1What's really going on in the glass
Two ideas: water tugs, and grains hold on
You only need two ideas. Watch each one move:
Water is a tugging crowd
Water is millions of tiny jiggling pieces (molecules). They never hold still, and each one has a slightly sticky side. They bump and tug on anything you drop in.
A grain is held by a grip
A grain contains enormous numbers of tiny building blocks. Dissolving separates those blocks from a crystal; it does not just move whole visible grains around.
2Two kinds of grain
Molecular crystals and strong networks
The two powders look alike, but their grains play tug-of-war in opposite ways:
Grabs water back
Water attracts sucrose molecules. It can separate whole molecules from a sugar crystal and surround them, while each molecule stays intact.
A strong mineral network
Water can wet a quartz sand grain, but ordinary water does not readily pull apart its strong network of silicon and oxygen atoms.
3Your turn — pull on one grain
Zoom way in and stir the water harder
Here's one sugar grain up close, with the water crowd around it. Slide to stir harder — watch the water molecules cluster on the grain and start peeling its tiny pieces away. The harder they tug, the faster pieces let go.
give it a stir →
4Now the real test
Can a paper filter catch what disappeared?
Now pour sugar water and a sand–water mixture through two identical paper filters. Their holes are much smaller than the sand grains but much larger than individual molecules. Predict what the filters keep.
Guess before you pour
The sand grains remain whole. The sugar has dissolved. What will be left in the two paper filters?