Chapter3-Water and the Fitness of the Environment

C-3 Water and the Fitness of the Environment.

 

Properties of Water

Most of the properties of water are because it a polar covalent molecule and because of its hydrogen bonds. Living organisms contain about 75% water.

Example: Water is a wonderful solvent because its charged atoms can attract other charged atoms. NaCl in water.

 

Cohesion- water molecules attract each other due to hydrogen bonding. This creates surface tension.

Adhesion- the clinging of one substance to another. Example: water is attracted the walls of plant vessels.

 

Specific Heat- water can absorb a lot of heat before its own temperature rises very much.

The costal regions are cooler because of this. A person out in the sun for a long time does not experience much of a rise in internal body temperature.

 

Heat of vaporization- the amount of heat required to convert liquid water to a gas.

So when you sweat and water evaporates (gas) from your skin it removes a lot of heat and cools you down.

It takes 580 Calories of energy to evaporate 1 gram of water.

 

Calories

Calorie- a measure of energy. It is the amount of heat energy necessary to raise the temperature of 1 gram of water by 1 degree centigrade.

Kilocalorie- used to measure the energy content of food is the amount of heat energy necessary to raise 1000 grams of water by 1 degree centigrade.

 

Water forms an unusual Solid:
Ice

Most liquids become more dense when they solidify and the solid sinks

Water does just the opposite. When it forms a solid (ice) it becomes less dense (expands) and floats.

Water reaches its greatest density at 4 degrees C. Below this temperature it begins to expand.

 

Because ice floats lakes and oceans don’t freeze solid because the floating ice insulates the liquid water below, preventing it from freezing and allowing life to exist under the frozen surface.

 

Water is the solvent of Life

Solution- A liquid that is a completely homogeneous mixture of 2 or more substances. Example: Water + Salt

Solvent- the dissolving agent of a solution

Solute- the substance that is dissolved

 

Hydrophilic- any substance that has an affinity for water. Example: Paper towel + Water

Hydrophobic- any substance that does not have an affinity for water. Example: Oil + Water

 

Moles and Molarity

Mole- the number of grams of a substance that is equal to it’s molecular weight. Example: C12H22O11 is table sugar.

If I weigh out 342 grams of sucrose I will have 1 mole of sucrose.

144 + 22 + 176 = 342

 

 

Molarity- the number of moles of solute per liter of solution.

Example: If I take 342 grams of sucrose ( 1 Mole) and dissolve it in 1 liter of water I will have a 1 molar solution of sucrose.

How many grams of sucrose would it take to make a 0.5 molar solution?________

 

Acids and Bases

Acid- any substance which when added to water increases the hydrogen ion concentration ( H+) or reduces the hydroxide ion (OH-) concentration

Example: HCl + H2O = H+ + Cl-

Acids – Taste sour, are electrolytes and can be detected by turning litmus paper red.

 

Base- any substance which when added to water increases the hydroxide ion concentration or reduces the hydrogen ion concentration.

Example: NaOH + Water = Na+ + OH-

Bases- taste bitter, are electrolytes and can be detected by turning litmus paper blue.

 

A neutral solution has neither acidic or basic properties. In this case the hydrogen ions and hydroxide ions are equal or have been cancelled out or removed.

Examples: Water is neutral. Salt water is neutral.

An acid + a base will neutralize each other. HCl + NaOH= NaCl + H2O

 

The Ph Scale
(Potential Hydrogen)

The Ph scale is a way to measure how acidic or basic a substance is. It ranges from 0-14

0-6 = Acid

7= neutral

8-14 = Base

 

Each number on the pH scale tells us how many hydrogen ions we have in the solution in moles per liter

If we know how many hydrogen ions we have we can also determine how many hydroxide ions we have.

Example: a pH of 1 means _____________

A pH of 2 means _____________

 

Each decrease or increase in number on the pH scale means a tenfold increase in either acidity or alkalinity.

Example pH 6 is 10 times more acid than pH 7

Example pH 12 is 10 times more basic than pH 11

 

Buffers

Buffer- a buffer is a substance that helps to maintain the proper pH inside our bodies.

Blood has a pH of about 7.4. If it drops above or below that we can be in serious trouble.

A buffer works by accepting (removing) hydrogen ions from a solution when they are in excess and donating (releasing) hydrogen ions to a solution when they have been depleted.

 

Acid Rain

Rain water is slightly acidic (pH 5.6) because of the reaction of the rainwater with carbon dioxide in the atmosphere.

H2O + CO2 >>>>> H2CO3 (carbonic acid)

When rainwater, snow or fog becomes more acidic than this (pH number < 5.6) it is referred to as acid rain.

 

Acid rain is caused by the presence of sulfur and nitrogen oxides in the atmosphere which react with water in the air to form sulfuric and nitric acids.

These oxides come from burning coal, oil and gas in factories, automobiles and electric power plants.

 

Damage due to acid rain

Increases the acidity of soil which destroys plants

Increases the acidity of water which destroys aquatic life.