Water

The search for life is really a search for

water.

We’ve met and discovered many organisms that have surprised us in their abilities to survive and even thrive in many harsh and inhospitable environments.

But we’ve never met one that can survive without water.

Mars video

Why?

The water molecule

1. Water is clingy

Polarity

Due to the unequal sharing of electrons in a covalent bond

Hydrogen bonding…

Leading to the property of cohesion

(the attraction among like molecules)

Surface Tension

How difficult it is to stretch or break the surface of a liquid.

Water is not the only substance with charged regions, so the charged areas of the water molecule can be attracted to oppositely charged areas on other types of molecules resulting in adhesion…

the attraction of unlike molecules.

Capillarity (capillary action)

Water conducting cells

100 µmFigure 3.3

The combined effects of cohesion and adhesion

2. Water absorbs and releases heat s-l-o-w-l-y.

HeatIs a measure of the total amount of kinetic energy of the molecules

TemperatureMeasures the average speed of one molecule of the substance.

Think of a cool swimming pool vs a hot cup of tea. Which has the higher temperature?Which has more heat energy?

The specific heat of a substance:The amount of heat that must be absorbed or lost for 1 gram of that substance to change its temperature by 1’C.

The specific heat of water is 1 cal/g ‘CSo…

A calorie is the amount of heat required to raise 1g of water by 1’C.

A kilocalorie (kcal, or Calorie) is the amount of heat required to raise 1kg of water by 1’C.

Water has an extremely high specific heat, which allows it to minimize temperature fluctuations on the Earth and in your body.

Have you ever gone in the ocean on a nice, hot day in early June?

What about a mildly cool day in mid-September?

Most deserts are extremely hot during the day, but what happens when the sun goes down?

Water moderates air temperature by absorbing heat from air that is warmer and releasing the stored

heat to air that is cooler.

The Gulf Stream redistributes the heat

energy from the equator to the higher latitudes of

Western Europe.

A similar thing happens when you perspire…each droplet of sweat contains a large amount of heat energy that is sent to the surface of your body to be released to the air around you as it turns to a gas. You warm up the air when you sweat!

Evaporative Cooling

In order for one gram of liquid water to turn into a gas at room temp, 580 calories of heat energy must be absorbed. Again, water has a very high heat of vaporization compared to most other substances.

Remember…breaking bonds releases energy (exergonic) and forming bonds requires energy (endergonic).

Heat of Vaporization

3. Solid water FLOATS!!!

Ice is less dense than liquid water and therefore floats on it. It serves to insulate the water below the

ice. (Ponds are not frozen to the bottom!...life continues all winter).

Liquid water

Hydrogen bonds constantly break and re-form

IceHydrogen bonds are stable

Hydrogen bond

Figure 3.5

To form the crystal lattice, water molecules “push away” from each other.

4. Universal Solvent

Because water is polar it can dissolve most charged solutes and ionic compounds.

+

+

+

+Cl –

–

–

–

–

Na+Positive hydrogen regions

of water molecules cling to chloride anions

(Cl–).

+

+

+

+

–

–

–

–

–

–

Na+

Cl–

Figure 3.6

Negative oxygen regions of polar water molecules are attracted to sodium cations (Na+).

Hydration Shell

(a) Lysozyme molecule

in a nonaqueous

environment

(b) Lysozyme molecule (purple)

in an aqueous environment

such as tears or saliva

(c) Ionic and polar regions on the protein’s

Surface attract water molecules.

+

–

Figure 3.7

This oxygen is attracted to a slight positive charge on the lysozyme molecule.

This hydrogen is attracted to a slight negative charge on the lysozyme molecule.

Hydrophilic; “water-loving”polar or ionic substancescarbohydrates, proteins, ionic compounds

Hydrophobic; “water-fearing”non-polar substancesfats, oils, lipids

So what if I need to dissolve a non-polar compound?

Use a non-polar solvent.

Remember the rule “Like dissolves like”.

A moleRepresents an exact number of molecules of a substance in a given mass

How many moles are there in 20 grams of hydrogen? H’s atomic mass ~ 1 Moles = mass/relative mass = 20/1 = 20 moles.

MolarityIs the number of moles of solute per liter of solution

Measures concentration

5. Acidity and Alkalinity

H

Hydroniumion (H3O+)

H

Hydroxideion (OH–)

H

H

H

H

H

H

+ –

+

Figure on p. 34 of water dissociating

Water dissociates into hydronium and hydroxide ions

Acids; increase hydrogen ion concentration

Bases; decrease hydrogen ion concentration (increase hydroxide ion concentration)

Remember that pH is based on the negative log of the concentration of hydrogen ions, so a higher pH number is a lower concentration of ions.

Buffers in the blood…

The internal pH of most cells is 7Human blood is about 7.4

Buffers are substances that minimize changes in the concentrations of hydrogen and hydroxide ions in a solution

They consist of an acid-base pair that either donates or accepts hydrogen ions in reversible reactions.

Living things make the buffers they need to maintain homeostasis.