Rocket sets a glass of ice water on the Bench. Within minutes, the outside of the glass is beaded with drops.
"The glass is leaking," he says. Raven shakes her head. "Touch the drops. Then look at the water level inside."
Rocket checks. "Same level. So the water outside did not come from inside. Where did it come from?"
Nova hovers near the glass, her light cool and blue. "It came from the air," she says. "Think about which direction the state change went."
"Gas to liquid," Rocket says slowly. "Water vapor in the air touched the cold glass and turned to liquid."
"What do you notice about the puddle on Monday's plate now?" Raven asks. Rocket looks. The plate is dry. "Liquid to gas. It went the other way."
Raven writes two arrows in the Bench Log, one up and one down.
Evaporation is the process that changes liquid water to water vapor. It happens below the boiling point, from puddles, lakes and oceans.
Evaporation needs energy. Heat breaks the bonds that hold water molecules together, and they escape into the air as vapor.
Only the fastest molecules escape, so the liquid left behind has less kinetic energy. Evaporation cools the liquid that remains.
That is how sweat cools you. Water on your skin absorbs body heat and evaporates, leaving your skin cooler.
A liquid evaporates faster when it is warmer, because more of its molecules have enough energy to escape.
About 90 percent of the moisture in the air came from water evaporating from oceans, seas, lakes and rivers.
Condensation is the process of water vapor turning into liquid water. It is the opposite of evaporation.
The drops on the outside of a cold glass on a humid day are condensation. Water vapor in the air cooled on the glass and became liquid.
Clouds form the same way. Air containing water vapor rises and cools, and the water condenses out into tiny droplets.
Freezing is the opposite of melting. Condensation is the opposite of vaporization. Each pair is the same change running in opposite directions.
Some substances skip the liquid state. Carbon dioxide sublimes, going straight from solid to gas at standard pressure.
This week you learned that temperature measures the average kinetic energy of particles and that heat flows from warm to cold.
You read three temperature scales, ran a fair ice melt test, and read the melting and boiling points of real substances.
Next week you will measure why ice floats. Ice is less dense than liquid water, which is why ice cubes float in a glass.
| Week review | True or false? |
|---|---|
| Temperature measures the average kinetic energy of particles. | ? |
| Heat flows from a cold object to a hot one. | ? |
| Evaporation cools the liquid left behind. | ? |
| Condensation is the opposite of evaporation. | ? |
| Water needs to boil before any of it can become a gas. | ? |
Strong week, chemist. Tomorrow is Bench Day: a family freeze and melt with the ice tray.