10.3
The Water Cycle
Key Question: How does water move through
the water cycle?
You may see water as ice, snow, rain, and steam. On
very humid days, you can sense it as water vapour.
Water is the only substance that exists on Earth in its
three states.
CHANGES OF STATE
Water changes state when it gains or loses thermal
energy. Figure 1 shows how water particles behave
when they change state.
increasing thermal energy of particles
sublimation
melting
evaporation
freezing
condensation
solid
gas
deposition
decreasing thermal energy of particles
requires energy (heat)
releases energy (heat)
Figure 1 Water gains or loses thermal energy as it changes state.
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
first becoming a liquid;
occurs when a solid
gains thermal energy
214 Chapter 10 Worksheet 10.3-1
Water changes state in six different ways:
• melting: When ice absorbs enough thermal energy, it
changes to liquid water. For example, ice cubes melt
in a drink.
• sublimation: Sometimes the addition of thermal
energy causes ice to change to a gas without
becoming a liquid first. For example, snow may seem
to disappear without melting into puddles.
Copyright © 2010 by Nelson Education Ltd.
evaporation
the change of state
from a liquid to a gas;
occurs when a liquid
gains thermal energy
• evaporation: When liquid water absorbs enough
thermal energy, it changes to water vapour. For
example, wet clothes on a washing line dry because
the water evaporates into the air.
freezing
the change of state
from a liquid to a solid;
occurs when a liquid
loses thermal energy
• freezing: When liquid water loses enough thermal
energy, it changes to ice. For example, ponds and
lakes freeze in cold weather.
condensation
the change of state
from a gas to a liquid;
occurs when a gas
loses thermal energy
deposition
the change of state
from a gas to a solid;
occurs when a gas
loses thermal energy
• condensation: When water vapour loses enough
thermal energy, it changes to liquid water. For
example, water vapour condenses on cool grass to
form dew.
• deposition: Sometimes the loss of thermal energy
causes water vapour to change directly to a solid. For
example, water vapour creates ice in the form of frost
on cold windows.
THE WATER CYCLE
water cycle
a continuous pattern
in nature in which
water moves as it
changes state above,
on, and below the
surface of Earth
Water changes state in a continuous pattern called the
water cycle. The water cycle is powered by the Sun.
It moves water around Earth (Figure 2).
condensation
Sun
cloud formation
(water storage in the atmosphere)
deposition ice
sublimation
precipitation
freezing
(water storage
in ice and snow)
water vapour
evaporation
surface runoff
(snowmelt into
streams)
surface
water
surface
runoff
groundw
ater
Figure 2 Water changes
state in a pattern called the
water cycle.
Copyright © 2010 by Nelson Education Ltd.
Chapter 10 Worksheet 10.3-1 215
TURN
MELTING, EVAPORATION, AND SUBLIMATION
Melting, evaporation, and sublimation happen when
water particles gain enough thermal energy.
Melting
Solid water exists in winter as ice and snow. It also
exists as permanent ice and snow in glaciers and polar
regions.
When ice and snow absorb enough thermal energy,
they melt to form liquid water. Liquid water also falls
to the ground as rain. The water from ice and snow,
together with rain, flows to different places.
runoff
water from
precipitation and
snowmelt that flows
over Earth’s surface
Some liquid water becomes runoff. Runoff flows over
Earth’s surface to rivers, lakes, and oceans. All of the
liquid water on the surface of Earth is called surface
water.
groundwater
water that seeps
through soil and
cracks in rock; the
source of water for
underground springs
and wells
Some liquid water flows down through openings in
soil and rock until it hits bedrock. Once the liquid water
hits bedrock, it cannot flow down any farther. This
water is called groundwater.
aquifer
a geological formation
of loose rock or soil
that is saturated with
groundwater
water table
the depth at which
loose rock and soil
below Earth’s surface
are saturated with
water; the upper
boundary of an aquifer
Groundwater fills, or saturates, the spaces in the rock
and soil above the bedrock. This saturated area is
called an aquifer (Figure 3). The top surface of the
aquifer is the water table.
water table
soil
groundwater
aquifer
surface water
bedrock
Figure 3 Groundwater saturates loose rock and soil to form an aquifer.
216 Chapter 10 Worksheet 10.3-1
Copyright © 2010 by Nelson Education Ltd.
Evaporation and Sublimation
Solid and liquid water on the surface of Earth move
back into the atmosphere as water vapour.
Water vapour forms in two ways:
• surface water evaporates
• snow and ice sublime
Water vapour in Earth’s atmosphere acts like a blanket.
It traps thermal energy close to Earth.
CONDENSATION, DEPOSITION, AND FREEZING
Condensation, deposition, and freezing happen when
water particles lose enough thermal energy.
Condensation and Deposition
When water vapour in the atmosphere loses thermal
energy, it can condense into water droplets. It can also
undergo deposition to form ice crystals.
Water droplets and ice crystals in the atmosphere form
clouds (Figure 4).
Figure 4 Water vapour forms clouds through condensation and deposition.
precipitation
solid or liquid water
that falls to Earth’s
surface
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These cloud particles fall to the ground as
precipitation. Water droplets collide with each other to
form larger droplets. These large droplets fall as rain.
Ice crystals fall as snow.
Chapter 10 Worksheet 10.3-1 217
TURN
Name:
_________________________________________
Date: ________________________
polar ice sheet
a frozen field of ice
covering either the
North Pole or the
South Pole
When snow falls, it may accumulate on Earth in
different ways:
icecap
a large area of ice that
permanently covers
land
• as ice caps: permanent ice that covers land
glacier
a river of ice, formed
from snow
accumulated over
hundreds of years,
that moves slowly
downhill under the
force of gravity
Freezing
• as polar ice sheets: areas of ice at the North and
South Poles
• as glaciers: rivers of ice that slowly flow down
mountains
In cold temperatures, Earth’s surface water freezes and
forms solid ice.
Ice is slightly less dense than liquid water. For this
reason, a layer of ice forms on top of the liquid water
in lakes and ponds. The organisms in the water below
the ice do not freeze solid.
CHECK YOUR UNDERSTANDING
1. What causes water to change state?
2. What happens to liquid water after it seeps into the ground?
3. How do solid and liquid water change to water vapour?
4. How does water in Earth’s atmosphere return to Earth’s surface?
5. Think back to the Key Question. Name three ways you can see the water
cycle in action in your community.
218 Chapter 10 Worksheet 10.3-1
END
Copyright © 2010 by Nelson Education Ltd.