3. Water Flashcards
The polarity of water molecules and the hydrogen bond that results explain most of water’s life-supporting properties.
Water and polarity
Oxygen is more electronegative then hydrogen, so the electrons of the covalent bonds spend more time closer to oxygen than to hydrogen: these are polar covalent bonds.
The unequal sharing of electrons and water molecule’s v like shape make it a polar molecule meaning that its overall charge is unevenly distributed.
In water, the oxygen of the molecule has two regions of partial negative charge, and each hydrogen has a partial positive charge
Water and Hydrogen Bonds
The properties of water arise from attractions between oppositely charged atoms of different water molecules.
The partially positive hydrogen of one water molecule is attracted to the partially negative oxygen of a nearby molecule. The two molecules are thus held together by a Hydrogen Bond
Are water’s hydrogen bonds strong or weak in liquid form?
The extraordinary properties of water emerge from?
When water is in liquid form its hydrogen bonds are very fragile each only about 1/20 as strong as a covalent bond.
The hydrogen bonds form, break, and reform with great frequency. Each lasts only a few trillionths of a second but the molecules are constantly forming new hyrdogen bonds with a succession of partners.
The extraordinary properties of water emerge from hydrogen bonding which organizes water molecules into a higher level of structural order.
Adhesion
Interaction between H2O and other molecules
Adhesion of water by hydrogen bonds to the molecules of cell walls helps counter the downward pull of gravity.
Cohesion
Interaction between H2O and H2O
At any given moment many of the molecules are linked by multiple hydrogen bonds. These linkages make water more structured then most other liquids. Collectively, the hyrdrogen bonds hold the substance together, this is called cohesion.
Example: Cohesion due to hydrogen bonding contributes to the transport of water and dissolved nutrients against gravity in plants. Water from the roots reaches the leaves through a network of water-conducting cells (Figure 3.3). As water evapo- rates from a leaf, hydrogen bonds cause water molecules leaving the veins to tug on molecules farther down, and the upward pull is transmitted through the water-conducting cells all the way to the roots.
Surface Tension
A measure of how difficult it is to stretch or break the surface of a liquid.
Cohesion at the surface of liquid
Water has ___ Specific Heat
What does that mean?
What is specific heat?
Water has a strong resistance to change in temperature.
Water moderates air temperature by absorbing heat from air that is warmer and releasing the stored heat to air that is cooler.
This ability stems from its relatively high specific heat. The specific heat of a substance is defined as the amount heat that must be absorbed or lost for 1g of that substance to change its temperature by 1°C.
The specific heat of water is 1 calorie per gram and per degree celsius.
Compare that to iron which is .1 calorie per gram and per degree celsius
Evaporative Cooling
- As a liquid evaporates, the surface of the liquid that remains behind cools down (its temperature decreases).
- This evaporative cooling occurs because the “hottest” molecules, those with the greatest kinetic energy, are the most likely to leave as gas.
- Analogy: It is as if the 100 fastest runners at a college transferred to another school; the average speed of the remaining students would decline.
- Evaporative cooling of water contributes to the stability of temperature in lakes and ponds and also provides a mechanism that prevents terrestrial organisms from overheating.
- For example, evaporation of water from the leaves of a plant helps keep the tissues in the leaves from becoming too warm in the sunlight. Evaporation of sweat from human skin dissipates body heat and helps prevent overheating on a hot day or when excess heat is generated by strenuous activity. High humidity on a hot day increases discomfort because the high concentration of water vapor in the air inhibits the evaporation of sweat from the body.
Frozen Water Floats
HYDROPHILIC VS HYDROPHOBIC
