Water’s Unique Properties

Question 1

What is the chemical composition of water?

Answer 1

Two atoms of hydrogen and one of oxygen, which readily bond, make up each water molecule. Once the hydrogen and oxygen atoms join in covalent bonds, they are difficult to separate, thereby producing a water molecule that remains stable in Earth’s environment.

Question 2

Why do water molecules attract each other?

Answer 2

The nature of the hydrogen–oxygen bond gives the hydrogen side of a water molecule a positive charge and the oxygen side a negative charge. As a result of this polarity, water molecules attract each other: The positive (hydrogen) side of a water molecule attracts the negative (oxygen) side of another—an interaction called hydrogen bonding.

Question 3

Why is water able to dissolve many substances?

Answer 3

The polarity of water molecules also explains why water is able to dissolve many substances; pure water is rare in nature because of its ability to dissolve other substances within it. Without hydrogen bonding to make the molecules in water and ice attract each other, water would be a gas at normal surface temperatures.

Question 4

What is surface tension?

Answer 4

The effects of hydrogen bonding in water are observable in everyday life, creating the surface tension that allows a steel needle to float lengthwise on the surface of water, even though steel is much denser than water. This surface tension allows you to slightly overfill a glass with water; webs of millions of hydrogen bonds hold the water slightly above the rim

Question 5

What is Capillarity?

Answer 5

Hydrogen bonding is also the cause of capillarity, which you observe when you “dry” something with a paper towel. The towel draws water through its fibres because hydrogen bonds make each molecule pull on its neighbour.

Question 6

What must occur for water to change from one state to another?

Answer 6

For water to change from one state to another, heat energy must be added to it or released from it. The amount of heat energy absorbed or released must be sufficient to affect the hydrogen bonds between molecules.

Question 7

Why is the heat exchanged between states if water important?

Answer 7

the heat exchanged between physical states of water provides more than 30% of the energy that powers the general circulation of the atmosphere.

Question 8

What is a Phase change?

Answer 8

The change in phase, or state, among ice, water, and water vapor; involves the absorption or release of latent heat. (See Latent heat.)

Question 9

What are the different phase changes?

Answer 9

Melting and freezing describe the familiar phase changes between solid and liquid. Condensation is the process through which water vapour in the air becomes liquid water—this is the process that forms clouds. Evaporation is the process through which liquid water becomes water vapour—the cooling process discussed in Chapter 4. This phase change is called vaporization when water is at boiling temperature.

Question 10

What is Deposition?

Answer 10

The phase changes between solid ice and gaseous water vapour may be less familiar. Deposition is the process through which water vapour attaches directly to an ice crystal, leading to the formation of frost. You may have seen this on your windows or car windshield on a cold morning. It also occurs inside your freezer

Question 11

What is sublimation?

Answer 11

Sublimation is the process by which ice changes directly to water vapour. A classic sublimation example is the water-vapour clouds associated with the vaporization of dry ice (frozen carbon dioxide) when it is exposed to air.

Question 12

How dose water behave differently below temperatures of 4

Answer 12

Below a temperature of 4°C, water behaves differently from other compounds. Continued cooling makes it expand as more hydrogen bonds form among the slowing molecules, creating the hexagonal (six-sided) crystalline structure characteristic of ice (see Figure 7.2c). This six-sided preference applies to ice crystals of all shapes: plates, columns, needles, and dendrites (branching or treelike forms). Ice crystals demonstrate a unique interaction of chaos (all ice crystals are different) and the determinism of physical principles (all have a six-sided structure).

Question 13

What happens to volume and density of water as it descends below freezing temperature?

Answer 13

As temperatures descend further below freezing, ice continues to expand in volume and decrease in density to a temperature of
–29°C—up to a 9% increase in volume is possible. Pure ice has 0.91 times the density of water, so it floats. Without this unusual pattern of density change, much of Earth’s freshwater would be bound in masses of ice on the ocean floor

Question 14

What must happen for ice to change to water?

Answer 14

For ice to change to water, heat energy must increase the motion of the water molecules enough to break some of the hydrogen bonds

Question 15

What is latent heat?

Answer 15

the heat energy of a phase change is latent heat and is hidden within the structure of water’s physical state.

Question 16

How many calories of heat energy must be absorbed to change 1g of ice to 1 g of water?

Answer 16

In total, 80 calories* of heat energy must be absorbed for the phase change of 1 g of ice melting to 1 g of water—this latent heat transfer occurs despite the fact that the sensible temperature remains the same: Both ice and water measure 0°C

Question 17

What happens with latent head when water freezes?

Answer 17

When the phase change is reversed and a gram of water freezes, latent heat is released rather than absorbed. The latent heat of melting and the latent heat of freezing are each 80 cal·g−1.

Question 18

Definition of water vapor.

Answer 18

Water vapour is an invisible and compressible gas in which each molecule moves independently of the others

Question 19

What is the latent heat of vaporization?

Answer 19

The heat energy absorbed from the environment in a phase change from liquid to water vapour at the boiling point; under normal sea-level pressure, 540 calories must be added to each gram of boiling water to achieve a phase change to water vapour.

Question 20

What is the Latent heat of condensation

Answer 20

Latent heat of condensation
The heat energy released to the environment in a phase change from water vapour to liquid; under normal sea-level pressure, 540 calories are released from each gram of water vapour that changes phase to water at boiling, and 585 calories are released from each gram of water vapour that condenses at 20°C.

Question 21

For example, what is the latent heat energy total of changing 1 g of ice to water and then to vapour?

Answer 21

In summary, the changing of 1 g of ice at 0°C to water and then to water vapour at 100°C—from a solid to a liquid to a gas—absorbs 720 cal (80 cal + 100 cal + 540 cal). Reversing the process, or changing the phase of 1 g of water vapour at 100°C to water and then to ice at 0°C, releases 720 cal into the surrounding environment.

Question 22

Latent heat of sublimation

Answer 22

The heat energy absorbed or released in the phase change from ice to water vapour or water vapour to ice—no liquid phase. The change from water vapour to ice is also called deposition.