Grades 3–5 · Materials & Matter
Why can salt remain after saltwater evaporates?
A puddle of salty water can disappear and leave salt behind. Dissolving did not destroy the salt: it spread the salt through the water so finely that separate grains could no longer be seen.
In the diagramEvaporation can leave dissolved salt behindWater vapor enters the surrounding air.Circles mark dissolved salt spread through water.Water leaves as vapor; salt stays in the dish.
During evaporation, water leaves the liquid surface as water vapor and mixes with the air. The dissolved salt does not leave along with that vapor in this example. Less liquid water remains, while the same amount of salt is still present.
As water leaves, the remaining solution becomes more concentrated: more salt is crowded into each equal amount of water. Eventually the remaining water cannot keep all the salt dissolved. Some salt joins together into solid crystals. If all the water evaporates, the salt remains as a solid deposit.
The picture keeps four salt symbols while the number of water symbols decreases. They track what stays and what leaves, not the shapes or sizes of actual particles. The salt left behind explains why drying a salty splash can reveal a pale mark where the water used to be.
About this explanation
Starting knowledge: Know that dissolved material remains present and distinguish liquid water from water vapor.
Sources
- American Chemical Society — Dissolving and Back Again