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/ | The energy needed per unit mass to change liquid into vapour. |
| hydrolysis/haɪˈdrɒləsɪs/ | Breaking chemical bonds by reaction with water. |
| condensation/kɒndenˈseɪʃn/ | A chemical reaction in which molecules join and a small molecule, commonly water, is eliminated. |
A stable pond and a cooling wet leaf
- A pond can warm slowly while an evaporating wet leaf loses heat. Both depend on water, but they involve different changes: warming a liquid and changing liquid into vapour.
- Water is polar 极性, and its molecules form hydrogen bonds 氢键. Explain how those features produce a property, then link the property to a biological role.
Locate the hydrogen bond correctly
- Oxygen attracts the shared electrons in an O–H bond more strongly than hydrogen. The oxygen has a partial negative charge (δ−), and the hydrogens have partial positive charges (δ+); these are not complete ionic charges.
- A hydrogen bond attracts a δ+ hydrogen on one water molecule to a δ− oxygen on another. The O–H bonds within each molecule are covalent. Do not draw the intermolecular bond between two hydrogens or imply that water must split into ions to become polar.
Dashed bonds connect different molecules; solid bonds connect atoms within one molecule.
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 is a polar molecule that forms hydrogen bonds.
Difficulty 1/5 · 100 coins + a small bonus on first completion today
Polarity (a slightly negative oxygen and slightly positive hydrogens) lets neighbouring molecules form hydrogen bonds, which explain water's properties.
Water molecules attract each other because water is:
Difficulty 1/5 · 100 coins + a small bonus on first completion today
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.
Difficulty 1/5 · 100 coins + a small bonus on first completion today
Each δ+ hydrogen is attracted to a δ− oxygen of a neighbour — a hydrogen bond. Many of these give water its special properties.
Draw two water molecules on paper and label partial charges. Show a hydrogen bond between them with a dashed line and an O–H covalent bond with a solid line. Describe the endpoints of each here.
Within each molecule, solid covalent bonds join O to its two H atoms. Oxygen is δ− and hydrogen δ+. The dashed hydrogen bond joins a hydrogen of one molecule to the oxygen of the other, with no intervening hydrogen atom on its drawn path. It is not H···H, O···O or an extra covalent O–H bond. Keep the drawing for teacher review.
Use polarity to explain solvent 溶剂 action
- Water surrounds dissolved ions: oxygen ends face positive ions and hydrogen ends face negative ions. Water can also interact with polar molecules such as glucose, allowing them to dissolve.
- Dissolved substances can move in blood plasma, cytoplasm and xylem, and can meet in solution for reactions. Water does not dissolve every substance: non-polar triglycerides remain largely insoluble.
A positive sodium ion and a negative chloride ion dissolve in water. Which ends of nearby water molecules face each ion? Link this to one biological use of water as a solvent.
Water’s δ− oxygen ends face sodium ions; δ+ hydrogen ends face chloride ions. Interactions with ions help keep them dispersed in solution. Water can therefore transport dissolved mineral ions, for example in xylem, or provide a medium where dissolved reactants meet. The water molecules remain polar molecules rather than becoming sodium or chloride ions.
High specific heat capacity 比热容
- Specific heat capacity is the energy needed to raise the temperature of a unit mass by one degree. Water's value is high: some supplied energy overcomes intermolecular attractions, so a large heat input is needed for a given temperature rise.
- Water-rich bodies and habitats therefore resist rapid temperature changes. This helps keep conditions stable for enzymes and cells; it does not mean that water can never become hot or that no energy is transferred.
Water has a high specific heat capacity, which means it:
Difficulty 1/5 · 100 coins + a small bonus on first completion today
A high specific heat capacity means temperature changes slowly, protecting organisms from sudden changes.
High latent heat of vaporisation 汽化潜热
- Latent heat of vaporisation is the energy needed per unit mass to change liquid into vapour. Many hydrogen bonds must be overcome for water molecules to leave the liquid; covalent O–H bonds remain intact.
- Evaporation takes energy from the leaf, skin and surrounding liquid, producing cooling. Transpiration and sweating can therefore remove much heat per mass evaporated; sweat that merely stays on the skin has not yet supplied this evaporative cooling.
A leaf loses heat as water evaporates from moist cell walls. Link all three steps: hydrogen bonds between water molecules must be overcome → evaporation needs energy → energy leaves the leaf, cooling it. “Water has hydrogen bonds” alone stops before explaining the biological effect.
Why does sweating cool the body?
Difficulty 1/5 · 100 coins + a small bonus on first completion today
Evaporating water needs a lot of energy (high latent heat of vaporisation); it takes that heat from the skin, cooling it.
Two equal wet cloths cover warm objects under otherwise matching conditions. One cloth loses more water by evaporation. Explain the expected difference in cooling using the bonds involved, without claiming that water molecules split apart.
The cloth losing more water by evaporation can remove more heat, if the other conditions are equal. Energy is needed to overcome hydrogen bonds between water molecules as they leave the liquid; this energy is taken from the cloth, object and surroundings. The covalent O–H bonds within each molecule stay intact. The argument concerns latent heat, not a chemical decomposition into hydrogen and oxygen.
Choose the property that fits the evidence
| Observation | Relevant property | Reasoning |
|---|---|---|
| Liquid water warms slowly for its mass and heat input | High specific heat capacity | Much energy is needed for a given temperature rise |
| A wet surface cools as water becomes vapour | High latent heat of vaporisation | Evaporation removes much energy |
| Glucose travels dissolved in plasma | Solvent action | Polar water interacts with the solute |
- Both thermal properties involve intermolecular attractions, but the physical changes differ. Choose using the observation, rather than using either property name for every temperature question.
- Water remains H₂O when it evaporates. Boiling or evaporation is not hydrolysis 水解 and does not break each molecule into hydrogen and oxygen.
Match each property of water to what it does for life.
Difficulty 2/5 · 140 coins + a small bonus on first completion today
All of water's life-supporting properties come back to its polarity and the hydrogen bonds it forms.
Water in condensation 缩合 and hydrolysis
- In condensation, linking units can produce water. In hydrolysis, water is a reactant used to break an inter-unit bond. Identify which side of the reaction water belongs on.
- These are chemical reactions, unlike evaporation's physical change of state. Keep the bond broken in a polymer separate from hydrogen bonds between nearby water molecules.
A diagram shows a polymer being split into smaller units with water on the arrow’s reactant side. Another shows liquid water turning into vapour. Name each process and distinguish what happens to chemical bonds.
The first is hydrolysis: water is used to break an inter-unit covalent bond in the polymer. The second is evaporation: intermolecular attractions between water molecules are overcome, while each H₂O molecule’s covalent bonds remain intact. Condensation would join units and release water, so it is not the first process.
You've got it
- Unequal electron sharing makes water polar; hydrogen bonds connect H on one molecule to O on another.
- Solvent action supports reactions and transport of dissolved ions and polar molecules.
- High specific heat capacity limits temperature change; high latent heat of vaporisation supports evaporative cooling.
- Evaporation overcomes intermolecular attractions; the covalent bonds within H₂O stay intact.
- Water is produced in condensation and used in hydrolysis.
Equal masses of water and another liquid start at the same temperature and receive the same heat input. Neither evaporates. Water’s temperature rises less. Explain which property this demonstrates, how intermolecular attractions contribute and one biological benefit.
Choose the property from the physical change: warming a liquid uses specific heat capacity; changing liquid into vapour involves latent heat.
Which statement explains how evaporating sweat cools the skin?
Difficulty 1/5 · 100 coins + a small bonus on first completion today
The energy for evaporation is removed from the skin and nearby liquid. Intermolecular hydrogen bonds are overcome; water molecules remain H₂O.