Unit 1: Chapter 3

Question 1

Describe a water molecule

Answer 1

• Shaped like a wide V, with its two hydrogen atoms joined to the oxygen atom by single covalent bonds.
• Oxygen is more electronegative than hydrogen, so electrons of the covalent bonds spend more time closer to oxygen than to hydrogen; these are polar covalent bonds (see Figure 2.11).
• a polar molecule
• In water, the oxygen of the molecule has two regions of partial negative charge (δ-), and each hydrogen has a partial positive charge (δ+).

Question 2

Describe a hydrogen bond

Answer 2

The properties of water arise from attractions between oppositely charged atoms of different water molecules:

• partially positive hydrogen of one molecule is attracted to the partially negative oxygen of a nearby molecule.
• Held together by a hydrogen bond (Figure 3.2).
• When water is in its liquid form, its hydrogen bonds are very fragile - form, break, and re-form with great frequency.
• Each lasts only a few trillionths of a second, but the molecules are constantly forming new hydrogen bonds

Question 3

What is cohesion?

Answer 3

• Water molecules stay close to each other as a result of hydrogen bonding.
• At any given moment many of the molecules are linked by multiple hydrogen bonds.
  
  • These linkages make water more structured than most other liquids.
• Collectively, the hydrogen bonds hold the substance together, a phenomenon called cohesion.

Question 4

What is adhesion?

Answer 4

• Clinging of one substance to another, also plays a role.
• Adhesion of water by hydrogen bonds to the molecules of cell walls helps counter the downward pull of gravity

Question 5

What is surface tension?

Answer 5

Related to cohesion - is a measure of how difficult it is to stretch or break the surface of a liquid.

Question 6

What is kinetic energy?

Answer 6

• Anything that moves has kinetic energy, the energy of motion.
• Atoms and molecules have kinetic energy because they are always moving, although not necessarily in any particular direction.
• The faster a molecule moves, the greater its kinetic energy.

Question 7

What is thermal energy?

Answer 7

• The kinetic energy associated with the random movement of atoms or molecules
• is related to temperature, but they are not the same thing.

Question 8

What is temperature and how is it different form thermal energy?

Answer 8

• Temperature represents the average kinetic energy of the molecules in a body of matter, regardless of volume,
• thermal energy of a body of matter reflects the total kinetic energy, and thus depends on the matter’s volume.

Question 9

Define heat

Answer 9

Thermal energy in transfer from one body of matter to another is defined as heat.

Question 10

Specific heat of water

Answer 10

• water has an unusually high specific heat due to it’s many hydrogen bonds.
• water will change its temperature less than other

liquids when it absorbs or loses a given amount of heat.

• Heat must be absorbed in order to break hydrogen bonds; by the same token, heat is released when hydrogen bonds form

Question 11

Heat of vaporization of water

Answer 11

• Water’s high heat of vaporization is from the strength of its hydrogen bonds, which must be broken before the molecules can exit from the liquid in the form of water vapor.

Question 12

Water as ice

Answer 12

• less dense as a solid than as a liquid. In other words,
• ice floats on liquid water.
  
  • While other materials contract and become denser when they solidify, water expands.
• At temperatures above 4°C, water behaves like other liquids, expanding as it warms and contracting as it cools.
• As the temperature falls from 4°C to 0°C, water begins to freeze because more and more of its molecules are moving too slowly to break hydrogen bonds.
• floating ice insulates the liquid water below, preventing it from freezing and allowing life to exist under the frozen surface,

Question 13

What is a solution

Answer 13

A liquid that is a completely homogeneous mixture of two or more substances

Question 14

What is a solvent and solute?

Answer 14

• Solvent: dissolving agent of a solution
• Solute: the substance dissolved is the .

Question 15

What is a hydration shell?

Answer 15

The sphere of water molecules around each dissolved ion

Question 16

Define hydrophilic

Answer 16

Any substance that has an affinity for water is said to be hydrophilic (from the Greek hydro, water, and philos, loving).

Question 17

Define hydrophobic

Answer 17

• substances that do not have an affinity for water.
• Substances that are nonionic and nonpolar (or otherwise cannot form hydrogen bonds) actually seem to repel water;
• (from the Greek phobos, fearing).

Question 18

What is Molarity?

Answer 18

the number of moles of solute per liter of solution

Question 19

Describe how properties of water contribute to the upward movement of water in a tree.

Answer 19

Because of the properties of cohesion and adhesion

• Cohesion due to hydrogen bonds between water molecules helps hold together the column of water within the cells.
• Adhesion of the water to cell walls by hydrogen bonds helps resist the downward pull of gravity.
• Evaporation from leaves pulls water upward from the roots through water-conducting cells.

Question 20

Why is it unlikely that two neighboring water molecules would be arranged like this?

Answer 20

The hydrogen atoms of one molecule, with their partial positive charges, would repel the hydrogen atoms of the adjacent molecule.

Question 21

Define specific heat

Answer 21

• the amount of heat that must be absorbed or lost for 1 g of that substance to change its temperature by 1°C.