Matter exists in one of four different categories or states – solid, liquid, gas, or plasma. Some molecules can exist in multiple states, while others will only ever be in one. When we think of water, we usually think of the liquid that comes out of our faucet, falls from the sky as rain, or fills creeks, rivers, ponds, lakes, and oceans. The water in these examples is in liquid form, but if you heat up water, it changes. It stops being a liquid and becomes a gas called water vapor. Another common name for this vapor is steam. Similarly, when liquid water gets really cold, it changes state again. This time it becomes a solid – ice.
The water that exists on the surface of the Earth is the same water that has been here for millions of years. On the surface of the planet, water can be found in liquid form (water), or as a solid (snow and ice). Water can also be found in the atmosphere as water vapor. Water moves from the atmosphere to the planet as precipitation. Water moving from the planet to the atmosphere is called evaporation. When there is more energy from the sun present, like on a hot, sunny day, water evaporates more quickly. Humidity refers to the amount of water vapor in the air. High humidity is when a large amount of water vapor is present. Low humidity is when little water vapor is present.
The Experiment
Supplies: A hot sunny day, chalk, a paper cup of water, a flat portion of concrete, a watch or clock.
What to do: Pour some water on the concrete or asphalt to make a puddle about 18 inches in diameter – preferably in full sunlight. Use the chalk to make a circle around the edge of the puddle. Every 15 minutes or so come back to examine the puddle and draw a new circle around it.
What happened to the puddle each time you checked on it? How long did it take for the puddle to disappear entirely?
What is happening: Energy from the sun, in the form of heat, changes the water from liquid to water vapor, causing it to evaporate into the atmosphere.
Links
To learn more about evaporation and the water cycle, head over to the National Geographic Kids website.