Water – H2O

Wow – Water! All living things require water more than any other substance. Organisms may survive up to 3 weeks without food, but usually less than a week without water. Water participates in most chemical reactions important for life. Cells are made up of 70-95% water. 75% of Earth is covered in water.

What does this mean?

Polarity Water is polar because of the great difference in electronegativity between its atoms. This allows water to form hydrogen bonds between its molecules. This simple fact explains most of water’s properties.

Hydrogen bonding at work…

Cohesion The “stickiness” of water, the attraction of water molecules to each other due to H-bonds This property allows plants to send water up against gravity (capillary action) Related to adhesion: clinging of water to other substances.

Surface Tension A measure of how difficult it is to stretch or break the surface of a liquid. Also a result of H-bonds: Water molecules bond to each other on the surface and to ones below forming a kind of invisible film. Over fill a glass, water will stand above the rim. Insect walking on water

Surface Tension in Water

Heat and Temperature Water has a high specific heat. This means it can absorb a lot of heat energy without a temperature change. This is also due to H-bonds, most of the heat absorbed is used to break these bonds before it can excite the molecules themselves. Large bodies of water can moderate temperature, both in water and on land.

Evaporative Cooling Water has a high heat of vaporization. This helps moderate climate. This also contributes to cooling of bodies of water and bodies of organisms.

Density Solid water (ice) is less dense than liquid water. This allows life to survive in the harshest of winters in bodies of water. The whole lake does not freeze, just a small layer on top.

The Solvent of Life Solute = substance being dissolved Solvent = substance doing the dissolving Water’s polarity makes it a very useful solvent, but it is not universal. (like dissolves…)

Hydrophobic vs. Hydrophilic Hydrophilic – molecules that “love water” tend to be ionic or polar (salt) Hydrophobic – molecules that “fear water” tend to be not ionic and nonpolar (oil, cell membranes)

Acids & Bases Acids: release protons (hydrogen ions in water) Bases: release hydroxide ions in water This will shift the pH of an environment.

Buffers A buffer is used to minimize changes in pH. Buffers in human blood maintain pH at about 7.4. If it were to stray too far from this number, that human would die :o( Thank you bicarbonate, our body’s buffer friend

Summary Water’s polarity allows it to make hydrogen bonds between molecules. This accounts for cohesion, surface tension, water’s high specific heat and vaporization, and its ability to act as a solvent for life’s reactions.