Choosing the right way to separate a mixture

Science · Matter & Materials · ages 11-13

Name ______________________ Date ____________

Why does a filter catch sand but let dissolved salt pass through?
- Sand grains are too big to fit through the paper's holes, but salt has dissolved into particles small enough to pass through
- Sand dissolves in water, but salt does not
- The filter paper is sized to trap salt but let sand through
Which technique would you use to separate sand from water?
- Filtration
- Crystallisation
- Chromatography
In simple distillation, the liquid with the higher boiling point evaporates first.
Circle one: True False
This chromatogram shows how far each colored spot traveled up the paper. How many separate substances were in the original mixture?
Answer: ______________
You've already filtered sand out of a salty water mixture. The water that passed through still has salt dissolved in it. Which technique gets solid salt back?
- Crystallisation, by evaporating the water
- Filtration again, with smaller holes
- Chromatography, to sort out the salt
Salty water is heated in a distillation setup. Why does the collected liquid come out as pure water, not salty water?
- Water evaporates and turns back to liquid in the condenser, but the dissolved salt is left behind in the flask
- Salt evaporates first and gets trapped in the condenser
- The heat destroys the salt so only water remains
A dye spot travels 4 cm up the paper while the solvent front travels 8 cm. What is the Rf value?
Answer: ______________
Tom poured salty water through a filter, hoping to catch the salt on the paper. Nothing was left behind except a wet paper. What went wrong?
- The salt was dissolved into particles too small for the filter to catch; he needed to evaporate the water instead
- He used the wrong shape of filter paper
- The water washed the salt away before it could touch the paper

Answer key

For grown-ups. Fold this page away before handing over the rest.

Choosing the right way to separate a mixture W1-mt_U_tJvy3cbB-s1

Sand grains are too big to fit through the paper's holes, but salt has dissolved into particles small enough to pass through · Dissolved salt breaks apart into particles tiny enough to travel through the same gaps that water passes through.
Filtration · Sand is an insoluble solid, so pouring the mixture through filter paper traps the sand while the water flows through.
False · The liquid with the lower boiling point evaporates first, since it needs less heat to turn to vapor.
3 · Each spot on a chromatogram is a different substance, so three spots means three substances were mixed together.
Crystallisation, by evaporating the water · No filter is fine enough to catch dissolved particles. Evaporating the water is what leaves the salt crystals behind.
Water evaporates and turns back to liquid in the condenser, but the dissolved salt is left behind in the flask · Salt does not evaporate at water's boiling point, so only water vapor travels to the condenser and drips out as pure water.
0.5 · Rf is the distance the spot traveled divided by the distance the solvent traveled: 4 divided by 8 is 0.5.
The salt was dissolved into particles too small for the filter to catch; he needed to evaporate the water instead · Filtration only catches particles bigger than the holes in the paper. Dissolved salt needs evaporation, not filtration.