Putting things together
When you make trail mix, you combine nuts, raisins and chocolate chips. Each piece keeps its own properties. You can pick out the raisins if you want. This is a mixture: two or more substances combined but not chemically joined.
Types of mixtures
| Type | What you see | Examples |
|---|---|---|
| Mechanical (uneven) mixture | You can see the different parts | Trail mix, salad, sand and gravel, cereal and milk |
| Solution (even mixture) | Looks like one substance | Salt water, sugar water, air, lemonade |
Solutions
In a solution, one substance dissolves completely into another. The pieces become so tiny you can’t see them.
- The solute is what dissolves (sugar).
- The solvent does the dissolving (water).
Water is called the "universal solvent" because it dissolves so many things.
What dissolves in water?
| Dissolves (soluble) | Does not dissolve (insoluble) |
|---|---|
| Sugar | Sand |
| Salt | Oil |
| Drink crystals | Pepper |
| Baking soda | Chalk |
Making things dissolve faster
- Stir it.
- Use warmer water.
- Use smaller pieces (granulated sugar dissolves faster than a sugar cube).
Separating mixtures
Because the parts of a mixture aren’t chemically joined, they can be separated.
| Method | How it works | Example |
|---|---|---|
| Picking by hand | Remove pieces you can see | Taking raisins out of trail mix |
| Sieving | Small bits fall through the holes | Separating sand from gravel |
| Magnet | Pulls out magnetic pieces | Removing iron filings from sand |
| Filtration | A filter traps solids; liquid passes through | Coffee filter separates grounds from coffee |
| Evaporation | The liquid turns to gas, leaving the solid behind | Getting salt from salt water |
| Floating | Light things float, heavy things sink | Separating wood chips from pebbles |
Filtration won’t separate a solution. Salt water passes right through a filter. To get the salt back, you need evaporation.
Simple chemical changes
A mixture can be separated, but a chemical change makes a brand-new substance. The old substances can’t easily be separated back out.
Signs of a chemical change:
- bubbles of gas
- a colour change
- a new smell
- heat or light
- a new solid forming
Example: baking soda and vinegar
When you mix baking soda and vinegar, it fizzes. The bubbles are a new gas, carbon dioxide. You can’t get the baking soda back. That’s a chemical change.
Other examples: rusting iron, burning wood, baking bread, a cut apple turning brown.
Mixture or chemical change?
| What you do | Result |
|---|---|
| Mixing salt and pepper | Mixture: you could separate them |
| Mixing salt and water | Mixture (solution): evaporate to get the salt |
| Mixing baking soda and vinegar | Chemical change: new gas forms |
| Mixing flour, eggs and sugar, then baking | Chemical change: a cake forms |
In the real world
Water treatment plants filter water to remove dirt. Sea salt is made by evaporating ocean water in shallow ponds. Gold miners used to swirl gravel in pans to separate heavy gold from lighter sand.
Common mistakes
- Thinking dissolved sugar is gone. It’s still there; you can taste it!
- Thinking filtration can separate a solution.
Try it yourself
- Mix sand, salt and water in a cup.
- Filter the mixture through a coffee filter. What stays in the filter?
- Pour the liquid onto a plate and leave it in a sunny spot for a few days. What’s left?
Quick recap
- A mixture is substances combined but not chemically joined.
- A solution is a mixture where a solute dissolves in a solvent.
- Mixtures can be separated by sieving, filtering, magnets and evaporation.
- A chemical change makes a new substance.
- Bubbles, colour changes and new smells are signs of chemical change.