Structure of water and hydrogen bonding

Question 1

What is a hydrogen bond?

Answer 1

1. Weaker than covalent
2. Between H and N, O, F (electronegative atoms)

Question 2

Is water a perfect tetrahedral shape?

Answer 2

No
1. Lone pairs repel more than H bonds
2. Angle between H-O is 104 degrees (perfect is 109)

Question 3

Hydrophilic vs Hydrophobic?

Answer 3

1. Philic = Dissolves in water
2. Phobic = Doesn’t dissolve in water

Question 4

What is a meniscus?

Answer 4

Curved liquid surface
Concave = Inwards
Convex = Outwards

Question 5

What is capillary action?

Answer 5

Adhesion in very thin and polar tubes

Question 6

In water what molecules have stronger intermolecular forces?

Answer 6

Surface molecules because they are not being pulled from above - as have less bonding partners, form stronger bonds with partners they do have
(More closely packed and tighter)
(More attracted to each other than surrounding air)

Question 7

What causes surface tension (convex meniscus)?

Answer 7

Cohesion (more attracted to each other than surrounding air)

Question 8

Heat capacity of water

Why does sweating cool you down?

Answer 8

1. Water takes a high amount of heat to break H bonds (high heat of vaporisation)
2. Because of this is can greatly resist changes in temperature
3. When water molecules evaporate - surface they evaporate from gets cooler (evaporative cooling) - molecules with highest kinetic energy are lost to evaporation

Question 9

Density

Why does ice float?

Answer 9

in ice the water molecules are pushed further apart than in liquid water (expands when it freezes) (most other molecules become more dense - making water an anomaly)

Question 10

Polar molecule

Answer 10

A neutral, or uncharged molecule that has an asymmetric internal distribution of charge, leading to partially positive and partially negative regions

Question 11

Cohesion

Answer 11

The attraction of molecules for other molecules of the same kind

Question 12

Adhesion

Answer 12

The attraction of molecules for other molecules of a different kind

Question 13

Density

Answer 13

The mass per unit volume of a substance

Question 14

Specific heat capacity

Answer 14

The amount of heat needed to raise the temperature of one gram of a substance by one degree Celsius

Question 15

Heat of vaporization

Answer 15

The amount of energy needed to change one gram of a liquid substance to a gas at constant temperature

Question 16

Unique properties of water

Answer 16

1. Water is polar
2. Water is an excellent solvent
3. Water has high heat capacity
4. Water has high heat of vaporization
5. Water has cohesive and adhesive properties
6. Water is less dense as a solid than as a liquid

Question 17

Exaplain water’s polarity

Answer 17

Water molecules are polar, with partial positive charges on the hydrogens, a partial negative charge on the oxygen, and a bent overall structure. This is because oxygen is more electronegative, meaning that it is better than hydrogen at attracting electrons.

Question 18

Why is water an excellent solvent?

+ how does this help life?

Answer 18

Water has the unique ability to dissolve many polar and ionic substances. This is important to all living things because, as water travels through the water cycle, it takes many valuable nutrients along with it!

Question 19

Explain water’s high heat capacity

+ how does this help life?

Answer 19

It takes a lot of energy to raise the temperature of a certain amount of water by a degree, so water helps with regulating temperature in the environment. For example, this property allows the temperature of water in a pond to stay relatively constant from day to night, regardless of the changing atmospheric temperature.

Question 20

Explain water’s high heat of vaporization

+ how does this help life?

Answer 20

Humans (and other animals that sweat) use water’s high heat of vaporization to cool off. Water is converted from its liquid form to steam when the heat of vaporization is reached. Since sweat is made mostly of water, the evaporating water absorbs excess body heat, which is released into the atmosphere. This is known as evaporative cooling.

Question 21

Explain water’s cohesive and adhesive properties

+ how does this help life?

Answer 21

Water molecules have strong cohesive forces due to their ability to form hydrogen bonds with one another. Cohesive forces are responsible for surface tension, the tendency of a liquid’s surface to resist rupture when placed under tension or stress. Water also has adhesive properties that allow it to stick to substances other than itself.

These cohesive and adhesive properties are essential for fluid transport in many forms of life. For example, they allow nutrients to be transported to the top of a tree against the force of gravity.

Question 22

Explain why water is less dense as a solid than as a liquid

Answer 22

As water freezes, the molecules form a crystalline structure that spaces the molecules further apart than in liquid water. This means that ice is less dense than liquid water, which is why it floats.
This property is important, as it keeps ponds, lakes, and oceans from freezing solid and allows life to continue to thrive under the icy surface.

Question 23

Common mistake:

Does water dissolve everything because it is the “universal solvent?”

Answer 23

No
Water has the ability to dissolve many substances but the term “universal solvent” is misleading. Water is able to dissolve other polar molecules and ions, such as sugars and salts. However, nonpolar molecules like oils lack partial positive or partial negative charges, so they are not attracted to water molecules. This is why nonpolar substances like oil remain separate when added to water.