Chapter 3 Water - Georgetown ISD
AP Biology Fall 2013
Water is a polar molecule
Hydrogen bonds hold water molecules together
Properties of Water
Cohesion – tendency of similar molecules to stick together. Water molecules stick to each other.
Water molecules will bead up as well as follow each other in a line
Mostly due to cohesion, water molecules will follow each other up a tube against gravity… how water gets from roots to leaf.
Related to Cohesion
Surface tension
Because the water molecules stick together, they will make an ordered arrangement at the interface between water and air. This makes water behave as if it has a film on top. Think about overfilling a glass slightly. Surface tension is a measure
of how hard it is to break the interface between water and air.
Properties of Water due to hydrogen bonding
Adhesion – tendency of molecules to stick to other substances. Water molecules stick to other surfaces.
Water will temporarily aid in gluing two surfaces together.
Due to adhesion, water molecules will stick to the side of a tube and defy gravity. This is called capillary action. This also aids in water moving from the roots to the leaf of a plant.
Moderation of Temperature
Water resists changes in temperature
Water has a high specific heat
Water will retain its temperature after absorbing or releasing large amount of heat
Why? Because to make a hydrogen bond, water must release heat and to break the hydrogen bond, water must absorb heat.
Examples and application of high specific heat
Metal pot will burn you before water in the pot will.
Swimming pools, lakes, etc in the summer are cooler than the surrounding air during the day and can be warmer after the air has cooled.
Coastal cities tend to be cooler in the summer and warmer in the winter.
Water’s high specific heat helps with temperature moderation for living things on the land as well as in the ocean and within the body.
Temperature Moderation
Water has a high heat of vaporization –
it takes more heat to cause water to vaporize (turn to a gas).
As a liquid evaporates, the “hottest” molecules are the first to leave and thus what’s left behind is the cooler molecules.
This is called “evaporative cooling” and it’s how sweating helps cool you down.
Ice, Ice Baby
Water is most dense at 4o C.
As water forms ice crystals the hydrogen bonds are stable and the water molecules have more space in between them
The density of ice is less than that of the liquid form of water
So why is this important?
Water will freeze and float to the top of a body of water and insulate what’s below it.
If frozen water sank, eventually all bodies of water would freeze from the bottom up meaning that only the top few inches of water in the summer would be liquid.
Frozen water vs. frozen benzene
Solvent
Water is an excellent solvent
Ions are surrounded by water molecules.
These structures are called hydration shells
Many ions are dissolved in sea water as well as tears, blood, sweat, etc.
Polar Molecules
Remember that polar molecules are hydrophilic molecules and hydrophilic molecules dissolve in water (like sugar)
Well, I lied.
Actually, I just want to point out some exceptions: large (we’re talking really large) molecules (for example: cotton) don’t dissolve because large portions of them are hydrophobic
Dissociation of water
Water can dissociate into 2 kinds of ions
H2O (( H+ + OH-
hydrogen hydroxide
ion ion
Acids will increase the volume of hydrogen ions
Bases will increase the volume of hydroxide ions
We use a pH scale to measure how acidic or basic a solution is.
Scale from 0 to 14. 7 is neutral Above 7 is basic Below 7 is acidic
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