What is a mixture?
A mixture is two or more substances combined physically, not chemically. Each part keeps its own properties, and the parts can be separated again.
Examples are everywhere: air, salad, salt water, milk, granite and soil are all mixtures.
Pure substances vs. mixtures
| Pure substance | Mixture |
|---|---|
| Made of only one kind of particle | Made of two or more kinds |
| Fixed composition | Composition can vary |
| Sharp melting and boiling points | Melts and boils over a range |
| Elements (gold, oxygen) and compounds (water, sugar) | Lemonade, sand and iron filings, air |
Types of mixtures
Heterogeneous mixtures
A heterogeneous mixture has parts you can see.
- Trail mix
- Pizza
- Sand and water
- Granite (you can see the different mineral grains)
Homogeneous mixtures
A homogeneous mixture looks the same throughout. You can’t see the separate parts.
- Salt water
- Air
- Vinegar
- Brass (a mixture of copper and zinc)
Homogeneous mixtures are also called solutions.
Solutions
In a solution, one substance dissolves completely in another.
- The solute is what dissolves (salt).
- The solvent does the dissolving (water).
Solutions don’t have to be liquids:
| Solution | Solute | Solvent | State |
|---|---|---|---|
| Salt water | Salt | Water | Liquid |
| Soda pop | Carbon dioxide | Water | Liquid |
| Air | Oxygen, argon, others | Nitrogen | Gas |
| Brass | Zinc | Copper | Solid |
| Steel | Carbon | Iron | Solid |
Metal mixtures like brass and steel are called alloys.
Concentration
A dilute solution has a little solute; a concentrated solution has a lot. A saturated solution has dissolved as much solute as it can at that temperature. Warm water can usually dissolve more sugar than cold water.
Suspensions and colloids
| Type | Particle size | Do particles settle? | Scatters light? | Examples |
|---|---|---|---|---|
| Solution | Tiny (molecules or ions) | No | No | Salt water, sugar water |
| Colloid | Medium | No | Yes | Milk, fog, whipped cream, jelly |
| Suspension | Large | Yes, over time | Yes | Muddy water, salad dressing, orange juice with pulp |
Shine a flashlight through a colloid and you can see the beam, like car headlights in fog. This is the Tyndall effect. A beam passes invisibly through a true solution.
"Shake well before using" on a bottle means it’s a suspension.
Mixtures vs. compounds
This is a common source of confusion.
| Mixture | Compound |
|---|---|
| Substances physically combined | Elements chemically bonded |
| Parts keep their properties | New properties |
| Any proportions | Fixed proportions |
| Separated by physical methods | Separated only by chemical reactions |
| Iron filings + sulfur powder | Iron sulfide |
If you mix iron filings and sulfur, a magnet can still pull out the iron. If you heat them, they react to form iron sulfide, a compound, and the magnet no longer works.
Mixtures in daily life
- Air is about 78% nitrogen, 21% oxygen and 1% other gases.
- Blood is a mixture of plasma, red and white blood cells, and platelets.
- Concrete is cement, sand, gravel and water.
- Seawater is about 3.5% salt.
Common mistakes
- Thinking all mixtures are easy to see. Solutions look like one substance.
- Thinking air is a compound. It’s a mixture of gases.
- Thinking dissolving is a chemical change.
Try it yourself
- Classify: orange juice with pulp, tap water, a chocolate chip cookie, air, salt water, milk.
- Shine a flashlight through glasses of salt water, milk diluted with water, and muddy water. Which show the beam?
- Stir sugar into cold water until no more dissolves. Then try warm water. Which holds more?
(Answers to 1: suspension, solution, heterogeneous, solution, solution, colloid)
Quick recap
- A mixture combines substances physically; each keeps its properties.
- Heterogeneous mixtures have visible parts; homogeneous mixtures (solutions) don’t.
- Solutions have a solute and a solvent and can be solids, liquids or gases.
- Colloids scatter light; suspensions settle.
- Mixtures differ from compounds, which are chemically bonded.