Getting Sand and Salt Into Separate Dishes
A group is handed a beaker holding 20 g of sand, 10 g of table salt, and 150 mL of water, all stirred together. Their goal is to finish with dry sand in one dish and dry salt in another. The tools on the table are filter paper and a funnel, a hot plate, a stirring rod, an empty beaker, a thermometer, and tweezers. Group 1 pours the mixture through the filter paper first, then heats the liquid that passed through. Group 2 sets the whole beaker on the hot plate and heats until no liquid is left, then plans to pick the salt out of the sand with tweezers.
1. Which group's plan can actually reach the goal? Explain using what happens to sand and to salt in water.
Read the worked answer
Group 1's plan works. Salt dissolves in water, so it passes through the filter paper along with the water; sand does not dissolve, so its grains get trapped in the paper. That separates the two solids. Heating the liquid that came through drives the water off as vapor and leaves salt crystals behind. Group 2's plan fails at the end: heating the whole mixture to dryness leaves salt crystals grown all around and between the sand grains, and the grains are far too small and too stuck together to pick apart with tweezers. Once you dry a mixture of two solids, the easy separation step is gone.
2. Group 1 wants to say it has 'pure salt' at the end. Is the salt dish pure, and does the sand dish need more work? Explain.
Read the worked answer
The salt dish is salt plus whatever was dissolved in the original water - if the water was clean, that is just salt, so it is reasonably pure, though the dish will also contain any dissolved material that was already in the water. The sand dish is not finished: the sand is still wet with salt water. If the group simply dries that wet sand, the salt that was in the liquid will crystallize onto the sand again and re-mix the two solids. The sand should be rinsed with clean water on the filter paper and then dried, so the salt goes down the drain and the sand is left behind.
3. Someone asks where the water went. What should the group say, and why does the answer matter?
Read the worked answer
The water did not disappear - it became water vapor and spread into the air. Matter is not destroyed by heating; it changes state. This matters because it explains where the missing mass went and predicts what the group would see if they put a cool lid over the hot beaker: droplets would form as the vapor condenses. It also warns them that anything dissolved in the water is left behind, not carried away, which is exactly why evaporation recovers the salt.