Worksheets and lesson ideas to challenge students aged 11 to 16 to think hard about filtration and crystallisation (GCSE and Key Stage 3)
Where to start?
Ask students to label a set-up showing apparatus for filtration but purposely omit filter paper. Ask students to spot the error. Then add filter paper and ask students to make a prediction – what will happen when we filter seawater contaminated with bits of visible plastic? Why won’t we be able to separate the salt? Draw particle pictures to explain how the filter paper works. Avoid using single particles to represent substances in the residue – it’s important students get the idea that the residue has a solid arrangement and is not just one massive particle!
Carrying out a simple filtration
Making and separating a precipitate, made from adding sodium hydroxide to copper sulfate solution, can be a great practical to use. If you want a context to frame this practical around, copper hydroxide is used as a fungicide when growing fruits or vegetables, including wine.
Design a filter to purify dirty water for drinking (potable water)
This activity is taken from the secondary resource pack made for British Science Week where students design a filter to purify dirty water. There is an excellent paper that you can then discuss to develop interest and understanding outlining how scientists have designed filter paper made of cellulose (plant cell walls!) that removes viruses from water.
Filtration and crystallisation – separating soluble salts
KS3 practical activity to separate salt and sand. This practical is an open activity that asks students to come up with their own method to separate salt from sand. To increase the challenge, students should be allowed to select the apparatus they want to use. It’s worth pointing out the difference between evaporation and crystallisation – i.e. leave some water for crystallisation to take place in. At the end of the practical the percentage yield of each group is calculated and a winner declared (if you want to!). You could use this method for teaching about preparation of solution salts. (PDF)
- Why can you not separate salt from seawater using filter paper?
- What are the three main types of contaminant in drinking water? How could you ‘remove’ biological ones?
- Where do you find filters in the human body?