond with an uneven distribution of electric charge that creates partial positive and negative regions. |
| hydrogen bond/ˈhaɪdrədʒn bɒnd/ | An attraction between a hydrogen atom bonded to an electronegative atom and a suitable electronegative atom with a lone pair. |
| solvent/ˈsɒlvənt/ | The substance in which a solute dissolves to form a solution. |
| specific heat capacity/spəˈsɪfɪk hiːt kəˈpæsɪti/ | The energy needed to raise the temperature of one kilogram of a substance by one kelvin or one degree Celsius. |
| latent heat of vaporisation/ˈleɪtənt hiːt ɒv ˌveɪpəraɪˈzeɪʃn/ | latent heat of vaporisation |
The molecule life can't do without
- Water seems ordinary, but life depends on its unusual behaviour.
- Almost everything about it comes from one fact: the water molecule is polar 极性.
- Polarity lets water molecules grip each other with hydrogen bonds 氢键.
Water is a polar molecule that forms hydrogen bonds.
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Polarity (a slightly negative oxygen and slightly positive hydrogens) lets neighbouring molecules form hydrogen bonds, which explain water's properties.
Polar molecule, hydrogen bonds
- In water the oxygen end is slightly negative and the hydrogen ends slightly positive — it is a polar molecule.
- Opposite charges attract, so each molecule forms weak hydrogen bonds with its neighbours.
- These many small bonds are the reason for water's special properties.
Water is polar, so each molecule forms hydrogen bonds with its neighbours
- Explore the molecule below — then see why this matters.
- Water is also a key reactant: it is removed when molecules join (condensation) and added when they are split (hydrolysis).
Water as a reactant: condensation removes it to join molecules; hydrolysis adds it to split them.
Why water is polar
Tap each part. Oxygen pulls the shared electrons closer, so it is slightly negative and the hydrogens slightly positive — and these opposite charges form hydrogen bonds.
Water molecules attract each other because water is:
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Water is polar (slightly negative oxygen, slightly positive hydrogens), so neighbouring molecules form hydrogen bonds.
The weak attractions between water molecules, caused by its polarity, are called ______ bonds.
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Each δ+ hydrogen is attracted to a δ− oxygen of a neighbour — a hydrogen bond. Many of these give water its special properties.
Three life-giving properties
- Solvent 溶剂 — many substances dissolve in water, so reactions can happen and substances can be transported around the body.
- High specific heat capacity 比热容 — water needs a lot of energy to warm up, so temperatures stay steady and protect living things.
- High latent heat of vaporisation 汽化潜热 — water needs a lot of energy to evaporate, so sweating carries away a lot of heat and cools the body.
Match each property of water to what it does for life.
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All of water's life-supporting properties come back to its polarity and the hydrogen bonds it forms.
Water has a high specific heat capacity, which means it:
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A high specific heat capacity means temperature changes slowly, protecting organisms from sudden changes.
Why does sweating cool the body?
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Evaporating water needs a lot of energy (high latent heat of vaporisation); it takes that heat from the skin, cooling it.
Water as transport and coolant
- Water's solvent property lets it carry dissolved substances in blood, xylem and cytoplasm.
- Its high specific heat capacity resists temperature change, and evaporation (sweating, transpiration) cools organisms.
You've got it
- water is a polar molecule → forms hydrogen bonds between molecules
- solvent: dissolves substances for reactions and transport
- high specific heat capacity: temperature stays stable
- high latent heat of vaporisation: evaporation (sweat) cools the body