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.
Understanding Biology: The Properties of Water
The attractions between water molecules are temporary. They break and reform constantly as molecules move. A single attraction is weak, but a large group produces a strong effect.
This helps explain why water stays liquid over the range of temperatures found in most living environments. To evaporate, water molecules must gain enough energy to escape their neighbours. Sweat removes heat from skin because the fastest moving water molecules are most likely to leave as vapour.
Their departure lowers the average energy of the water left behind. Plants use the same process when water evaporates from leaves, although excessive water loss can cause wilting.
Water behaves unusually when it freezes. In liquid water, molecules can pack fairly close together. In ice, the attractions hold molecules in a more open arrangement.
This makes ice less dense than liquid water, so ice floats. A floating layer of ice acts as insulation on a pond or lake. The water below may remain liquid even during cold weather.
Fish, insects, and microorganisms can survive beneath it. If ice sank, bodies of water could freeze from the bottom upwards and many aquatic habitats would be far less stable.
Water is useful inside cells because it provides a place where many chemical reactions can happen. Ions such as sodium and chloride separate in water because water molecules surround them. The same happens with many small polar molecules, including sugars.
Once dissolved, particles can move through cytoplasm, blood plasma, and tissue fluid. This movement allows cells to receive raw materials and remove wastes. Not every substance mixes with water.
Oils and fats are mostly nonpolar, so water pushes them together rather than surrounding them. Cells use this effect to build membranes. Membrane lipids arrange with their water-friendly ends facing water and their water-avoiding parts tucked inside.
Water movement across cell membranes is especially important. If a cell is placed in a solution with more dissolved particles than its cytoplasm, water tends to move out of the cell. The cell may shrink.
In a more dilute solution, water tends to move into the cell. Animal cells can swell too much and burst, while plant cells become firm because their cell wall resists expansion. This process affects red blood cells in medical fluids, food preservation with salt or sugar, and the water balance of freshwater organisms.
When learning this topic, separate the movement of water from the movement of dissolved particles. Pay close attention to concentration, membrane permeability, and whether the solution is inside or outside the cell.
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.