Water is one of the most important molecules in biology because its structure gives it unusual properties that support life. Each water molecule is polar, meaning it has a slightly negative oxygen side and slightly positive hydrogen sides. This polarity lets water molecules attract each other and form hydrogen bonds.
These weak attractions explain many features of water, from surface tension to temperature stability.
Key Facts
- Water is polar because oxygen pulls shared electrons more strongly than hydrogen.
- Hydrogen bonds form between the δ+ hydrogen of one water molecule and the δ- oxygen of another.
- Cohesion is attraction between water molecules, and it helps create surface tension.
- Adhesion is attraction between water and other polar or charged surfaces, helping water climb plant xylem.
- Specific heat capacity of water is about 4.18 J/g°C, so q = mcΔT is often used for heating or cooling water.
- Water dissolves many ionic and polar substances by surrounding charged particles in hydration shells.
Vocabulary
- Polarity
- Polarity is the uneven distribution of charge in a molecule caused by unequal sharing of electrons.
- Hydrogen bond
- A hydrogen bond is a weak attraction between a slightly positive hydrogen atom and a slightly negative atom such as oxygen.
- Cohesion
- Cohesion is the attraction between molecules of the same substance, such as water molecules attracting each other.
- Adhesion
- Adhesion is the attraction between molecules of different substances, such as water sticking to glass or plant cell walls.
- Specific heat
- Specific heat is the amount of energy needed to raise the temperature of 1 gram of a substance by 1°C.
Common Mistakes to Avoid
- Calling hydrogen bonds strong covalent bonds is wrong because hydrogen bonds are much weaker attractions between separate molecules, not shared electron bonds within one molecule.
- Assuming water is nonpolar because its total charge is zero is wrong because the molecule has an uneven charge distribution even though it is electrically neutral overall.
- Confusing cohesion with adhesion is wrong because cohesion is water-to-water attraction, while adhesion is water-to-other-surface attraction.
- Forgetting hydrogen bonds when explaining high specific heat is wrong because energy must disrupt many hydrogen bonds before water molecules move much faster.
Practice Questions
- 1 A 200 g sample of water is heated from 20°C to 35°C. Using q = mcΔT and c = 4.18 J/g°C, how much heat energy is absorbed?
- 2 If 50 g of water releases 1045 J of heat, and c = 4.18 J/g°C, what is the temperature change of the water?
- 3 Explain why water can move upward through narrow tubes in plant stems even though gravity pulls it downward.