CoachWhen warmth makes liquid water fly off into the air as a gas, the water evaporates — the puddle shrinks, the laundry dries. When water vapor touches something cold and turns back into liquid droplets, it condenses — mist on the cold window, fog on the bathroom mirror. Again two opposite doors: warmth sends water into the gas state, cold calls it back to the liquid.
Examples
Example 9.5 (Try it: catch the invisible water)
Take a can or a glass straight from the refrigerator, dry its outside carefully with a towel, and set it on the table. Wait while you count to . The outside turns misty, then wet — drops appear from nowhere! They are not leaking through the glass: the invisible water vapor in the room’s air touched the cold wall and condensed. You just caught the invisible gas.
Example 9.6 (Breath clouds)
Your breath always carries invisible water vapor. On a warm day it stays invisible. On a freezing morning, the cold outdoor air condenses it at once into a little cloud in front of your mouth — and the same happens on a cold window when you breathe on it to draw with your finger.
Example 9.8 (The shrinking puddle)
After the rain, a puddle shines on the playground. By afternoon it has shrunk; by tomorrow it is gone. Nobody drank it and it did not leak through the tarmac: the Sun’s warmth made it evaporate, drop by drop, into the air. Draw a chalk line around a puddle in the morning and check after lunch: the water has retreated inside your line.