Rocket keeps adding salt to the same cup of water. Spoon after spoon disappears. Then one does not.
"It stopped!" he says. A white layer sits on the bottom no matter how hard he stirs.
Raven leans in. "What do you notice? The water took a lot, and then it was full."
Nova hovers low, her light glinting off the grains. "Every solvent has a limit," she says. "Think about what full means here."
"The water cannot hold any more salt," Rocket says. "It is like a parking lot with no spaces left."
Raven writes "saturated" in the Bench Log and underlines it. "That is the word for a full solution."
"So how much salt fits in a cup of water?" Rocket asks. Nova dims. "Chemists have measured it. Read on."
Mixtures have different properties depending on the size of their particles. Chemists sort them into solutions, colloids and suspensions.
A solution is a homogeneous mixture with tiny particles. The particles are too small to see and too small to settle or be filtered out.
When salt is thoroughly mixed into water, it forms a solution. The salt is no longer visible, and it will not settle to the bottom.
A suspension is a heterogeneous mixture with large particles. The particles are large enough to see, to settle and to be filtered out.
Throw a handful of sand into a glass of water and the water turns cloudy. Left standing, the sand gradually sinks to the bottom.
A colloid is in between: a homogeneous mixture with medium-sized particles that you can see but that do not settle. Gelatin is a colloid.
| Type of mixture | Particle size | Settles? | Can be filtered out? | Example |
|---|---|---|---|---|
| Solution | Too small to see | No | No | salt water |
| Colloid | Medium, visible | No | No | gelatin |
| Suspension | Large, visible | Yes | Yes | sand in water |
A solution is a homogeneous mixture consisting of a solute dissolved into a solvent.
The solute is the substance that is being dissolved. The solvent is the dissolving medium. Salt is the solute in salt water, and water is the solvent.
The solvent is the material present in the larger amount, and the solute is present in the smaller amount.
Water is called the universal solvent because it dissolves more substances than any other liquid. It cannot dissolve everything, but the name fits well.
Solutions are not always liquids. Air is a solution of gases. About 78 percent of the atmosphere is nitrogen, which makes nitrogen the solvent.
| Solution | Solvent | Solute |
|---|---|---|
| Salt water, seawater | water | salts |
| Fizzy water | water | carbon dioxide gas |
| Vinegar | water | acetic acid |
| Air | nitrogen | oxygen and other gases |
Water molecules move about continuously. When a salt crystal is placed in water, the water molecules collide with the crystal.
Water molecules pull the sodium and chloride particles apart. After coming apart from the crystal, each particle is surrounded by water molecules.
The salt particles spread evenly among the water particles. That is why the solution is the same throughout.
A saturated solution contains the maximum amount of solute that is capable of dissolving. An unsaturated solution contains less than that maximum.
At 20 degrees Celsius, the maximum amount of table salt that will dissolve in 100 grams of water is 36.0 grams.
If more salt is added past this point, it will not dissolve, because the solution is saturated. That is the white layer Rocket saw.
| Statement | True or false? |
|---|---|
| The solute is the substance that is being dissolved. | ? |
| The solvent is present in the smaller amount. | ? |
| Water dissolves more substances than any other liquid. | ? |
| A saturated solution can still dissolve more solute. | ? |
Clear thinking. Tomorrow is Bench Lab: you will make a solution, a suspension and a two-layer mixture, and measure how much salt dissolves.