1What's really happening at the ice
Ice is a tug-of-war — and loose pieces can jam it
You only need two ideas. Watch each one move:
Ice is never sitting still
Even cold ice is a tug-of-war. Water molecules keep leaving the ice (melting) and snapping back on (refreezing). When the snap-backs keep up with the leavers, the ice looks like it's just sitting there.
Loose pieces get in the way
Dissolve something in the wet film and it crowds the surface. Those loose pieces sit in the empty spots, so fewer water molecules can snap back on. Melting keeps going — refreezing falls behind.
2Zoom far inside a grain
Salt splits in two · sugar stays whole
A visible grain contains huge numbers of tiny units. These pictures zoom in on just one salt formula unit and one sugar molecule:
Splits into two pieces
One sodium–chloride formula unit supplies two ions when its crystal dissolves. A real grain supplies enormous numbers of ions.
Stays one whole piece
Each sucrose molecule stays one whole molecule when it leaves the crystal. Its atoms stay joined together.
3Your turn — tip the tug-of-war
Crowd the surface and watch the ice lose the war
No salt or sugar yet — just slide more loose pieces into the wet film and watch. Few pieces: melting and refreezing stay even, so the ice holds. More pieces: refreezing can't keep up, and the ice starts to shrink.
4Now race salt against sugar
Change the counts. Can sugar catch up?
Two matching ice samples have equal amounts of water at the same temperature. In our magnified model, each tap adds one salt formula unit to the left and two sugar molecules to the right. Predict their freezing-point effect.
Guess before you sprinkle
After these unequal counts dissolve, which side lowers the freezing point more in this dilute-solution model?