Biology Now, 2e

46 ■ CHAPTER 03 Chemistry of Life

CELLS

Water can exist in all three states of matter:

liquid, solid, and gas. Hydrogen bonds explain

the physical properties of water in these three

states. Though water is composed of two

elements that are gases at room temperature

(hydrogen and oxygen), it forms a liquid at room

temperature because hydrogen bonds stick water

molecules together, keeping them close. Those

hydrogen bonds are constantly forming and

breaking in water, creating a nonstop jostling

that gives water its liquid form (Figure 3.6, left).

When water chills, its molecules cannot

move about as vigorously (Figure 3.6, right).

As water turns into ice at 0°C (32°F), a stable

Figure 3.6

Water molecules change state as hydrogen bonds increase or decrease

Japanese snow macaques escape the cold with a daily dip in natural hot springs. Water can be seen

here in its liquid, solid, and gas states.

Q1: Identify where in the picture water can be seen in its liquid, solid, and gas states.

Q2: In the gas state, water molecules move too rapidly and are too far apart to form hydrogen

bonds. Compare the volumes occupied by an equal number of water molecules in the liquid, solid,

and gas states.

Q3: Explain in your own words how ice floats on water.

Hydrogen bonds develop readily

between adjacent water molecules,

with the partially positive pole of one

water molecule tugging at, and being

tugged upon by, the partially negative

pole of a neighboring water molecule.

When water freezes,

hydrogen bonds

become more rigid,

and water molecules

become stacked in a

3D network, forming a

crystal lattice.

Water molecules are

spaced farther apart in

an ice crystal, making

ice less dense than

water and able to float

on it.

When water heats up, energy from the

heat breaks the hydrogen bonds, and

the number of intact hydrogen bonds

decreases as the temperature rises.

As a result, some water molecules

escape as vapor, or steam.

When water is liquid,

hydrogen bonds form

between some water

molecules, but not all of

them, and the water

molecules can move

about.

Hydrogen bonds are

constantly forming...

...and breaking.

Hydrogen

bonds

Slightly

negative

Slightly

positive

+

–

H H

O

1

2

3

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