The Water Cycle: A Detailed Explanation with Illustrations | Lecture notes Water Resources Planning and Management

270 Chapter 10 • Safe Drinking Water NEL

10.3 The Water Cycle

Water is the only substance that exists on Earth in each of its

three states. Water easily changes from one state to another. Water

sometimes changes its location by changing state in a continuous

pattern called the water cycle. Th e water cycle is self-renewing and

constant. Th e Sun provides the energy to power the water cycle.

Changes of State

When water changes state in the water cycle, the total number of

water particles remains the same. Th e changes of state include melting,

sublimation, evaporation, freezing, condensation, and deposition. All

changes of state involve the transfer of energy. Figure 1 shows how the

water particles in each state behave as energy is added or removed.

When solid ice gains thermal energy, it changes state from solid ice

to liquid water in a process called melting. Ice cubes in a cold drink,

for example, gradually melt. Each spring you see snow melt into slush

and puddles.

Sometimes adding thermal energy to solid ice causes a change

of state from a solid to a gas. Th is change, directly from a solid to a

gas without becoming a liquid, is called sublimation. On crisp, dry

winter days you might notice that snow banks shrink, or ice gradually

disappears, without fi rst becoming slushy and wet.

When water absorbs enough thermal energy, it becomes a gas

(water vapour). Th is process is called evaporation. Water vapour

mixes with the air and seems to disappear. For example, wet clothes

on a washing line dry because the water evaporates into the air.

water cycle: a continuous pattern

in nature in which water moves as it

changes state above, on, and below

the surface of Earth

melting: the change of state from a

solid to a liquid; occurs when a solid

gains thermal energy

sublimation: the change of state from

a solid to a gas without fi rst becoming

a liquid; occurs when a solid gains

thermal energy

evaporation: the change of state from

a liquid to a gas; occurs when a liquid

gains thermal energy

solid

sublimation

deposition

melting

gas

evaporation

releases energy (heat)

requires energy (heat)

freezing condensation

decreasing thermal energy of particles

increasing thermal energy of particles

Figure 1 Energy is gained or lost whenever water changes state.

Questioning the Text

To maintain your reading focus

and get more meaning from a

text, ask questions as you read.

Begin by scanning the

page and reading the title and

headings. What questions come

to mind about the water cycle?

Read the fi rst few

paragraphs. Stop and refl ect

on what you have read. What

questions do you have? What

more do you want to learn

about this topic?

Move to the next paragraph

and, again, stop to ask

questions. The conversation in

your head will help you think

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10.3 The Water Cycle

Water is the only substance that exists on Earth in each of its

three states. Water easily changes from one state to another. Water

sometimes changes its location by changing state in a continuous

pattern called the water cycle. The water cycle is self-renewing and

constant. The Sun provides the energy to power the water cycle.

When water changes state in the water cycle, the total number of

water particles remains the same. The changes of state include melting,

sublimation, evaporation, freezing, condensation, and deposition. All

changes of state involve the transfer of energy. Figure 1 shows how the

water particles in each state behave as energy is added or removed.

When solid ice gains thermal energy, it changes state from solid ice

to liquid water in a process called melting. Ice cubes in a cold drink,

for example, gradually melt. Each spring you see snow melt into slush

and puddles.

Sometimes adding thermal energy to solid ice causes a change

of state from a solid to a gas. This change, directly from a solid to a

gas without becoming a liquid, is called sublimation. On crisp, dry

winter days you might notice that snow banks shrink, or ice gradually

disappears, without first becoming slushy and wet.

When water absorbs enough thermal energy, it becomes a gas

(water vapour). This process is called evaporation. Water vapour

mixes with the air and seems to disappear. For example, wet clothes

on a washing line dry because the water evaporates into the air.

Questioning the Text

To maintain your reading focus

and get more meaning from a

text, ask questions as you read.

Begin by scanning the

page and reading the title and

headings. What questions come

to mind about the water cycle?

Read the fi rst few

paragraphs. Stop and refl ect

on what you have read. What

questions do you have? What

more do you want to learn

about this topic?

Move to the next paragraph

and, again, stop to ask

questions. The conversation in

your head will help you think

more deeply about your reading.

When water vapour loses thermal energy and becomes liquid

water, condensation has occurred. Rain and dew are examples of

condensation. A cold can of pop placed outside on a hot summer day

often collects water droplets. This is because water vapour in the air

condenses when it is cooled by the cold can.

Sometimes, removing thermal energy from water vapour causes it

to become a solid, rather than a liquid. Deposition occurs when water

vapour changes state directly from a gas to a solid. Deposition is the

reverse of sublimation. One example of deposition occurs high in the

atmosphere where the temperature is very low. In these conditions,

water vapour forms snow without becoming a liquid first.

Liquid water can also lose thermal energy and undergo freezing :

changing state from a liquid to a solid. We see many examples of this

in everyday life. Puddles, ponds, lakes, and even parts of oceans freeze

when the water becomes cold enough.

Water moves around Earth in the water cycle. Figure 2 shows where

water is found, the state in which it exists, and how it changes from

one state to another. Look at Figure 2 carefully to find water in each of

its three states.

Reading Visual Text:

The Cycle Map

A cycle map is used to illustrate

a process that repeats itself.

In nature, the water cycle and

the life cycle can be illustrated

using a cycle map. Can you

think of other processes that

are repeated in nature?

Condensation, Freezing, and Deposition in Nature

Once water vapour is in the atmosphere, low temperatures cause the

vapour to either condense into a liquid or undergo deposition to

form ice crystals. Water droplets and ice crystals in the atmosphere

form clouds. Air currents move the clouds around the planet. The