Rocket stares at Wednesday's salt water. "The salt is in there. How do I get it back out?"
Raven points at a shallow dish on the windowsill. A thin white crust rims the edge where water used to be.
"What do you notice?" she asks. "That was salt water on Monday."
"The water left and the salt stayed," Rocket says. "Because water evaporates and salt does not."
Nova hovers over the dish. "Every part of a mixture keeps its own properties," she says. "Use the property that is different."
Rocket grabs a coffee filter and a pinch of sand from the plant pot. "Then this is easy."
Raven grins. "Filtering for big particles, evaporating for dissolved ones. Two tools, one idea."
The components of a mixture keep their own identity when they combine, so they retain their physical properties.
A physical property is a characteristic of a substance that can be observed or measured without changing what the substance is.
Physical properties include color, density, hardness, solubility, melting point and boiling point.
When the parts of a mixture differ in a physical property, that difference can be used to separate them.
Filtration separates particles that are large enough to be caught by a porous material. Sand stays in a coffee filter, and water passes through.
Evaporation separates a dissolved solid from a liquid. The liquid leaves as vapor and the solid stays behind.
Think of a very salty lake. The water has a lower boiling point than the salt and evaporates in the sun. The salt is left behind on the shore.
Distillation separates two or more pure liquids. The mixture is heated, and the liquid with the lowest boiling point turns to vapor first.
Chromatography separates a mixture by passing it through a material in which the parts move at different rates. It works well for colors.
| Method | Property it uses | Separates |
|---|---|---|
| Filtration | Particle size | large particles from a liquid |
| Evaporation | The liquid turns to vapor, the solid does not | a dissolved solid from its solvent |
| Distillation | Boiling point | two or more liquids |
| Chromatography | How fast each part moves through a material | mixed colors or pigments |
You live inside a mixture. Air is a solution of gases, with nitrogen as the solvent and oxygen as a solute.
The ocean is a solution of salts in water, about 35,000 parts per million. Rivers carry dissolved minerals wherever water flows.
Water takes along valuable chemicals, minerals and nutrients wherever it goes, through the ground and through living things.
Next week you will build models of molecules and see how atoms team up to make water, carbon dioxide and salt.
| Week review | True or false? |
|---|---|
| The substances in a mixture keep their own properties. | ? |
| A solution has particles large enough to filter out. | ? |
| The solvent is the dissolving medium. | ? |
| Evaporation leaves a dissolved solid behind. | ? |
| Most solids dissolve less in warmer water. | ? |
Strong week, chemist. Tomorrow is Bench Day: your family makes and separates a mixture together.