The Water Cycle

Grade 7 · Science · Study Guide

Follow water as it moves continuously between Earth's surface, atmosphere, and back through evaporation, condensation, and precipitation.

Water is one of the few substances on Earth that naturally exists in all three states — solid (ice), liquid (water), and gas (water vapor) — at temperatures found on our planet's surface. The water cycle, also called the hydrological cycle, describes how water continuously moves through Earth's systems: the atmosphere, the land, the oceans, and living organisms.

The cycle begins with evaporation — liquid water from oceans, lakes, and rivers absorbs solar energy and transforms into water vapor, rising into the atmosphere. Plants also release water vapor through their leaves in a process called transpiration. Together, evaporation and transpiration are called evapotranspiration. This water vapor rises, cools, and condenses around tiny dust particles to form clouds and fog — this is condensation.

When water droplets in clouds become large enough and heavy enough, they fall back to Earth as precipitation — rain, snow, sleet, or hail, depending on temperature. Some precipitation soaks into the ground as infiltration, replenishing groundwater and aquifers. Some flows along the surface as runoff into streams, rivers, and eventually back to the ocean. Some is intercepted by plants and absorbed through roots. Ice and snow in cold regions accumulate as glaciers and ice caps — forming a temporary reservoir called the cryosphere.

The water cycle is crucial to life and climate. It distributes fresh water across continents, regulates temperature (water absorbs and releases large amounts of heat), shapes landscapes through erosion, and makes agriculture possible. Human activities — like over-pumping groundwater, deforestation, and climate change — are altering the water cycle in ways scientists are actively studying.

Key terms

Evaporation: The process by which liquid water at the surface absorbs energy and changes into water vapor.
Condensation: The process by which water vapor cools and changes into liquid droplets, forming clouds or dew.
Precipitation: Water falling from clouds to Earth's surface as rain, snow, sleet, or hail.
Transpiration: The release of water vapor by plants through their leaves.
Runoff: Water that flows over the land surface into streams and rivers after precipitation.
Groundwater: Water that has soaked into the soil and rock below Earth's surface, filling pores and cracks.

Flashcards

Water cycle: The continuous movement of water through Earth's systems via evaporation, condensation, precipitation, and runoff.
Evaporation: Liquid water transforms into water vapor using solar energy.
Condensation: Water vapor cools and forms liquid droplets — clouds, fog, and dew.
Precipitation: Water falling from clouds as rain, snow, sleet, or hail.
Infiltration: Water soaking into the ground to replenish soil moisture and groundwater.
Groundwater: Fresh water stored underground in aquifers below the soil surface.

Practice questions

What process converts liquid water at the surface into water vapor?
1. Condensation
2. Precipitation
3. Evaporation
4. Infiltration
Show answer
Answer: Evaporation
Evaporation occurs when solar energy heats water molecules enough to escape the liquid surface as vapor. Oceans are the biggest source of evaporated water.
Condensation is when water vapor cools and turns back into liquid water.
Show answer
Answer: True
Condensation is the phase change from gas to liquid. As moist air rises and cools, water vapor condenses around tiny particles to form cloud droplets.
The release of water vapor from plant leaves is called ___.
Show answer
Answer: transpiration
Plants pull water up from roots and release it as vapor through stomata in their leaves. Tropical forests transpire enormous amounts of water — essentially 'making' rain.
When precipitation soaks into the ground and refills underground water supplies, this is called:
1. Runoff
2. Condensation
3. Transpiration
4. Infiltration
Show answer
Answer: Infiltration
Infiltration is the process where rain or melted snow seeps into the soil. This water eventually reaches underground aquifers — vital freshwater sources for billions of people.
Most of Earth's fresh water is found in rivers and lakes.
Show answer
Answer: False
About 69% of Earth's fresh water is frozen in glaciers and ice sheets. Only about 0.3% of all fresh water is found in rivers, lakes, and swamps.

Fun facts

The water you drank today might have once been part of a dinosaur — water has been cycling on Earth for over 4 billion years.
About 97% of Earth's water is salt water in the oceans; only about 3% is fresh water, and most of that is frozen in glaciers.

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