AP BIO Properties of Water

Solution

A homogenous mixture which mainly comprises two components, namely solute and solvent

Solute

Any substance that is dissolved in a liquid solvent

Solvent

Substance (molecule) with the ability to dissolve other substances (solutes) to form a solution

Acid

A substance that increases the hydrogen ion concentration of a solution; has a pH lower than 7

Base

A substance that decreases the hydrogen ion concentration in a solution; has a pH higher than 7

Polar molecule

A molecule with an unequal distribution of charge, resulting in the molecule having a positive end and a negative end

Cohesion

The attraction between water molecules due to hydrogen bonding; causes water to collect as drops and fall from the sky

Adhesion

The ability of water to cling to other materials using hydrogen bonds; causes water to climb up the sides of a glass, forming a meniscus

Surface tension

A measure of how hard it is to break the surface of a liquid; insects can "walk" on water due to its high surface tension

Specific heat

The number of calories needed to raise the temperature of 1g of substance by 1ºC

Heat of vaporization

The quantity of heat that needs to be absorbed to vaporize a particular quantity of liquid at a constant temperature

Evaporative cooling

The process of liquid water evaporating from a surface, which decreases the surface's temperature

Hydrophilic

A molecule that can mix and interact with water; "water-loving"

Hydrophobic

A property of molecules that do not mix with water; "water-fearing"

Hydroxide ion

A diatomic anion with chemical formula OH− which consists of an oxygen and hydrogen atom held together by a single covalent bond and carries a negative electric charge

pH

A scale that ranges from 0 to 14 that indicates a substance's alkalinity or acidity determined by the presence of H+ and OH- molecules

Buffer

A solution composed of a weak acid and its conjugate base that can be used to stabilize the pH of a solution

Electronegativity

A measure of the tendency of an atom to attract a bonding pair of electrons

Ionic bonds

Formed after one atom transfers electrons to another; there must be a significant difference in electronegativity for ionic bonds to form

Define 5 characteristics of water that are explained by its ability to form hydrogen bonds

1) Cohesion: Attraction between water molecules due to hydrogen bonds.

2) Adhesion: Ability of water to cling to other materials thanks to hydrogen bonds

3) High surface tension: The measure of how hard it is to break the surface of a liquid (water) in which hydrogen bonds break.

4) High Specific Heat: The amount of energy required to raise 1g of a material by 1°C (water has a temperature that allows it to absorb energy without increasing much in heat)

5) Water as a solvent: water easily dissolves materials by forming hydrogen bonds with the atoms or molecules of the material

Explain how adhesion and cohesion allow plants to draw water from their roots to their leaves

Since water is attracted to other molecules, adhesive forces pull the water towards other molecules. Water is transported in plants through both cohesive and adhesive forces; these forces pull water and the dissolved minerals from the roots to the leaves and other parts of the plant. As water exits the leaves, the adhesion of water molecules pulls more molecules into the roots and upward. Transpiration from the tree's leaves causes tension (negative pressure) to increase in the tree's phloem. As water exits the leaves, the cohesion of water molecules pulls more molecules into the roots and upward.

Explain why it gets hotter during the day and colder at night in Eastlake (which is miles from the ocean) than it does in La Jolla (which is on the coast)

In Eastlake, which is away from the ocean, temperature shifts are extreme due to quick heating during the the day and rapid cooling during the night. In La Jolla, tempreture changes are milder because the ocean moderates the daytime heat and preserves heat at night due to hydrogen bonds. The ocean moderates the tempuratures by absorbing heat during the day and releasing it at night, preventing extreme temperature shifts.

Explain how evaporative cooling works.

Evaporative cooling uses evaporation to help cool the air. Based on the principles of evaporation, hot and dry outside air is drawn through water-soaked cooling pads. As the air is pushed through these pads, the water evaporates and the heat in the air is absorbed, which lowers the air temperature.

Explain why ice floats.

Ice floats because its density is much lower than that of water, which keeps it from sinking

Explain how water causes a salt crystal to dissolve.

Water molecules pull the sodium and chloride ions apart, breaking the ionic bond that held them together. After the salt compounds are pulled apart, the sodium and chloride atoms are surrounded by water molecules. Once this happens, the salt is dissolved, resulting in a homogeneous solution.

What is the difference between hydrophilic and hydrophobic? What types of molecules are in each category?

Hydrophilic molecules are attracted to water, while hydrophobic molecules repel from water. The following groups are hydrophilic: hydroxyl groups, carbonyl groups, carboxyl groups, amino groups, sulfhydryl groups. The following molecules are hydrophobic: alkanes, oils, fats, and greasy substances.

What is the pH of a neutral solution? An acid? A base? Is a solution with a pH of 3 higher or lower in H+ ion concentration than a solution with a pH of 5? How much higher or lower is it?

The pH of a neutral solution is 7. The pH of an acidic solution is below 7 and the pH of a base is above 7. A solution with a pH of 3 will have 100 times more concentration of H+ ions than a solution with a pH of 5.