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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