Concepts | Test 3 - Part III: Water and the pH Scale

Water

All living organisms require water more than any other substance

* Earth is 70% water
* Most cells are surrounded by water
* Cells themselves are about 70-95% water

Water’s polar molecules

Opposite ends have opposite charges, resulting in water molecules readily forming hydrogen bonds with other polar molecules, giving it properties of adhesion and making it “sticky”

Cohesion

Water sticking to water, which can help transport it against gravity

Adhesion

Water is attracted to another substance, which can also help transport it against gravity

Why does ice float?

Ice floats in liquid water as hydrogen bonds in ice are more “ordered,” making the ice less dense

* At 4°C, the molecules are as close together as possible, remaining in constant motion
* At 0°C, the molecules move apart and stop moving
* Since ice floats, the upper layer of ice will maintain heat in lower layers, allowing life to survive when it gets cold

Ice vs. liquid water

* Frozen water (ice): Hydrogen bonds are stable
* Liquid water: Hydrogen bonds break and re-form

pH Scale

Describes whether a solution is acidic or basic by way of its magnitude, with acids being represented by lower values and bases being represented by higher values

Solution

A solution is a liquid that is a mixture of substances, with a solute (eg. sugar) being dissolved in the solvent (eg. water)

Modern day acids and alkalines

Acids (highest to lowest):

* Hydrochloric acid
* Cola
* Wine
* Mineral water

Neutral:

* Blood

Alkaline:

* Soap
* Laundry detergent
* Lye

Acid levels in biological fluids

Most biological fluids are within pH 6-8, with human blood having a pH value of 7.4

* Pure water has equal amounts of hydrogen ions \[H⁺\] and hydroxide ions \[OH⁻\]
* If pH decreases or increases beyond a safe range, cell processes with no longer function correctly

Buffers

Substances that minimize changes in concentrations of H⁺ and OH⁻ in a solution

Air pollutants

Pose a threat to water quality on Earth, as air pollutants originating from urban areas, power plants, vehicles, and so on contain acid precursors that get swept up and transported into ecosystems through various means (air, acid precipitation)

* Water with a pH level outside a normal biological range can cause damage to life

Acid precipitation

Refers to rain, snow, or fog that has a pH value lower than 5.6, which is the threshold for normal rain

* Acid rain has been known to damage buildings and structures made of marble