From the ocean to the salt shaker
In coastal salt works, seawater is let into shallow ponds where the sun and wind evaporate the water, leaving crude salt behind. That salt still contains sand and grit, so it is dissolved, filtered, and evaporated again before it reaches your salt shaker. Water treatment plants, distilleries, and oil refineries all do the same basic job every day: separating the parts of a mixture.
The principle: use differences in properties
Each part of a mixture keeps its own properties. If you can find a difference in properties between the parts, you can find a way to separate them.
| Method | Difference it uses | Use it for | Example |
|---|---|---|---|
| Filtration过滤 | Whether a solid dissolves (whether its particles can pass through filter paper) | An insoluble solid mixed with a liquid | Removing sand from water |
| Evaporation蒸发 | Whether a part evaporates | Recovering a dissolved solid from a solution | Making sea salt |
| Distillation蒸馏 | Boiling point | Recovering a pure liquid from a solution; separating liquids with different boiling points | Making distilled water, distilling spirits |
| Magnetic separation | Whether a part is attracted to a magnet | Iron mixed with other solids | Pulling iron filings out of sand |
You learned how to filter and evaporate in lesson 1.1.3. This lesson adds distillation.
Distillation
In distillation, you heat a liquid until it turns into vapor, then cool the vapor so it condenses back into a liquid. Anything that doesn’t evaporate (such as salt) stays behind in the flask, so the liquid you collect is much purer.
A typical lab setup has a distilling flask, a thermometer, a condenser, and a receiving flask. Key points:
- Add a few boiling chipsboiling chipsA few small porous chips (bits of broken porcelain also work) added to a liquid before heating. Bubbles form smoothly on their surfaces, which prevents bumping. to the flask so the liquid doesn’t boil violently (“bumpbumpingSudden, violent boiling of a heated liquid instead of steady bubbling. It can throw hot liquid out of the flask, so it’s dangerous.”).
- Place the thermometer bulb level with the side arm of the flask, so it measures the temperature of the vapor.
- Cooling water enters the condenser at the bottom and leaves at the top, so the condenser stays completely full of cold water.
- Fill the flask no more than 2/3 full, and heat it gently and evenly.
Cooling water enters the condenser at the bottom and leaves at the top, flowing against the vapor, so the condenser stays full of cold water. The thermometer measures the temperature of the vapor, so its bulb sits level with the side arm. Boiling chips in the flask prevent bumping.
Try it: separate a mixture step by step
Pick a mixture, then click separation methods in order. Each step is worked out from the properties of the parts. See which methods get you what you want and which ones fail. Purifying rock salt takes several steps:
Example 1: Purifying rock salt
Rock salt contains sand that doesn’t dissolve in water. How can you get fairly pure salt from it? What does the glass stirring rod do at each step?
- Dissolve. Put the rock salt in water and stir with the rod to dissolve it faster. The salt dissolves; the sand doesn’t.
- Filter. Pour the cloudy liquid through filter paper, using the rod to guide the liquid into the funnel. The sand stays on the paper, and the liquid that passes through is salt water.
- Evaporate. Heat the salt water in an evaporating dish while stirring constantly, so no spot overheats and spatters. Stop heating when plenty of solid appears and let the leftover heat dry the rest.
- Summary. The order is dissolve → filter → evaporate. Finally, use the rod to scrape the salt out, so the rod gets used four times in all.
Example 2: Fresh water from seawater
On a ship far out at sea, how can you turn seawater into fresh drinking water?
- Try filtering. The salt is dissolved, and its particles pass right through filter paper. The filtered water is still salty, so filtering fails.
- Try evaporating. Evaporation gives you salt, but the water escapes as vapor, so you lose the very thing you wanted.
- Distill. Heat the seawater: the water turns into vapor while the salt stays in the flask. The vapor condenses in the condenser, and the liquid you collect is fresh water.
- Conclusion. If you want the liquid (the water), distill. If you only want the dissolved solid (the salt), evaporation is enough.
- Removing sand from water
Sand doesn't dissolve, so it stays on the filter paper.
- Getting salt back from salt water
The water evaporates and the salt, which doesn't evaporate, is left behind.
- Making distilled water from tap water
The water turns into vapor and condenses again, while dissolved impurities stay in the flask.
- Separating iron filings from copper powder
Iron is attracted to a magnet; copper isn't.
- Separating cooked pasta from its cooking water
The pasta is an insoluble solid. A colander is just a coarse filter.
- Getting fresh water from seawater
You want the water itself, so distill it.
- Making sea salt in sunny salt ponds
Sun and wind evaporate the water and leave the salt.
- Separating ethanol from a mixture of ethanol and water
Ethanol boils at 78 °C, lower than water, so it vaporizes first and is collected.
- Removing a few iron filings from table salt
The iron filings are attracted to a magnet; the salt isn't.
Common mistakes
Mistake 1: “Filtering removes every impurity from water.” Filtering only removes solids that don’t dissolve. Dissolved substances pass right through the paper, so filtered water is still a mixture.
Mistake 2: Putting the thermometer into the boiling liquid during distillation. The bulb belongs level with the side arm of the flask, where it measures the temperature of the vapor heading into the condenser.
Mistake 3: Running the cooling water in at the top. Water should enter at the bottom and leave at the top. If it enters at the top, it drains straight out the bottom, the condenser never fills, and cooling is poor.
Quick check
What is the correct order of steps for purifying rock salt?
First dissolve the salt so it goes into solution, then filter out the sand, and finally evaporate the water to get the salt.
Distillation can separate ethanol from water because the two differ in their
Ethanol boils at about 78 °C and water at 100 °C. When heated, the ethanol vaporizes first and is collected after condensing.
Summary
- To separate a mixture, take advantage of a difference in the properties of its parts.
- Filtration separates an insoluble solid from a liquid; evaporation recovers a dissolved solid; distillation uses differences in boiling point to collect a pure liquid.
- Purifying rock salt: dissolve → filter → evaporate. The stirring rod speeds dissolving, guides the liquid, prevents spattering, and finally scrapes out the solid.
- In distillation, the thermometer bulb sits at the side arm, cooling water goes in at the bottom and out at the top, and boiling chips prevent bumping.
On to AP (for later)
AP topic 3.9 covers separating mixtures, including chromatography as well as distillation. AP expects you to explain the separation with intermolecular forces: substances with weaker intermolecular forces have lower boiling points and distill over first, and in chromatography, the parts that are attracted less strongly to the stationary phase travel farther.