The Water Cycle
Grade 6 · Science · Study Guide
Follow a water molecule through evaporation, condensation, precipitation, and back again.
Water is constantly moving around our planet in a never-ending loop called the water cycle (or hydrological cycle). The same water molecules that fell as rain on the dinosaurs may be in your drinking glass today. There is no new water — it just keeps cycling.
The cycle begins with evaporation — heat from the sun causes liquid water from oceans, lakes, and rivers to turn into water vapor (a gas) and rise into the atmosphere. Plants also release water vapor through their leaves in a process called transpiration. Together, evaporation and transpiration are sometimes called evapotranspiration.
As water vapor rises, it cools down in the upper atmosphere. Cool air can hold less water vapor, so the water vapor condenses around tiny dust or pollen particles to form water droplets — this is condensation. These droplets cluster together to form clouds.
When clouds collect enough water droplets, the water falls back to Earth as precipitation — rain, snow, sleet, or hail, depending on temperature. Some precipitation soaks into the ground (infiltration) and refills groundwater reservoirs called aquifers. Some flows over the surface as runoff, feeding rivers and streams that eventually return to the ocean. Then the cycle starts again.
The water cycle is powered by the sun (which drives evaporation) and gravity (which pulls precipitation down and moves runoff toward the ocean). It distributes fresh water around the planet, regulates temperature, and supports all life.
| Stage | Process | Energy Source |
|---|---|---|
| Evaporation | Liquid water → water vapor | Solar energy (heat) |
| Transpiration | Plants release water vapor | Solar energy |
| Condensation | Water vapor → liquid droplets (clouds) | Cooling / altitude |
| Precipitation | Water falls to Earth | Gravity |
| Runoff/Infiltration | Water moves to oceans or underground | Gravity |
Key terms
- Evaporation
- The process of liquid water turning into water vapor due to heat from the sun.
- Condensation
- The process of water vapor cooling and turning back into liquid water droplets, forming clouds.
- Precipitation
- Water falling from clouds as rain, snow, sleet, or hail.
- Transpiration
- The release of water vapor from plant leaves into the atmosphere.
- Runoff
- Water that flows over the land surface into rivers and streams after precipitation.
- Infiltration
- The process of water soaking into the ground to replenish groundwater.
- Aquifer
- An underground layer of rock or soil that stores groundwater.
- Water cycle
- The continuous movement of water through evaporation, condensation, precipitation, and collection.
Flashcards
- Evaporation
- Solar heat turns liquid water into water vapor that rises into the atmosphere.
- Condensation
- Water vapor cools and turns back into liquid droplets, forming clouds.
- Precipitation
- Water falling from clouds as rain, snow, sleet, or hail.
- Transpiration
- Plants release water vapor through their leaves into the air.
- Runoff
- Water flowing over land surfaces into rivers and streams.
- Aquifer
- Underground layer of rock or soil that stores fresh groundwater.
Practice questions
Which process turns liquid water into water vapor?
- Condensation
- Precipitation
- Evaporation
- Infiltration
Show answer
Answer: Evaporation
Evaporation is the phase change from liquid to gas. Heat from the sun gives water molecules enough energy to escape into the atmosphere as vapor.
Clouds form through the process of condensation.
Show answer
Answer: True
When water vapor rises and cools, it condenses around tiny particles (dust, pollen) to form the tiny droplets that make up clouds.
Water that soaks into the ground instead of flowing over the surface is called ___.
Show answer
Answer: infiltration
Infiltration replenishes groundwater in aquifers, which can serve as drinking water sources for communities.
What is the main energy source that drives the water cycle?
- Wind
- The moon
- The sun
- Gravity
Show answer
Answer: The sun
Solar energy heats water, causing evaporation — the main driver of the water cycle. Gravity then pulls precipitation back to Earth.
Transpiration is the evaporation of water from plant leaves.
Show answer
Answer: True
Plants absorb water through their roots and release it as vapor through tiny pores in their leaves called stomata. This is transpiration.
Fun facts
- About 97% of Earth's water is in the oceans and is too salty to drink. Only about 3% is fresh water, and most of that is locked in glaciers and ice caps.
- A single large tree can transpire hundreds of liters of water per day — the Amazon rainforest creates its own rain this way.
- Water can remain underground in an aquifer for thousands of years before it evaporates again.
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