BackWater and Life: Properties, Acids/Bases, and Buffers (Chapter 3 Study Notes)
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Water and Life (Chapter 3)
Water's Polarity
Water (H2O) is a unique molecule with important chemical properties due to its polarity. The atoms in water are covalently bonded, but the electrons are not shared equally between hydrogen and oxygen.
Inorganic molecule: Water is classified as inorganic, but its covalent bonds are similar to those found in organic molecules.
Polarity: Oxygen is more electronegative than hydrogen, so electrons spend more time around oxygen, giving it a partial negative charge and hydrogen a partial positive charge.
Result: This polarity allows water molecules to form hydrogen bonds with each other and with other substances.
Diagram: Water molecules have a partial negative charge near the oxygen atom and partial positive charges near the hydrogen atoms.
Properties of Water
Water exhibits four key properties that are essential for life:
1. Cohesion and Surface Tension
Cohesion: Hydrogen bonds hold water molecules together, resulting in high cohesion.
Surface tension: Cohesion at the surface creates surface tension, allowing small insects like water striders to walk on water.
Example: Water striders walking on the surface of a pond.
2. Moderation of Temperature
Heat bank: Water moderates temperature by absorbing and releasing heat slowly.
Kinetic energy: Energy of motion in molecules.
Thermal energy: Total kinetic energy due to molecular motion.
Temperature: Average kinetic energy of molecules in a substance.
Calorie: Amount of heat needed to raise the temperature of 1g of water by 1°C.
Specific heat: Water has a high specific heat (), meaning it resists temperature changes.
Heat of vaporization: Water requires a lot of energy to change from liquid to gas.
3. Floating of Ice on Liquid Water
Density: Ice is less dense than liquid water due to hydrogen bonding, so it floats.
Ecological importance: Ice forms on the surface of bodies of water, insulating the liquid below and allowing aquatic life to survive.
4. Water as the Solvent of Life
Solution: A homogeneous mixture of solute and solvent.
Solute: The dissolved substance (e.g., NaCl).
Solvent: The dissolving substance (water in biological systems).
Aqueous solution: Solution in which water is the solvent.
Hydrophilic: Water-loving substances that dissolve easily in water.
Hydrophobic: Water-hating substances that do not dissolve in water.
Acids and Bases
Water can dissociate into ions, making it central to acid-base chemistry in biological systems.
Dissociation:
Acids: Molecules that release hydrogen ions () in water.
Bases: Molecules that release hydroxide ions () in water.
pH scale: Measures the concentration of hydrogen ions;
Scale: Each pH unit represents a tenfold change in concentration.
pH Value | Type |
|---|---|
0-6 | Acidic |
7 | Neutral |
8-14 | Basic (Alkaline) |
Buffers
Buffers are essential for maintaining stable pH in biological systems.
Definition: Chemicals that take up excess hydrogen ions () or hydroxide ions ().
Function: Help keep the pH constant in solutions, such as blood.
Example: Buffers maintain blood pH at 7.4.
Additional info: Buffers are crucial in homeostasis, preventing harmful changes in pH that could disrupt cellular processes.
