1Two things to notice first
Water goes in a loop — and rivers aren't perfectly pure
You only need two ideas. Watch each one:
Water travels in a loop
The sun lifts water off the sea as invisible vapor. It gathers into clouds, rains back down, and runs to the sea again — round and round, forever.
Rivers carry tiny specks of salt
River water looks clean, but it isn't pure. Rolling over rocks and soil, it picks up tiny, tasteless specks of dissolved salt and carries them to the sea.
2The big question about the sun
When the sun lifts water off the sea, what goes up?
Here's the thing nobody can see. The sun's heat lifts water off the sea every day — but does it carry the salt up too, or leave it behind? There are two ideas:
Could water and salt rise together?
Hypothesis A: heating lifts water and dissolved salt together. If so, the collected vapor would contain salt.
Could only the water rise?
Hypothesis B: heating lifts water while dissolved salt remains. If so, the collected vapor would contain little salt.
3Your turn — run the water cycle
Move the sun and watch the loop spin
Drag the slider to make the sun hotter or cooler. Watch water rise off the sea, become a cloud, rain down, and flow back through the river — the never-ending loop. (We'll worry about the salt next.)
4Now test an isolated sample
Evaporate half the water from one sample
Pause the rivers and isolate one sample. Removing water might change its salt amount, its concentration, or both. Predict the amount left, then watch the sample and collector.
Guess before evaporating half
A sample starts with 100 water units and 10 dissolved-salt units. No rivers add anything. Half the water evaporates. How many salt units remain in the sample?